1.
Mercury-vapor lamp
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A mercury-vapor lamp is a gas discharge lamp that uses an electric arc through vaporized mercury to produce light. The arc discharge is generally confined to a fused quartz arc tube mounted within a larger borosilicate glass bulb. Mercury vapor lamps are more efficient than incandescent and most fluorescent lights. Their other advantages are a long lifetime in the range of 24,000 hours. For these reasons, they are used for large area lighting, such as in factories, warehouses. Clear mercury lamps produce light with a bluish-green tint due to mercurys combination of spectral lines. This is not flattering to human skin color, so such lamps are not used in retail stores. Color corrected mercury bulbs overcome this problem with a phosphor on the inside of the bulb that emits white light. They offer better color rendition than the more efficient high or low-pressure sodium vapor lamps and they operate at an internal pressure of around one atmosphere and require special fixtures, as well as an electrical ballast. They also require a period of 4 –7 minutes to reach full light output. Mercury vapor lamps are becoming obsolete due to the higher efficiency, charles Wheatstone observed the spectrum of an electric discharge in mercury vapor in 1835, and noted the ultraviolet lines in that spectrum. In 1860, John Thomas Way used arc lamps operated in a mixture of air, the German physicist Leo Arons studied mercury discharges in 1892 and developed a lamp based on a mercury arc. In February 1896 Dowsing and Keating of England patented a mercury vapour lamp, the first mercury vapor lamp to achieve widespread success was invented in 1901 by American engineer Peter Cooper Hewitt. Patent 682,692 on September 17,1901, in 1903, Hewitt created an improved version that possessed higher color qualities which eventually found widespread industrial use. The ultraviolet light from mercury vapor lamps was applied to water treatment by 1910, the Hewitt lamps used a large amount of mercury. In the 1930s, improved lamps of the form, developed by the Osram-GEC company, General Electric company. The mercury in the tube is a liquid at normal temperatures and it needs to be vaporized and ionized before the tube will conduct electricity and the arc can start. So, like fluorescent tubes, mercury vapor lamps require a starter, a third electrode is mounted near one of the main electrodes and connected through a resistor to the other main electrode
2.
Chemistry
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Chemistry is a branch of physical science that studies the composition, structure, properties and change of matter. Chemistry is sometimes called the science because it bridges other natural sciences, including physics. For the differences between chemistry and physics see comparison of chemistry and physics, the history of chemistry can be traced to alchemy, which had been practiced for several millennia in various parts of the world. The word chemistry comes from alchemy, which referred to a set of practices that encompassed elements of chemistry, metallurgy, philosophy, astrology, astronomy, mysticism. An alchemist was called a chemist in popular speech, and later the suffix -ry was added to this to describe the art of the chemist as chemistry, the modern word alchemy in turn is derived from the Arabic word al-kīmīā. In origin, the term is borrowed from the Greek χημία or χημεία and this may have Egyptian origins since al-kīmīā is derived from the Greek χημία, which is in turn derived from the word Chemi or Kimi, which is the ancient name of Egypt in Egyptian. Alternately, al-kīmīā may derive from χημεία, meaning cast together, in retrospect, the definition of chemistry has changed over time, as new discoveries and theories add to the functionality of the science. The term chymistry, in the view of noted scientist Robert Boyle in 1661, in 1837, Jean-Baptiste Dumas considered the word chemistry to refer to the science concerned with the laws and effects of molecular forces. More recently, in 1998, Professor Raymond Chang broadened the definition of chemistry to mean the study of matter, early civilizations, such as the Egyptians Babylonians, Indians amassed practical knowledge concerning the arts of metallurgy, pottery and dyes, but didnt develop a systematic theory. Greek atomism dates back to 440 BC, arising in works by such as Democritus and Epicurus. In 50 BC, the Roman philosopher Lucretius expanded upon the theory in his book De rerum natura, unlike modern concepts of science, Greek atomism was purely philosophical in nature, with little concern for empirical observations and no concern for chemical experiments. Work, particularly the development of distillation, continued in the early Byzantine period with the most famous practitioner being the 4th century Greek-Egyptian Zosimos of Panopolis. He formulated Boyles law, rejected the four elements and proposed a mechanistic alternative of atoms. Before his work, though, many important discoveries had been made, the Scottish chemist Joseph Black and the Dutchman J. B. English scientist John Dalton proposed the theory of atoms, that all substances are composed of indivisible atoms of matter. Davy discovered nine new elements including the alkali metals by extracting them from their oxides with electric current, british William Prout first proposed ordering all the elements by their atomic weight as all atoms had a weight that was an exact multiple of the atomic weight of hydrogen. The inert gases, later called the noble gases were discovered by William Ramsay in collaboration with Lord Rayleigh at the end of the century, thereby filling in the basic structure of the table. Organic chemistry was developed by Justus von Liebig and others, following Friedrich Wöhlers synthesis of urea which proved that organisms were, in theory
3.
Ultraviolet
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Ultraviolet is an electromagnetic radiation with a wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. UV radiation constitutes about 10% of the light output of the Sun. It is also produced by electric arcs and specialized lights, such as lamps, tanning lamps. Consequently, the effects of UV are greater than simple heating effects. Suntan, freckling and sunburn are familiar effects of over-exposure, along with risk of skin cancer. Living things on dry land would be damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earths atmosphere. More-energetic, shorter-wavelength extreme UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground, Ultraviolet is also responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans. The UV spectrum thus has both beneficial and harmful to human health. Ultraviolet rays are invisible to most humans, the lens in a human eye ordinarily filters out UVB frequencies or higher, and humans lack color receptor adaptations for ultraviolet rays. Under some conditions, children and young adults can see ultraviolet down to wavelengths of about 310 nm, near-UV radiation is visible to some insects, mammals, and birds. Small birds have a fourth color receptor for ultraviolet rays, this gives birds true UV vision, reindeer use near-UV radiation to see polar bears, who are poorly visible in regular light because they blend in with the snow. UV also allows mammals to see urine trails, which is helpful for animals to find food in the wild. The males and females of some species look identical to the human eye. Ultraviolet means beyond violet, violet being the color of the highest frequencies of visible light, Ultraviolet has a higher frequency than violet light. He called them oxidizing rays to emphasize chemical reactivity and to them from heat rays. The terms chemical and heat rays were eventually dropped in favour of ultraviolet and infrared radiation, in 1878 the effect of short-wavelength light on sterilizing bacteria was discovered. By 1903 it was known the most effective wavelengths were around 250 nm, in 1960, the effect of ultraviolet radiation on DNA was established. The discovery of the ultraviolet radiation below 200 nm, named vacuum ultraviolet because it is absorbed by air, was made in 1893 by the German physicist Victor Schumann
4.
Wavelength
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In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the waves shape repeats, and thus the inverse of the spatial frequency. Wavelength is commonly designated by the Greek letter lambda, the concept can also be applied to periodic waves of non-sinusoidal shape. The term wavelength is also applied to modulated waves. Wavelength depends on the medium that a wave travels through, examples of wave-like phenomena are sound waves, light, water waves and periodic electrical signals in a conductor. A sound wave is a variation in air pressure, while in light and other electromagnetic radiation the strength of the electric, water waves are variations in the height of a body of water. In a crystal lattice vibration, atomic positions vary, wavelength is a measure of the distance between repetitions of a shape feature such as peaks, valleys, or zero-crossings, not a measure of how far any given particle moves. For example, in waves over deep water a particle near the waters surface moves in a circle of the same diameter as the wave height. The range of wavelengths or frequencies for wave phenomena is called a spectrum, the name originated with the visible light spectrum but now can be applied to the entire electromagnetic spectrum as well as to a sound spectrum or vibration spectrum. In linear media, any pattern can be described in terms of the independent propagation of sinusoidal components. The wavelength λ of a sinusoidal waveform traveling at constant speed v is given by λ = v f, in a dispersive medium, the phase speed itself depends upon the frequency of the wave, making the relationship between wavelength and frequency nonlinear. In the case of electromagnetic radiation—such as light—in free space, the speed is the speed of light. Thus the wavelength of a 100 MHz electromagnetic wave is about, the wavelength of visible light ranges from deep red, roughly 700 nm, to violet, roughly 400 nm. For sound waves in air, the speed of sound is 343 m/s, the wavelengths of sound frequencies audible to the human ear are thus between approximately 17 m and 17 mm, respectively. Note that the wavelengths in audible sound are much longer than those in visible light, a standing wave is an undulatory motion that stays in one place. A sinusoidal standing wave includes stationary points of no motion, called nodes, the upper figure shows three standing waves in a box. The walls of the box are considered to require the wave to have nodes at the walls of the box determining which wavelengths are allowed, the stationary wave can be viewed as the sum of two traveling sinusoidal waves of oppositely directed velocities. Consequently, wavelength, period, and wave velocity are related just as for a traveling wave, for example, the speed of light can be determined from observation of standing waves in a metal box containing an ideal vacuum. In that case, the k, the magnitude of k, is still in the same relationship with wavelength as shown above
5.
Light
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Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to light, which is visible to the human eye and is responsible for the sense of sight. Visible light is defined as having wavelengths in the range of 400–700 nanometres, or 4.00 × 10−7 to 7.00 × 10−7 m. This wavelength means a range of roughly 430–750 terahertz. The main source of light on Earth is the Sun, sunlight provides the energy that green plants use to create sugars mostly in the form of starches, which release energy into the living things that digest them. This process of photosynthesis provides virtually all the used by living things. Historically, another important source of light for humans has been fire, with the development of electric lights and power systems, electric lighting has effectively replaced firelight. Some species of animals generate their own light, a process called bioluminescence, for example, fireflies use light to locate mates, and vampire squids use it to hide themselves from prey. Visible light, as all types of electromagnetic radiation, is experimentally found to always move at this speed in a vacuum. In physics, the term sometimes refers to electromagnetic radiation of any wavelength. In this sense, gamma rays, X-rays, microwaves and radio waves are also light, like all types of light, visible light is emitted and absorbed in tiny packets called photons and exhibits properties of both waves and particles. This property is referred to as the wave–particle duality, the study of light, known as optics, is an important research area in modern physics. Generally, EM radiation, or EMR, is classified by wavelength into radio, microwave, infrared, the behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths, when EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. There exist animals that are sensitive to various types of infrared, infrared sensing in snakes depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, which is how these animals detect it, above the range of visible light, ultraviolet light becomes invisible to humans, mostly because it is absorbed by the cornea below 360 nanometers and the internal lens below 400. Furthermore, the rods and cones located in the retina of the eye cannot detect the very short ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses are able to detect ultraviolet, by quantum photon-absorption mechanisms, various sources define visible light as narrowly as 420 to 680 to as broadly as 380 to 800 nm
6.
Infrared
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It extends from the nominal red edge of the visible spectrum at 700 nanometers, to 1000000 nm. Most of the radiation emitted by objects near room temperature is infrared. Like all EMR, IR carries radiant energy, and behaves both like a wave and like its quantum particle, the photon, slightly more than half of the total energy from the Sun was eventually found to arrive on Earth in the form of infrared. The balance between absorbed and emitted infrared radiation has an effect on Earths climate. Infrared radiation is emitted or absorbed by molecules when they change their rotational-vibrational movements and it excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines absorption and transmission of photons in the infrared range, Infrared radiation is used in industrial, scientific, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected, Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatuses. Thermal-infrared imaging is used extensively for military and civilian purposes, military applications include target acquisition, surveillance, night vision, homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10 μm, Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers to 1 mm. This range of wavelengths corresponds to a range of approximately 430 THz down to 300 GHz. Below infrared is the portion of the electromagnetic spectrum. Sunlight, at a temperature of 5,780 kelvins, is composed of near thermal-spectrum radiation that is slightly more than half infrared. At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level, of this energy,527 watts is infrared radiation,445 watts is visible light, and 32 watts is ultraviolet radiation. Nearly all the radiation in sunlight is near infrared, shorter than 4 micrometers. On the surface of Earth, at far lower temperatures than the surface of the Sun, almost all thermal radiation consists of infrared in mid-infrared region, much longer than in sunlight. Of these natural thermal radiation processes only lightning and natural fires are hot enough to produce much visible energy, thermal infrared radiation also has a maximum emission wavelength, which is inversely proportional to the absolute temperature of object, in accordance with Wiens displacement law. Therefore, the band is often subdivided into smaller sections. Due to the nature of the blackbody radiation curves, typical hot objects, such as exhaust pipes, the three regions are used for observation of different temperature ranges, and hence different environments in space
7.
Excited state
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In quantum mechanics, an excited state of a system is any quantum state of the system that has a higher energy than the ground state. Excitation is an elevation in energy level above a baseline energy state. In physics there is a technical definition for energy level which is often associated with an atom being raised to an excited state. The temperature of a group of particles is indicative of the level of excitation and this return to a lower energy level is often loosely described as decay and is the inverse of excitation. Long-lived excited states are called metastable. Long-lived nuclear isomers and singlet oxygen are two examples of this, a simple example of this concept comes by considering the hydrogen atom. The ground state of the hydrogen atom corresponds to having the single electron in the lowest possible orbit. By giving the additional energy, the electron is able to move into an excited state. If the photon has too much energy, the electron will cease to be bound to the atom, after excitation the atom may return to the ground state or a lower excited state, by emitting a photon with a characteristic energy. Emission of photons from atoms in excited states leads to an electromagnetic spectrum showing a series of characteristic emission lines An atom in a high excited state is termed Rydberg atom. A system of highly excited atoms can form a long-lived condensed excited state e. g. a condensed phase made completely of excited atoms, hydrogen can also be excited by heat or electricity. This phenomenon has been studied in the case of a gas in some detail. These calculations are more difficult than non-excited state calculations, a further consequence is reaction of the atom in the excited state, as in photochemistry. Excited states give rise to chemical reaction, Rydberg formula Stationary state Repulsive state NASA background information on ground and excited states
8.
Photoelectrochemical process
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Photoelectrochemical processes are processes in photoelectrochemistry, they usually involve transforming light into other forms of energy. These processes apply to photochemistry, optically pumped lasers, sensitized solar cells, luminescence, electron excitation is the movement of an electron to a higher energy state. Within a semiconductor crystal lattice, thermal excitation is a process where lattice vibrations provide enough energy to move electrons to an energy band. When an excited electron falls back to an energy state again. This can be done by radiation of a photon or giving the energy to a third spectator particle as well, in physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state. The excited state, in general, is in relation to the ground state, photoexcitation is the mechanism of electron excitation by photon absorption, when the energy of the photon is too low to cause photoionization. The absorption of the takes place in accordance with Plancks quantum theory. Photoexcitation is exploited in dye-sensitized solar cells, photochemistry, luminescence, optically pumped lasers, in chemistry, photoisomerization is molecular behavior in which structural change between isomers is caused by photoexcitation. Both reversible and irreversible photoisomerization reactions exist, however, the word photoisomerization usually indicates a reversible process. Photoisomerizable molecules are already put to use, for instance, in pigments for rewritable CDs, DVDs. In addition, recent interest in photoisomerizable molecules has been aimed at molecular devices, such as switches, molecular motors. Photoisomerization behavior can be categorized into several classes. Two major classes are trans-cis conversion, and open-closed ring transition, examples of the former include stilbene and azobenzene. This type of compounds has a bond, and rotation or inversion around the double bond affords isomerization between the two states. Examples of the latter include fulgide and diarylethene and this type of compounds undergoes bond cleavage and bond creation upon irradiation with particular wavelengths of light. Still another class is the Di-pi-methane rearrangement, Photoionization is the physical process in which an incident photon ejects one or more electrons from an atom, ion or molecule. This is essentially the process that occurs with the photoelectric effect with metals. In the case of a gas or single atoms, the term photoionization is more common, the ejected electrons, known as photoelectrons, carry information about their pre-ionized states
9.
Theodor Grotthuss
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Freiherr Christian Johann Dietrich Theodor von Grotthuss was a German chemist known for establishing the first theory of electrolysis in 1806 and formulating the first law of photochemistry in 1817. His theory of electrolysis is considered the first description of the so-called Grotthuss mechanism, Grotthuss was born in 1785 in Leipzig, Germany, during an extended stay of his parents away from their home in northern Lithuania. He showed interest in sciences and went to study first in Leipzig. Several renowned scientists taught at the École Polytechnique at that time, including Antoine François, comte de Fourcroy, Claude Louis Berthollet, because of some tensions in the relations between Russia and France, Grotthuss had to leave for Italy where he stayed at Naples for one year. The discovery of the first electric cell in 1800 by Alessandro Volta provided the scientists a source of electricity which was used in laboratory experiments around Europe. The electrolysis of water, acids and salt solutions was reported, Grotthuss actively contributed to this area both in terms of electrolysis experiments and their interpretation. During his stay in Italy, he published his work on electrolysis in 1806, the following two years Grotthuss spent in Rome, some other Italian cities, and Paris, and then went back to Russia via Munich and Vienna. From 1808 on he lived at the estate of his mother in northern Lithuania, there he conducted research on electricity and light with the limited research equipment he could assemble. Grotthuss committed suicide in the spring of 1822 during a depression caused by health problems, grotthuss–Draper law List of photochemists Stradins, J. Theodore von Grotthuss, 1785-1822. In the Wake of Voltas Challenge, The Electrolysis Theory of Theodor Grotthuss,1805, Leipzig, Verlag von Veit and Company. Theodor von Grotthuss -- A Trail Blazer, Bulletin of History of Chemistry, vol.32 no
10.
John William Draper
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John William Draper was an English-American scientist, philosopher, physician, chemist, historian and photographer. He is credited with producing the first clear photograph of a female face and he was also the first president of the American Chemical Society and a founder of the New York University School of Medicine. One of Drapers books, the History of the Conflict between Religion and Science, popularised the conflict thesis proposing intrinsic hostility in the relationship between religion and science and it was widely read, and was translated into several languages. His son, Henry Draper, and his granddaughter, Antonia Maury, were astronomers, her sister, Carlotta Maury was a paleontologist. John William Draper was born May 5,1811 in St. Helens, Lancashire, England to John Christopher Draper and he also had three sisters, Dorothy Catherine, Elizabeth Johnson, and Sarah Ripley. On June 23, he was baptized by the Wesleyan minister Jabez Bunting and his father often needed to move the family due to serving various congregations throughout England. John Wm. Draper was home tutored until 1822, when he entered Woodhouse Grove School and he returned to home instruction prior to entering University College London in 1829. While at University College London, Draper studied chemistry under the direction of Edward Turner, on September 13,1831, John William Draper married Antonia Coetana de Paiva Pereira Gardner, the daughter of Daniel Gardner, a court physician to John VI of Portugal and Charlotte of Spain. Antonia was born in Brazil after the family fled Portugal with Napoleons invasion. There is dispute as to the identity of Antonias mother, around 1830, Antonia was sent with her brother Daniel to live with their aunt in London. Following his fathers death in July 1831, John Williams mother was urged to move with her children to the US state of Virginia, John William hoped to acquire a teaching position at a local Methodist college. In 1832, the family settled in Mecklenburg County, Virginia,7 1⁄2 miles east of Christiansville, although he arrived too late to obtain the prospective teaching position, John William established a laboratory in Christiansville. Here he conducted experiments and published eight papers before entering medical school and his sister, Dorothy Catherine Draper provided finances through teaching drawing and painting for his medical education. In March 1836, he graduated from the University of Pennsylvania School of Medicine and that same year, he began teaching at Hampden–Sydney College in Virginia. In 1837, Draper accepted an appointment to be head of chemistry in a medical school at New York University. In 1839, Draper was elected professor of chemistry and botany at the university. In 1839–1840, Draper produced clear photographs, which at time were regarded as the first life photographs of a human face. Draper took a series of pictures, with a 65-second exposure in sunlight, the first ones, of a female assistant whose face was covered with a thin layer of flour to increase contrast, were not preserved
11.
Absorption (electromagnetic radiation)
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In physics, absorption of electromagnetic radiation is the way in which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the energy is transformed into internal energy of the absorber. The reduction in intensity of a wave propagating through a medium by absorption of a part of its photons is often called attenuation. The mass attenuation coefficient, also called mass extinction coefficient, which is the absorption coefficient divided by density, the absorption cross section and scattering cross-section are closely related to the absorption and attenuation coefficients, respectively. Extinction in astronomy is equivalent to the attenuation coefficient, absorbance and optical depth are two related measures All these quantities measure, at least to some extent, how well a medium absorbs radiation. However, practitioners of different fields and techniques tend to use different quantities drawn from the list above. The absorbance of an object quantifies how much of the incident light is absorbed by it and this may be related to other properties of the object through the Beer–Lambert law. A few examples of absorption are ultraviolet–visible spectroscopy, infrared spectroscopy, understanding and measuring the absorption of electromagnetic radiation has a variety of applications. Here are a few examples, In meteorology and climatology, global and local temperatures depend in part on the absorption of radiation by gases and land. In medicine, X-rays are absorbed to different extents by different tissues, for example, see computation of radiowave attenuation in the atmosphere used in satellite link design. In chemistry and materials science, because different materials and molecules will absorb radiation to different extents at different frequencies, in optics, sunglasses, colored filters, dyes, and other such materials are designed specifically with respect to which visible wavelengths they absorb, and in what proportions. In physics, the D-region of Earths ionosphere is known to significantly absorb radio signals that fall within the electromagnetic spectrum. Optical Propagation in Linear Media, Atmospheric Gases and Particles, Solid-State Components, physics Archive - Ask a scientist
12.
Mechanistic organic photochemistry
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Mechanistic organic photochemistry is that aspect of organic photochemistry which seeks to explain the mechanisms of organic photochemical reactions. The absorption of light by organic molecules very often leads to reactions. In the earliest days sunlight was employed while in modern times ultraviolet lamps are employed. Organic photochemistry has proven to be a useful synthetic tool. Complex organic products can be obtained simply, over the last century and earlier an immense number of photochemical reactions have been uncovered. In modern times the field is well understood and is used in organic synthesis. One of the earliest photochemical studies dealt with the natural product santonin, in the 19th century it had been observed by Ciamician that in Italian sunlight santonin gave several photoproducts. The structure of santonin was first correctly described by Clemo and Hayworth in 1929, the initial photoproduct obtained from santonin is lumisantonin. 1, the photoreaction involves a rearrangement, using steroid numbering, we note that the C-3 carbonyl group has moved to C-2, the C-4 methyl has moved to C-1, and the C-10 carbon has been inverted. A comparatively bizarre example was uncovered by Egbert Havinga in 1956, the curious result was activation on photolysis by a meta nitro group in contrast to the usual activation by ortho and para groups. Over the decades, many interesting but puzzling organic photochemical reactions were discovered that did not proceed by ordinary organic ground state processes, rather, they arose from the excited states of electrons in the compounds. The real problem was that, at the time, organic chemists were not versed in quantum mechanics, real mechanistic treatments were not possible. Starting in 1961 it was found that one could understand organic photochemical reactions in the context of the relevant excited states, one example is the n-pi* excitation of mono-carbonyl compounds, the simplest being that of formaldehyde. The structure was first described by Mulliken, the three-dimensional representation is simplified in the second line using a two-dimensional representation, which facilitates arrow pushing. In this early research simple Hückel computations were used to get excited state electron densities, the stereochemistry in Scheme 1 is shown three-dimensionally. The Hückel computations revealed that the beta-carbons of the ring had a large bond-order. As seen in the scheme a beta-beta bond is formed, subsequent to this, radiationless decay leads to a zwitterion ground state. The final rearrangement leads to lumisantonin as can be discerned by comparing the three-dimensional drawing with the earlier two-dimensional representation and this is followed by intersystem crossing to form the singlet ground state which is seen to be a zwitterion
13.
Johannes Stark
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Johannes Stark was a German physicist and Physics Nobel Prize laureate, who was closely involved with the Deutsche Physik movement under the Nazi regime. Born in Schickenhof, Kingdom of Bavaria, Stark was educated at the Bayreuth Gymnasium and his collegiate education began at the University of Munich, where he studied physics, mathematics, chemistry, and crystallography. His tenure at that college began in 1894, he graduated in 1897, with his dissertation titled Untersuchung über einige physikalische. Stark worked in positions at the Physics Institute of his alma mater until 1900. An extraordinary professor at Hanover by 1906, in 1908 he became professor at the RWTH Aachen University and he worked and researched at physics departments of several universities, including the University of Greifswald, until 1922. In 1919, he won the Nobel Prize in Physics for his discovery of the Doppler effect in canal rays, from 1933 until his retirement in 1939, Stark was elected President of the Physikalisch-Technische Reichsanstalt, while also President of the Deutsche Forschungsgemeinschaft. It was Stark who, as the editor of Jahrbuch der Radioaktivität und Elektronik, asked in 1907, then rather unknown. While working on his article, Einstein began a line of thought that would lead to his generalized theory of relativity. This is heavily ironic, given Starks later work as an anti-Einstein, Stark published more than 300 papers, mainly regarding electricity and other such topics. Probably his best known contribution to the field of physics is the Stark effect and he married Luise Uepler, and they had five children. His hobbies were the cultivation of fruit trees and forestry and he worked in his private laboratory on his country estate in Upper Bavaria after the war. There he studied the deflection of light in an electric field, during the Nazi regime, Stark attempted to become the Führer of German physics through the Deutsche Physik movement against the Jewish physics of Albert Einstein and Werner Heisenberg. After Werner Heisenberg defended Albert Einsteins theory of relativity Stark wrote an article in the SS newspaper Das Schwarze Korps. On August 21,1934 Stark wrote to physicist and fellow Nobel laureate Max von Laue to toe the party line or else, the letter was signed off with a Heil Hitler. He attacked theoretical physics as Jewish and stressed that scientific positions in Nazi Germany should only be held by pure-blooded Germans. In 1947, following the defeat of Germany in World War II, Stark was classified as a Major Offender, die Entladung der Elektricität von galvanisch glühender Kohle in verdünntes Gas. Leipzig,1899 Der elektrische Strom zwischen galvanisch glühender Kohle und einem Metall durch verdünntes Gas, leipzig,1899 Aenderung der Leitfähigkeit von Gasen durch einen stetigen elektrischen Strom. Leipzig,1900 Ueber den Einfluss der Erhitzung auf das elektrische Leuchten eines verdünnten Gases, leipzig,1900 Ueber elektrostatische Wirkungen bei der Entladung der Elektricität in verdünnten Gasen
14.
Albert Einstein
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Albert Einstein was a German-born theoretical physicist. He developed the theory of relativity, one of the two pillars of modern physics, Einsteins work is also known for its influence on the philosophy of science. Einstein is best known in popular culture for his mass–energy equivalence formula E = mc2, near the beginning of his career, Einstein thought that Newtonian mechanics was no longer enough to reconcile the laws of classical mechanics with the laws of the electromagnetic field. This led him to develop his theory of relativity during his time at the Swiss Patent Office in Bern. Briefly before, he aquired the Swiss citizenship in 1901, which he kept for his whole life and he continued to deal with problems of statistical mechanics and quantum theory, which led to his explanations of particle theory and the motion of molecules. He also investigated the properties of light which laid the foundation of the photon theory of light. In 1917, Einstein applied the theory of relativity to model the large-scale structure of the universe. He was visiting the United States when Adolf Hitler came to power in 1933 and, being Jewish, did not go back to Germany and he settled in the United States, becoming an American citizen in 1940. This eventually led to what would become the Manhattan Project, Einstein supported defending the Allied forces, but generally denounced the idea of using the newly discovered nuclear fission as a weapon. Later, with the British philosopher Bertrand Russell, Einstein signed the Russell–Einstein Manifesto, Einstein was affiliated with the Institute for Advanced Study in Princeton, New Jersey, until his death in 1955. Einstein published more than 300 scientific papers along with over 150 non-scientific works, on 5 December 2014, universities and archives announced the release of Einsteins papers, comprising more than 30,000 unique documents. Einsteins intellectual achievements and originality have made the word Einstein synonymous with genius, Albert Einstein was born in Ulm, in the Kingdom of Württemberg in the German Empire, on 14 March 1879. His parents were Hermann Einstein, a salesman and engineer, the Einsteins were non-observant Ashkenazi Jews, and Albert attended a Catholic elementary school in Munich from the age of 5 for three years. At the age of 8, he was transferred to the Luitpold Gymnasium, the loss forced the sale of the Munich factory. In search of business, the Einstein family moved to Italy, first to Milan, when the family moved to Pavia, Einstein stayed in Munich to finish his studies at the Luitpold Gymnasium. His father intended for him to electrical engineering, but Einstein clashed with authorities and resented the schools regimen. He later wrote that the spirit of learning and creative thought was lost in strict rote learning, at the end of December 1894, he travelled to Italy to join his family in Pavia, convincing the school to let him go by using a doctors note. During his time in Italy he wrote an essay with the title On the Investigation of the State of the Ether in a Magnetic Field
15.
Ground state
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The ground state of a quantum mechanical system is its lowest-energy state, the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with greater than the ground state. In the quantum theory, the ground state is usually called the vacuum state or the vacuum. If more than one ground state exists, they are said to be degenerate, many systems have degenerate ground states. Degeneracy occurs whenever there exists a unitary operator which acts non-trivially on a ground state, according to the third law of thermodynamics, a system at absolute zero temperature exists in its ground state, thus, its entropy is determined by the degeneracy of the ground state. Many systems, such as a crystal lattice, have a unique ground state. It is also possible for the highest excited state to have zero temperature for systems that exhibit negative temperature. In 1D, the state of the Schrödinger equation has no nodes. This can be proved considering the energy of a state with a node at x =0, i. e. ψ =0. Consider the average energy in this state ⟨ ψ | H | ψ ⟩ = ∫ d x where V is the potential, now consider a small interval around x =0, i. e. x ∈. Take a new wave function ψ ′ to be defined as ψ ′ = ψ, x < − ϵ and ψ ′ = − ψ, x > ϵ, if ϵ is small enough then this is always possible to do so that ψ ′ is continuous. Note that the energy density | d ψ ′ d x |2 < | d ψ d x |2 everywhere because of the normalization. For definiteness let us choose V ≥0, then it is clear that outside the interval x ∈ the potential energy density is smaller for the ψ ′ because | ψ ′ | < | ψ | there. Therefore, the energy is unchanged up to order ϵ2 if we deform the state with a node ψ into a state without a node ψ ′. We can therefore remove all nodes and reduce the energy, which implies that the wave function cannot have a node. The wave function of the state of a particle in a one-dimensional well is a half-period sine wave which goes to zero at the two edges of the well. The wave function of the state of a hydrogen atom is a spherically-symmetric distribution centred on the nucleus. The electron is most likely to be found at a distance from the equal to the Bohr radius
16.
Spin (physics)
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In quantum mechanics and particle physics, spin is an intrinsic form of angular momentum carried by elementary particles, composite particles, and atomic nuclei. Spin is one of two types of angular momentum in mechanics, the other being orbital angular momentum. In some ways, spin is like a vector quantity, it has a definite magnitude, all elementary particles of a given kind have the same magnitude of spin angular momentum, which is indicated by assigning the particle a spin quantum number. The SI unit of spin is the or, just as with classical angular momentum, very often, the spin quantum number is simply called spin leaving its meaning as the unitless spin quantum number to be inferred from context. When combined with the theorem, the spin of electrons results in the Pauli exclusion principle. Wolfgang Pauli was the first to propose the concept of spin, in 1925, Ralph Kronig, George Uhlenbeck and Samuel Goudsmit at Leiden University suggested an physical interpretation of particles spinning around their own axis. The mathematical theory was worked out in depth by Pauli in 1927, when Paul Dirac derived his relativistic quantum mechanics in 1928, electron spin was an essential part of it. As the name suggests, spin was originally conceived as the rotation of a particle around some axis and this picture is correct so far as spin obeys the same mathematical laws as quantized angular momenta do. On the other hand, spin has some properties that distinguish it from orbital angular momenta. Although the direction of its spin can be changed, a particle cannot be made to spin faster or slower. The spin of a particle is associated with a magnetic dipole moment with a g-factor differing from 1. This could only occur if the internal charge of the particle were distributed differently from its mass. The conventional definition of the quantum number, s, is s = n/2. Hence the allowed values of s are 0, 1/2,1, 3/2,2, the value of s for an elementary particle depends only on the type of particle, and cannot be altered in any known way. The spin angular momentum, S, of any system is quantized. The allowed values of S are S = ℏ s = h 4 π n, in contrast, orbital angular momentum can only take on integer values of s, i. e. even-numbered values of n. Those particles with half-integer spins, such as 1/2, 3/2, 5/2, are known as fermions, while particles with integer spins. The two families of particles obey different rules and broadly have different roles in the world around us, a key distinction between the two families is that fermions obey the Pauli exclusion principle, that is, there cannot be two identical fermions simultaneously having the same quantum numbers
17.
Angular momentum
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In physics, angular momentum is the rotational analog of linear momentum. It is an important quantity in physics because it is a conserved quantity – the angular momentum of a system remains constant unless acted on by an external torque. The definition of momentum for a point particle is a pseudovector r×p. This definition can be applied to each point in continua like solids or fluids, unlike momentum, angular momentum does depend on where the origin is chosen, since the particles position is measured from it. The angular momentum of an object can also be connected to the angular velocity ω of the object via the moment of inertia I. However, while ω always points in the direction of the rotation axis, Angular momentum is additive, the total angular momentum of a system is the vector sum of the angular momenta. For continua or fields one uses integration, torque can be defined as the rate of change of angular momentum, analogous to force. Applications include the gyrocompass, control moment gyroscope, inertial systems, reaction wheels, flying discs or Frisbees. In general, conservation does limit the motion of a system. In quantum mechanics, angular momentum is an operator with quantized eigenvalues, Angular momentum is subject to the Heisenberg uncertainty principle, meaning only one component can be measured with definite precision, the other two cannot. Also, the spin of elementary particles does not correspond to literal spinning motion, Angular momentum is a vector quantity that represents the product of a bodys rotational inertia and rotational velocity about a particular axis. Angular momentum can be considered an analog of linear momentum. Thus, where momentum is proportional to mass m and linear speed v, p = m v, angular momentum is proportional to moment of inertia I. Unlike mass, which only on amount of matter, moment of inertia is also dependent on the position of the axis of rotation. Unlike linear speed, which occurs in a line, angular speed occurs about a center of rotation. Therefore, strictly speaking, L should be referred to as the angular momentum relative to that center and this simple analysis can also apply to non-circular motion if only the component of the motion which is perpendicular to the radius vector is considered. In that case, L = r m v ⊥, where v ⊥ = v sin θ is the component of the motion. It is this definition, × to which the moment of momentum refers
18.
HOMO/LUMO
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In chemistry, HOMO and LUMO are types of molecular orbitals. The acronyms stand for highest occupied molecular orbital and lowest unoccupied molecular orbital, the energy difference between the HOMO and LUMO is termed the HOMO–LUMO gap. HOMO and LUMO are sometimes called frontier orbitals in frontier molecular orbital theory, the difference in energy between these two frontier orbitals can be used to predict the strength and stability of transition metal complexes, as well as the colors they produce in solution. Roughly, the HOMO level is to organic semiconductors what the valence band maximum is to inorganic semiconductors, the same analogy exists between the LUMO level and the conduction band minimum. In organometallic chemistry, the size of the LUMO lobe can help predict where addition to pi ligands will occur, a SOMO is a singly occupied molecular orbital such as half-filled HOMO of a radical. This abbreviation may also be extended to semi occupied molecular orbital, frontier molecular orbital theory Diels–Alder reaction Electron configuration Koopmans theorem Ligand Organic semiconductor OrbiMol Molecular orbital database
19.
Kasha's rule
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Kashas rule is a principle in the photochemistry of electronically excited molecules. The rule states that photon emission occurs in appreciable yield only from the lowest excited state of a given multiplicity and it is named for American spectroscopist Michael Kasha, who proposed it in 1950. The rule is relevant in understanding the spectrum of an excited molecule. Upon absorbing a photon, a molecule in its ground state may – depending on the photon wavelength – be excited to any of a set of higher electronic states. However, according to Kashas rule, photon emission is expected in appreciable yield only from the lowest excited state, since only one state is expected to yield emission, an equivalent statement of the rule is that the emission wavelength is independent of the excitation wavelength. The rule can be explained by the Franck–Condon factors for vibronic transitions, for a given pair of energy levels that differ in both vibrational and electronic quantum number, the Franck–Condon factor expresses the degree of overlap between their vibrational wavefunctions. The greater the overlap, the quicker the molecule can undergo transition from the higher to the lower level. In most molecules, the levels of the excited states all lie close together, so molecules in upper states quickly reach the lowest excited state, S1. However, the gap between S1 and S0 is greater, so here fluorescence occurs, since it is now kinetically competitive with internal conversion. Exceptions to Kashas rule arise when there are large gaps between excited states. An example is azulene, the explanation is that the S1. A corollary of Kashas rule is the Vavilov rule, which states that the yield of luminescence is generally independent of the excitation wavelength. This can be understood as a consequence of the tendency – implied by Kashas rule – for molecules in upper states to relax to the lowest excited state non-radiatively, again there are exceptions, for example benzene vapour. Stokes shift, the difference between the absorption and emission frequencies, related to Kashas rule
20.
Internal conversion (chemistry)
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Internal conversion is a transition from a higher to a lower electronic state in a molecule or atom. It is sometimes called radiationless de-excitation, because no photons are emitted, the energy of the electronically excited state is given off to vibrational modes of the molecule. The excitation energy is transformed into heat, a classic example of this process is the quinine sulfate fluorescence, which can be quenched by the use of various halide salts. The excited molecule can de-excite by increasing the energy of the surrounding solvated ions. Several natural molecules perform a fast internal conversion and this ability to transform the excitation energy of photon into heat can be a crucial property for photoprotection by molecules such as melanin. Fast internal conversion reduces the excited state lifetime, and thereby prevents bimolecular reactions, bimolecular electron transfer always produces a reactive chemical species, free radicals. Nucleic acids have a short lifetime due to a fast internal conversion. Both melanin and DNA have internal conversion rates that are orders of magnitude faster than any man-made molecule. In applications that use of bimolecular electron transfer the internal conversion is undesirable. For example, it is advantageous to have a long lived excited states in Grätzel cells, fluorescence spectroscopy Förster resonance energy transfer
21.
Fluorescence
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Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence, in most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. Fluorescent materials cease to glow immediately when the radiation source stops, unlike phosphorescence, Fluorescence also occurs frequently in nature in some minerals and in various biological states in many branches of the animal kingdom. An early observation of fluorescence was described in 1560 by Bernardino de Sahagún and it was derived from the wood of two tree species, Pterocarpus indicus and Eysenhardtia polystachya. The chemical compound responsible for this fluorescence is matlaline, which is the product of one of the flavonoids found in this wood. The name was derived from the fluorite, some examples of which contain traces of divalent europium. In a key experiment he used a prism to isolate ultraviolet radiation from sunlight, the specific frequencies of exciting and emitted light are dependent on the particular system. S0 is called the state of the fluorophore, and S1 is its first excited singlet state. A molecule in S1 can relax by various competing pathways and it can undergo non-radiative relaxation in which the excitation energy is dissipated as heat to the solvent. Excited organic molecules can also relax via conversion to a triplet state, relaxation from S1 can also occur through interaction with a second molecule through fluorescence quenching. Molecular oxygen is an extremely efficient quencher of fluorescence just because of its unusual triplet ground state, in most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation, this phenomenon is known as the Stokes shift. The emitted radiation may also be of the wavelength as the absorbed radiation. Molecules that are excited through light absorption or via a different process can transfer energy to a second sensitized molecule, the fluorescence quantum yield gives the efficiency of the fluorescence process. It is defined as the ratio of the number of photons emitted to the number of photons absorbed, Φ = Number of photons emitted Number of photons absorbed The maximum fluorescence quantum yield is 1.0, each photon absorbed results in a photon emitted. Compounds with quantum yields of 0.10 are still considered quite fluorescent, thus, if the rate of any pathway changes, both the excited state lifetime and the fluorescence quantum yield will be affected. Fluorescence quantum yields are measured by comparison to a standard, the quinine salt quinine sulfate in a sulfuric acid solution is a common fluorescence standard. The fluorescence lifetime refers to the time the molecule stays in its excited state before emitting a photon. This is an instance of exponential decay, various radiative and non-radiative processes can de-populate the excited state
22.
Jablonski diagram
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A Jablonski diagram is a diagram that illustrates the electronic states of a molecule and the transitions between them. The states are arranged vertically by energy and grouped horizontally by spin multiplicity, nonradiative transitions are indicated by squiggly arrows and radiative transitions by straight arrows. The vibrational ground states of each state are indicated with thick lines. The diagram is named after the Polish physicist Aleksander Jabłoński, radiative transitions involve the absorption, if the transition occurs to a higher energy level, or the emission, in the reverse case, of a photon. Nonradiative transitions arise through several different mechanisms, all differently labeled in the diagram, relaxation of the excited state to its lowest vibrational level is called Vibrational relaxation. This process involves the dissipation of energy from the molecule to its surroundings, a third type is intersystem crossing, this is a transition to a state with a different spin multiplicity. In molecules with large spin-orbit coupling, intersystem crossing is more important than in molecules that exhibit only small spin-orbit coupling. This type of transition can give rise to phosphorescence. Spectroscopy Franck–Condon principle Grotrian diagram Florida State University, Jablonski diagram primer
23.
Intersystem crossing
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Intersystem crossing is a radiationless process involving a transition between two electronic states with different spin multiplicity. When an electron in a molecule with a ground state is excited to a higher energy level. A singlet state is an electronic state such that all electron spins are paired. That is, the spin of the electron is still paired with the ground state electron. In a triplet state the excited electron is no longer paired with the ground state electron, since excitation to a triplet state involves an additional forbidden spin transition, it is less probable that a triplet state will form when the molecule absorbs radiation. When a singlet state nonradiatively passes to a state, or conversely a triplet transitions to a singlet. In essence, the spin of the electron is reversed. The probability of this occurring is more favorable when the vibrational levels of the two excited states overlap, since little or no energy must be gained or lost in the transition. As the spin/orbital interactions in such molecules are substantial and a change in spin is more favourable. This process is called spin-orbit coupling, simply-stated, it involves coupling of the electron spin with the orbital angular momentum of non-circular orbits. In addition, the presence of species in solution enhances intersystem crossing. The radiative decay from an excited triplet state back to a state is known as phosphorescence. Since a transition in spin multiplicity occurs, phosphorescence is a manifestation of intersystem crossing, the time scale of intersystem crossing is on the order of 10−8 to 10−3 s, one of the slowest forms of relaxation
24.
Hund's rule of maximum multiplicity
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Hunds rule of maximum multiplicity is a rule based on observation of atomic spectra, which is used to predict the ground state of an atom or molecule with one or more open electronic shells. The rule states that for an electron configuration, the lowest energy term is the one with the greatest value of spin multiplicity. This implies that if two or more orbitals of equal energy are available, electrons will occupy them singly before filling them in pairs. The rule, discovered by Friedrich Hund in 1925, is of important use in chemistry, spectroscopy, and quantum chemistry. The multiplicity of a state is defined as 2S +1, a high multiplicity state is therefore the same as a high-spin state. The lowest-energy state with maximum multiplicity usually has unpaired electrons all with parallel spin, since the spin of each electron is 1/2, the total spin is one-half the number of unpaired electrons, and the multiplicity is the number of unpaired electrons +1. For example, the nitrogen atom ground state has three unpaired electrons of parallel spin, so that the spin is 3/2 and the multiplicity is 4. As a result of Hunds rule, constraints are placed on the way atomic orbitals are filled in the state using the Aufbau principle. Before any two electrons occupy an orbital in a subshell, other orbitals in the same subshell must first each contain one electron, also, the electrons filling a subshell will have parallel spin before the shell starts filling up with the opposite spin electrons. As a result, when filling up atomic orbitals, the number of unpaired electrons is assured. For example, in the atom, the 2p4 subshell arranges its electrons as rather than or. The manganese atom has a 3d5 electron configuration with five unpaired electrons all of parallel spin, the superscript 6 is the value of the multiplicity, corresponding to five unpaired electrons with parallel spin in accordance with Hunds rule. An atom can have a state with two incompletely filled subshells which are close in energy. The lightest example is the atom with a 3d54s electron configuration. Here there are six unpaired electrons all of parallel spin for a 7S ground state, although most stable molecules have closed electron shells, a few have unpaired electrons for which Hunds rule is applicable. The most important example is the molecule, O2, which has two degenerate pi bond pi antibonding molecular orbitals occupied by only two electrons. In accordance with Hunds rule, the state is triplet oxygen with two unpaired electrons in singly occupied orbitals. The singlet oxygen state with one doubly occupied and one empty π* is a state with different chemical properties
25.
Phosphorescence
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Phosphorescence is a type of photoluminescence related to fluorescence. Unlike fluorescence, phosphorescent material does not immediately re-emit the radiation it absorbs, the slower time scales of the re-emission are associated with forbidden energy state transitions in quantum mechanics. As these transitions occur very slowly in certain materials, absorbed radiation is re-emitted at an intensity for up to several hours after the original excitation. Typically, the glow slowly fades out, sometimes within a few minutes or up to a few hours in a dark room, the study of phosphorescent materials led to the discovery of radioactivity in 1896. In simple terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light and this is in some cases the mechanism used for glow-in-the-dark materials which are charged by exposure to light. Most photoluminescent events, in which a chemical substrate absorbs and then re-emits a photon of light, are fast, light is absorbed and emitted at these fast time scales in cases where the energy of the photons involved matches the available energy states and allowed transitions of the substrate. In the special case of phosphorescence, the photon energy undergoes an unusual intersystem crossing into an energy state of higher spin multiplicity. As a result, the energy can become trapped in the state with only forbidden transitions available to return to the lower energy state. These transitions, although forbidden, will occur in quantum mechanics but are kinetically unfavored. Most phosphorescent compounds are relatively fast emitters, with triplet lifetimes on the order of milliseconds. However, some compounds have triplet lifetimes up to minutes or even hours, if the phosphorescent quantum yield is high, these substances will release significant amounts of light over long time scales, creating so-called glow-in-the-dark materials. S0 + h ν → S1 → T1 → S0 + h ν ′ where S is a singlet, transitions can also occur to higher energy levels, but the first excited state is denoted for simplicity. Some examples of glow-in-the-dark materials do not glow by phosphorescence, for example, glow sticks glow due to a chemiluminescent process which is commonly mistaken for phosphorescence. In chemiluminescence, a state is created via a chemical reaction. The light emission tracks the progress of the underlying chemical reaction. The excited state will then transfer to a dye molecule, also known as a sensitizer or fluorophor, common pigments used in phosphorescent materials include zinc sulfide and strontium aluminate. Use of zinc sulfide for safety related products dates back to the 1930s, however, the development of strontium aluminate, with a luminance approximately 10 times greater than zinc sulfide, has relegated most zinc sulfide based products to the novelty category. Strontium aluminate based pigments are now used in signs, pathway marking
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LED
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A light-emitting diode is a two-lead semiconductor light source. It is a p–n junction diode, which emits light when activated, when a suitable voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light is determined by the band gap of the semiconductor. LEDs are typically small and integrated optical components may be used to shape the radiation pattern, appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared light. Infrared LEDs are still used as transmitting elements in remote-control circuits. The first visible-light LEDs were also of low intensity and limited to red, modern LEDs are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness. Early LEDs were often used as indicator lamps for electronic devices and they were soon packaged into numeric readouts in the form of seven-segment displays and were commonly seen in digital clocks. Recent developments in LEDs permit them to be used in environmental, LEDs have allowed new displays and sensors to be developed, while their high switching rates are also used in advanced communications technology. LEDs have many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. Light-emitting diodes are now used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, as of 2017, LED lights home room lighting are as cheap or cheaper than compact fluorescent lamp sources of comparable output. They are also more energy efficient and, arguably, have fewer environmental concerns linked to their disposal. Electroluminescence as a phenomenon was discovered in 1907 by the British experimenter H. J. Round of Marconi Labs, using a crystal of silicon carbide, russian inventor Oleg Losev reported creation of the first LED in 1927. His research was distributed in Soviet, German and British scientific journals, rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide and other semiconductor alloys in 1955. Braunstein observed infrared emission generated by simple diode structures using gallium antimonide, GaAs, indium phosphide, in 1957, Braunstein further demonstrated that the rudimentary devices could be used for non-radio communication across a short distance. As noted by Kroemer Braunstein …had set up a simple optical communications link, the emitted light was detected by a PbS diode some distance away. This signal was fed into an amplifier and played back by a loudspeaker. Intercepting the beam stopped the music and we had a great deal of fun playing with this setup. This setup presaged the use of LEDs for optical communication applications, by October 1961, they had demonstrated efficient light emission and signal coupling between a GaAs p-n junction light emitter and an electrically-isolated semiconductor photodetector
27.
Schlenk flask
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A Schlenk flask, or Schlenk tube is a reaction vessel typically used in air-sensitive chemistry, invented by Wilhelm Schlenk. It has a side arm fitted with a PTFE or ground glass stopcock and these flasks are often connected to Schlenk lines, which allow both operations to be done easily. Schlenk flasks and Schlenk tubes, like most laboratory glassware, are made from borosilicate glass such as Pyrex, Schlenk flasks are round-bottomed, while Schlenk tubes are elongated. They may be purchased off-the-shelf from laboratory suppliers or made from round-bottom flasks or glass tubing by a skilled glassblower, typically, before solvent or reagents are introduced into a Schlenk flask, the flask is dried and the atmosphere of the flask is exchanged with an inert gas. A common method of exchanging the atmosphere of the flask is to flush the flask out with an inert gas, the gas can be introduced through the side arm of the flask, or via a wide bore needle. The contents of the exit the flask through the neck portion of the flask. The needle method has the advantage that the needle can be placed at the bottom of the flask to better flush out the atmosphere of the flask, flushing a flask out with an inert gas can be inefficient for large flasks and is impractical for complex apparatus. An alternative way to exchange the atmosphere of a Schlenk flask is to use one or more cycles, typically using a vacuum-gas manifold. This involves pumping the air out of the flask and replacing the vacuum with an inert gas. For example, evacuation of the flask to 1 mm and then replenishing the atmosphere with 760 mm inert gas leaves 0. 13% of the original atmosphere, two such vac-refill cycles leaves 0. 000173%. Most Schlenk lines easily and quickly achieve a vacuum of 1 mm Hg, when using Schlenk systems, including flasks, the use of grease is often necessary at stopcock valves and ground glass joints to provide a gas tight seal and prevent glass pieces from fusing. In contrast, teflon plug valves may have a trace of oil as a lubricant, in the following text any connection is assumed to be rendered mostly air free through a series of vac-refill cycles. The standard Schlenk flask is a bottom, pear-shaped, or tubular flask with a ground glass joint. The side arm contains a valve, usually a greased stopcock and this allows a material to be added to a flask through the ground glass joint, which is then capped with a septum. This operation can, for example, be done in a glove box, the flask can then be removed from the glove box and taken to a Schlenk line. Once connected to the Schlenk line, the inert gas and/or vacuum can be applied to the flask as required, while the flask is connected to the line under a positive pressure of inert gas, the septum can be replaced with other apparatus, for example a reflux condenser. Once the manipulations are complete, the contents can be dried and placed under a static vacuum by closing the side arm valve. These evacuated flasks can be back into a glove box for further manipulation or storage of the flasks contents
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Functional group
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In organic chemistry, functional groups are specific groups of atoms or bonds within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of, however, its relative reactivity can be modified by other functional groups nearby. The atoms of functional groups are linked to other and to the rest of the molecule by covalent bonds. Any subgroup of atoms of a compound also may be called a radical, and if a covalent bond is broken homolytically, Functional groups can also be charged, e. g. in carboxylate salts, which turns the molecule into a polyatomic ion or a complex ion. Complexation and solvation is also caused by interactions of functional groups. In the common rule of thumb like dissolves like, it is the shared or mutually well-interacting functional groups give rise to solubility. For example, sugar dissolves in water because both share the functional group and hydroxyls interact strongly with each other. Combining the names of groups with the names of the parent alkanes generates what is termed a systematic nomenclature for naming organic compounds. In traditional nomenclature, the first carbon atom after the carbon that attaches to the group is called the alpha carbon, the second, beta carbon. IUPAC conventions call for numeric labeling of the position, e. g. 4-aminobutanoic acid, in traditional names various qualifiers are used to label isomers, for example isopropanol is an isomer is n-propanol. The following is a list of functional groups. In the formulas, the symbols R and R usually denote an attached hydrogen, or a side chain of any length. Functional groups, called hydrocarbyl, that only carbon and hydrogen. Each one differs in type of reactivity, there are also a large number of branched or ring alkanes that have specific names, e. g. tert-butyl, bornyl, cyclohexyl, etc. Hydrocarbons may form charged structures, positively charged carbocations or negative carbanions, examples are tropylium and triphenylmethyl cations and the cyclopentadienyl anion. Haloalkanes are a class of molecule that is defined by a carbon–halogen bond and this bond can be relatively weak or quite stable. In general, with the exception of fluorinated compounds, haloalkanes readily undergo nucleophilic substitution reactions or elimination reactions, the substitution on the carbon, the acidity of an adjacent proton, the solvent conditions, etc. all can influence the outcome of the reactivity. Compounds that contain nitrogen in this category may contain C-O bonds, compounds that contain sulfur exhibit unique chemistry due to their ability to form more bonds than oxygen, their lighter analogue on the periodic table
29.
Quartz
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Quartz is the second most abundant mineral in Earths continental crust, behind feldspar. There are many different varieties of quartz, several of which are semi-precious gemstones, since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Eurasia. The word quartz is derived from the German word Quarz and its Middle High German ancestor twarc, the Ancient Greeks referred to quartz as κρύσταλλος derived from the Ancient Greek κρύος meaning icy cold, because some philosophers apparently believed the mineral to be a form of supercooled ice. Today, the rock crystal is sometimes used as an alternative name for the purest form of quartz. Quartz belongs to the crystal system. The ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end, well-formed crystals typically form in a bed that has unconstrained growth into a void, usually the crystals are attached at the other end to a matrix and only one termination pyramid is present. However, doubly terminated crystals do occur where they develop freely without attachment, a quartz geode is such a situation where the void is approximately spherical in shape, lined with a bed of crystals pointing inward. α-quartz crystallizes in the crystal system, space group P3121 and P3221 respectively. β-quartz belongs to the system, space group P6222 and P6422. These space groups are truly chiral, both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks. The transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, although many of the varietal names historically arose from the color of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Color is an identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. Pure quartz, traditionally called rock crystal or clear quartz, is colorless and transparent or translucent, common colored varieties include citrine, rose quartz, amethyst, smoky quartz, milky quartz, and others. The most important distinction between types of quartz is that of macrocrystalline and the microcrystalline or cryptocrystalline varieties, the cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline. Chalcedony is a form of silica consisting of fine intergrowths of both quartz, and its monoclinic polymorph moganite. Other opaque gemstone varieties of quartz, or mixed rocks including quartz, often including contrasting bands or patterns of color, are agate, carnelian or sard, onyx, heliotrope, amethyst is a form of quartz that ranges from a bright to dark or dull purple color. The worlds largest deposits of amethysts can be found in Brazil, Mexico, Uruguay, Russia, France, Namibia, sometimes amethyst and citrine are found growing in the same crystal. It is then referred to as ametrine, an amethyst is formed when there is iron in the area where it was formed
30.
Pyrex
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Pyrex is a brand introduced by Corning Inc. in 1908 for a line of clear, low-thermal-expansion borosilicate glass used for laboratory glassware and kitchenware. Pyrex sold in the United States is made of tempered soda-lime glass, Corning no longer manufactures or markets Pyrex-branded borosilicate glass kitchenware and bakeware in the US. Borosilicate glass was first made by German chemist and glass technologist Otto Schott, Schott AG sold the product under the name Duran. In 1908, Eugene Sullivan, director of research at Corning Glass Works, developed Nonex, Sullivan had learned about Schotts borosilicate glass as a doctoral student in Leipzig, Germany. Jesse Littleton of Corning discovered the potential of borosilicate glass by giving his wife a casserole dish made from a cut-down Nonex battery jar. Corning removed the lead from Nonex and developed it as a consumer product, Pyrex made its public debut in 1915 during World War I, positioned as an American-produced alternative to Duran. While some people have thought that it was made up from the Greek pyr, actually, we had a number of prior trade-marks ending in the letters ex. In 1958 an internal design department was started by John B. Ward and he redesigned the Pyrex ovenware and Flameware. Over the years, designers such as Penny Sparke, Betty Baugh, Smart Design, TEAMS Design, Corning divested its consumer products division in 1998, forming the company World Kitchen, LLC. Corning discontinued its production of Pyrex products, but still licensed the Pyrex brand name to other companies, including World Kitchen, france-based cookware maker Arc International acquired Newells European business in early 2006 and currently owns rights to the brand in Europe, the Middle East and Africa. Older clear-glass Pyrex manufactured by Corning before 1998, Arc Internationals Pyrex products, and Pyrex laboratory glassware is made of borosilicate glass. According to the National Institute of Standards and Technology, borosilicate Pyrex is composed of,4. 0% boron,54. 0% oxygen,2. 8% sodium,1. 1% aluminum,37. 7% silicon, and 0. 3% potassium. According to glass supplier Pulles and Hannique, borosilicate Pyrex is made of Corning 7740 glass and is equivalent in formulation to Schott Glass 8330 glass sold under the Duran brand name. Unlike borosilicate, it is not as heat-resistant, leading to the increase in breakage from heat stress. European Pyrex is still made from borosilicate, because of its low expansion characteristics, Pyrex borosilicate glass is often the material of choice for reflective optics in astronomy applications. In 1932, George Ellery Hale approached Corning with the challenge of fabricating the 200-inch telescope mirror for the California Institute of Technologys Palomar Observatory project, a previous effort to fabricate the optic from fused quartz had failed, the cast blank having voids. The mirror was cast by Corning during 1934–1936 out of borosilicate glass, after a year of cooling, during which it was almost lost to a flood, the blank was completed in 1935. The first blank now resides in the Corning Museum of Glass, new paper addresses causes of shattering glass cookware, margin of safety described as borderline
31.
Halogenation
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Halogenation is a chemical reaction that involves the addition of one or more halogens to a compound or material. Dehalogenation is the reverse of halogenation and results in the removal of a halogen from a molecule, the pathway and stoichiometry of halogenation depends on the structural features and functional groups of the organic substrate, as well as on the specific halogen. Inorganic compounds such as metals also undergo halogenation, several pathways exist for the halogenation of organic compounds, including free radical halogenation, ketone halogenation, electrophilic halogenation, and halogen addition reaction. The structure of the substrate is one factor that determines the pathway, saturated hydrocarbons typically do not add halogens but undergo free radical halogenation, involving substitution of hydrogen atoms by halogen. The regiochemistry of the halogenation of alkanes is usually determined by the weakness of the available C–H bonds. The preference for reaction at tertiary and secondary positions results from greater stability of the free radicals. Free radical halogenation is used for the production of chlorinated methanes. Unsaturated compounds, especially alkenes and alkynes, add halogens, RCH=CHR′ + X2 → RCHX–CHXR′ The addition of halogens to alkenes proceeds via intermediate halonium ions, in special cases, such intermediates have been isolated. Aromatic compounds are subject to electrophilic halogenation, RC6H5 + X2 → HX + RC6H4X The facility of halogenation is influenced by the halogen, fluorine and chlorine are more electrophilic and are more aggressive halogenating agents. Bromine is a weaker halogenating agent than fluorine and chlorine, while iodine is least reactive of them all. The facility of hydrogenolysis follows the trend, iodine is most easily removed from organic compounds. In the Hunsdiecker reaction, from carboxylic acids are converted to the chain-shortened halide, in the Hell–Volhard–Zelinsky halogenation, carboxylic acids are alpha-halogenated. Fluorination with elemental fluorine requires highly specialised conditions and apparatus, many commercially important organic compounds are fluorinated electrochemically using hydrogen fluoride as the source of fluorine. The method is called electrochemical fluorination, aside from F2 and its electrochemically generated equivalent, a variety of fluorinating reagents are known such as xenon difluoride and cobalt fluoride. Both saturated and unsaturated compounds react directly with chlorine, the former usually requiring UV light to initiate homolysis of chlorine, chlorination is conducted on a large scale industrially, major processes include routes to 1, 2-dichloroethane, as well as various chlorinated ethanes, as solvents. Bromination is more selective than chlorination because the reaction is less exothermic, most commonly bromination is conducted by the addition of Br2 to alkenes. An example of bromination is the synthesis of the anesthetic halothane from trichloroethylene, Organobromine compounds are the most common organohalides in nature. Their formation is catalyzed by the enzyme bromoperoxidase which utilizes bromide in combination with oxygen as an oxidant, the oceans are estimated to release 1–2 million tons of bromoform and 56,000 tons of bromomethane annually
32.
Cyclohexane
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Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is mainly used for the production of adipic acid and caprolactam. Cyclohexane is a colourless, flammable liquid with a distinctive detergent-like odor, on an industrial scale, cyclohexane is produced by hydrogenation of benzene. Producers of cyclohexane accounts for approximately 11. 4% of global demand for benzene, the reaction is highly exothermic, with ΔH =216.37 kJ/mol). Dehydrogenation commenced noticeably above 300 °C, reflecting the favorable entropy for dehydrogenation, unlike benzene, cyclohexane is not easily obtained from natural resources such as coal. For this reason, early investigators synthesized their cyclohexane samples, in 1867 Marcellin Berthelot reduced benzene with hydroiodic acid at elevated temperatures. In 1894 Baeyer synthesized cyclohexane starting with a Dieckmann condensation of pimelic acid followed by multiple reductions, In the same year E. Haworth, perkin Jr. prepared it via a Wurtz reaction of 1, 6-dibromohexane. Cyclohexane is rather unreactive, being a non-polar, hydrophobic hydrocarbon and it reacts with superacids, such as HF + SbF5, which will lead to cracking. Substituted cyclohexanes, however, may be reactive under a variety of conditions, commercially most of cyclohexane produced is converted into cyclohexanone–cyclohexanol mixture by catalytic oxidation. KA oil is used as a raw material for adipic acid. It is used as a solvent in some brands of correction fluid, cyclohexane is sometimes used as an organic solvent. Cyclohexane is also used for calibration of differential scanning calorimetry instruments, cyclohexane vapour is used in vacuum carburizing furnaces, in heat treating equipment manufacture. The 6-vertex edge ring does not conform to the shape of a perfect hexagon, therefore, to reduce torsional strain, cyclohexane adopts a three-dimensional structure known as the chair conformation. There are also two other intermediate conformers, half chair, which is the most unstable conformer, and twist boat and this was first proposed as early as 1890 by Hermann Sachse, but only gained widespread acceptance much later. The new conformation puts the carbons at an angle of 109. 5°, half of the hydrogens are in the plane of the ring while the other half are perpendicular to the plane. This conformation allows for the most stable structure of cyclohexane, another conformation of cyclohexane exists, known as boat conformation, but it interconverts to the slightly more stable chair formation. If cyclohexane is mono-substituted with a substituent, then the substituent will most likely be found attached in an equatorial position. Cyclohexane has the lowest angle and torsional strain of all the cycloalkanes, the high-temperature phase I, stable between 186 K and the melting point 280 K, is a plastic crystal, which means the molecules retain some rotational degree of freedom
33.
Flow chemistry
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In flow chemistry, a chemical reaction is run in a continuously flowing stream rather than in batch production. In other words, pumps move fluid into a tube, and where tubes join one another, if these fluids are reactive, a reaction takes place. Flow chemistry is a technique for use at a large scale when manufacturing large quantities of a given material. However, the term has only been coined recently for its application on a laboratory scale, comparing parameters in Batch vs Flow, Reaction stoichiometry. In batch production this is defined by the concentration of chemical reagents, in Flow this is defined by the concentration of reagents and the ratio of their flow rate. In batch production this is determined by how long a vessel is held at a given temperature, in flow the volumemetric residence time is used given by the ratio of volume of the reactor and the overall flow rate, as most often, plug flow reactors are used. Choosing to run a chemical reaction using flow chemistry, either in a microreactor or other mixing device offers a variety of pros, Reaction temperature can be raised above the solvents boiling point as the volume of the laboratory devices is typically small. Typically, non-compressible fluids are used with no gas volume so that the factor as a function of pressure is small. Mixing can be achieved within seconds at the scales used in flow chemistry. Mostly, because the area to volume ratio is large, thereby, endothermal and exothermal reaction can be thermostated. The temperature gradient can be steep, allowing efficient control over reaction time, safety is increased, Thermal mass of the system is dominated by the apparatus making thermal runaways unlikely. Smaller reaction volume is considered a safety benefit. The reactor operates under steady-state conditions, Flow reactions can be automated with far less effort than batch reactions. This allows for unattended operation and experimental planning, typical drivers are higher yields/selectivities, less needed manpower or a higher safety level. Multi step reactions can be arranged in a continuous sequence and this can be especially beneficial if intermediate compounds are unstable, toxic, or sensitive to air, since they will exist only momentarily and in very small quantities. Position along the stream and reaction time point are directly related to one another. This means that it is possible to arrange the system such that further reagents can be introduced into the flowing reaction stream at precisely the point in the reaction that is desired. It is possible to arrange a system such that purification is coupled with the reaction
34.
Pericyclic reaction
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In organic chemistry, a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion. Pericyclic reactions are usually rearrangement reactions, pericyclic reactions often have related stepwise radical processes associated with them. Some pericyclic reactions, such as the cycloaddition, are controversial because their mechanism is not definitively known to be concerted, a large photoinduced hydrogen sigmatropic shift was utilized in a corrin synthesis performed by Albert Eschenmoser containing a 16π system. Due to the principle of microscopic reversibility, there is a set of retro pericyclic reactions. Thus substituents in the diene can significantly affect the rate of the not only by their electronic character. Thus for example cis I -substituted butadiene I is less reactive than its trans isomer II since a bulky R disfavors the cisoid conformation, bulky 2-substituents in the diene favor the cisoid conformation more than the transoid and thus the diene in this case is more reactive
35.
Molecular orbital
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In chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region, the term orbital was introduced by Robert S. Mulliken in 1932 as an abbreviation for one-electron orbital wave function. At an elementary level, it is used to describe the region of space in which the function has a significant amplitude, Molecular orbitals are usually constructed by combining atomic orbitals or hybrid orbitals from each atom of the molecule, or other molecular orbitals from groups of atoms. They can be calculated using the Hartree–Fock or self-consistent field methods. A molecular orbital can be used to represent the regions in a molecule where an electron occupying that orbital is likely to be found, Molecular orbitals are obtained from the combination of atomic orbitals, which predict the location of an electron in an atom. A molecular orbital can specify the configuration of a molecule. Most commonly a MO is represented as a combination of atomic orbitals. They are invaluable in providing a model of bonding in molecules. Most present-day methods in computational chemistry begin by calculating the MOs of the system, a molecular orbital describes the behavior of one electron in the electric field generated by the nuclei and some average distribution of the other electrons. In the case of two electrons occupying the orbital, the Pauli principle demands that they have opposite spin. Necessarily this is an approximation, and highly accurate descriptions of the electronic wave function do not have orbitals. Molecular orbitals arise from allowed interactions between orbitals, which are allowed if the symmetries of the atomic orbitals are compatible with each other. Efficiency of atomic orbital interactions is determined from the overlap between two atomic orbitals, which is significant if the atomic orbitals are close in energy. Finally, the number of molecular orbitals that form must equal the number of orbitals in the atoms being combined to form the molecule. Here, the orbitals are expressed as linear combinations of atomic orbitals. Molecular orbitals were first introduced by Friedrich Hund and Robert S. Mulliken in 1927 and 1928, the linear combination of atomic orbitals or LCAO approximation for molecular orbitals was introduced in 1929 by Sir John Lennard-Jones. His ground-breaking paper showed how to derive the electronic structure of the fluorine and this qualitative approach to molecular orbital theory is part of the start of modern quantum chemistry. Linear combinations of atomic orbitals can be used to estimate the molecular orbitals that are formed upon bonding between the constituent atoms
36.
Photosensitizer
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A photosensitizer is a molecule that produces a chemical change in another molecule in a photochemical process. Photosensitizers are commonly used in chemistry in reactions such as photopolymerization, photocrosslinking. Photosensitizers generally act by absorbing ultraviolet or visible region of electromagnetic radiation, photosensitizers usually have large de-localized π systems, which lower the energy of HOMO orbitals and its absorption of light might be able to ionize the molecule. They help to produce oxygen to damage tumours. They can be divided into porphyrins, chlorophylls and dyes - See Photodynamic_therapy#Photosensitizers Phototoxicity Photosensitivity Artificial photosynthesis
37.
Quenching (fluorescence)
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Quenching refers to any process which decreases the fluorescence intensity of a given substance. A variety of processes can result in quenching, such as excited state reactions, energy transfer, complex-formation, as a consequence, quenching is often heavily dependent on pressure and temperature. Molecular oxygen, iodide ions and acrylamide are common chemical quenchers, the chloride ion is a well known quencher for quinine fluorescence. Quenching poses a problem for non-instant spectroscopic methods, such as laser-induced fluorescence, quenching is made use of in optode sensors, for instance the quenching effect of oxygen on certain ruthenium complexes allows the measurement of oxygen saturation in solution. Quenching is the basis for Förster resonance energy transfer assays, quenching and dequenching upon interaction with a specific molecular biological target is the basis for activatable optical contrast agents for molecular imaging. There are a few distinct mechanisms by which energy can be transferred nonradiatively between two dyes, a donor and an acceptor, Förster resonance energy transfer is a dynamic quenching mechanism because energy transfer occurs while the donor is in the excited state. FRET also depends on the spectral overlap and the relative orientation of the donor and acceptor transition dipole moments. FRET can typically occur over distances up to 100 Å, Dexter is another dynamic quenching mechanism. Dexter energy transfer is a phenomenon that falls off exponentially with distance and depends on spatial overlap of donor. In most donor-fluorophore–quencher-acceptor situations, the Förster mechanism is more important than the Dexter mechanism, with both Förster and Dexter energy transfer, the shapes of the absorption and fluorescence spectra of the dyes are unchanged. Exciplex formation is a dynamic quenching mechanism. The remaining energy transfer mechanism is static quenching, static quenching can be a dominant mechanism for some reporter-quencher probes. Unlike dynamic quenching, static quenching occurs when the form a complex in the ground state. The complex has its own properties, such as being nonfluorescent. Dye aggregation is often due to hydrophobic effects—the dye molecules stack together to minimize contact with water, planar aromatic dyes that are matched for association through hydrophobic forces can enhance static quenching. High temperatures and addition of surfactants tend to ground state complex formation. Dark quencher, for use in molecular biology, Förster resonance energy transfer, a phenomenon on which some quenching techniques rely
38.
Photosynthesis
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Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms activities. In most cases, oxygen is released as a waste product. Most plants, most algae, and cyanobacteria perform photosynthesis, such organisms are called photoautotrophs, in plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, in the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate. Using the ATP and NADPH produced by the light-dependent reactions, the compounds are then reduced and removed to form further carbohydrates. Cyanobacteria appeared later, the oxygen they produced contributed directly to the oxygenation of the Earth. Today, the rate of energy capture by photosynthesis globally is approximately 130 terawatts. Photosynthetic organisms also convert around 100–115 thousand million tonnes of carbon into biomass per year. Photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from carbon dioxide, however, not all organisms that use light as a source of energy carry out photosynthesis, photoheterotrophs use organic compounds, rather than carbon dioxide, as a source of carbon. In plants, algae, and cyanobacteria, photosynthesis releases oxygen and this is called oxygenic photosynthesis and is by far the most common type of photosynthesis used by living organisms. Although there are differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these organisms. There are also varieties of anoxygenic photosynthesis, used mostly by certain types of bacteria. Carbon dioxide is converted into sugars in a process called carbon fixation, photosynthesis provides the energy in the form of free electrons that are used to split carbon from carbon dioxide that is then used to fix that carbon once again as carbohydrate. Carbon fixation is a redox reaction, so photosynthesis supplies the energy that drives both process. In the first stage, light-dependent reactions or light reactions capture the energy of light and use it to make the energy-storage molecules ATP, during the second stage, the light-independent reactions use these products to capture and reduce carbon dioxide. Most organisms that utilize oxygenic photosynthesis use visible light for the light-dependent reactions, some organisms employ even more radical variants of photosynthesis. Some archea use a method that employs a pigment similar to those used for vision in animals. The bacteriorhodopsin changes its configuration in response to sunlight, acting as a proton pump and this produces a proton gradient more directly, which is then converted to chemical energy
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Solar energy
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Active solar techniques include the use of photovoltaic systems, concentrated solar power and solar water heating to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light-dispersing properties, the large magnitude of solar energy available makes it a highly appealing source of electricity. The United Nations Development Programme in its 2000 World Energy Assessment found that the potential of solar energy was 1. This is several times larger than the world energy consumption. In 2011, the International Energy Agency said that the development of affordable, inexhaustible, hence the additional costs of the incentives for early deployment should be considered learning investments, they must be wisely spent and need to be widely shared. The Earth receives 174,000 terawatts of incoming solar radiation at the upper atmosphere, approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum of light at the Earths surface is mostly spread across the visible. Most of the population live in areas with insolation levels of 150-300 watts/m². Solar radiation is absorbed by the Earths land surface, oceans – which cover about 71% of the globe –, warm air containing evaporated water from the oceans rises, causing atmospheric circulation or convection. When the air reaches an altitude, where the temperature is low, water vapor condenses into clouds. The latent heat of water condensation amplifies convection, producing atmospheric phenomena such as wind, sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 °C. By photosynthesis, green plants convert solar energy into stored energy, which produces food, wood. The total solar energy absorbed by Earths atmosphere, oceans and land masses is approximately 3,850,000 exajoules per year, in 2002, this was more energy in one hour than the world used in one year. Photosynthesis captures approximately 3,000 EJ per year in biomass, geography affects solar energy potential because areas that are closer to the equator have a greater amount of solar radiation. However, the use of photovoltaics that can follow the position of the sun can significantly increase the energy potential in areas that are farther from the equator. Time variation effects the potential of energy because during the nighttime there is little solar radiation on the surface of the Earth for solar panels to absorb. This limits the amount of energy that solar panels can absorb in one day, cloud cover can affect the potential of solar panels because clouds block incoming light from the sun and reduce the light available for solar cells. In addition, land availability has an effect on the available solar energy because solar panels can only be set up on land that is otherwise unused
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Oxygen
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Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the group on the periodic table and is a highly reactive nonmetal. By mass, oxygen is the third-most abundant element in the universe, after hydrogen, at standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. This is an important part of the atmosphere and diatomic oxygen gas constitutes 20. 8% of the Earths atmosphere, additionally, as oxides the element makes up almost half of the Earths crust. Most of the mass of living organisms is oxygen as a component of water, conversely, oxygen is continuously replenished by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone, strongly absorbs ultraviolet UVB radiation, but ozone is a pollutant near the surface where it is a by-product of smog. At low earth orbit altitudes, sufficient atomic oxygen is present to cause corrosion of spacecraft, the name oxygen was coined in 1777 by Antoine Lavoisier, whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle, Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries later Leonardo da Vinci built on Philos work by observing that a portion of air is consumed during combustion and respiration, Oxygen was discovered by the Polish alchemist Sendivogius, who considered it the philosophers stone. In the late 17th century, Robert Boyle proved that air is necessary for combustion, English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. From this he surmised that nitroaereus is consumed in both respiration and combustion, Mayow observed that antimony increased in weight when heated, and inferred that the nitroaereus must have combined with it. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract De respiratione. Robert Hooke, Ole Borch, Mikhail Lomonosov, and Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element. This may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, which was then the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, one part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. The fact that a substance like wood gains overall weight in burning was hidden by the buoyancy of the combustion products
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Bioluminescence
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Bioluminescence is the production and emission of light by a living organism. It is a form of chemiluminescence, Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some bioluminescent bacteria and terrestrial invertebrates such as fireflies. In some animals, the light is produced by organisms such as Vibrio bacteria. The principal chemical reaction in bioluminescence involves the light-emitting pigment luciferin, the enzyme catalyzes the oxidation of luciferin. In some species, the type of luciferin requires cofactors such as calcium or magnesium ions, in evolution, luciferins vary little, one in particular, coelenterazine, is found in nine different animal, though in some of these, the animals obtain it through their diet. Conversely, luciferases vary widely in different species, Bioluminescence has arisen over forty times in evolutionary history. It was not until the nineteenth century that bioluminescence was properly investigated. The phenomenon is widely distributed among animal groups, especially in environments where dinoflagellates cause phosphorescence in the surface layers of water. On land it occurs in fungi, bacteria and some groups of invertebrates, in the laboratory, luciferase-based systems are used in genetic engineering and for biomedical research. Other researchers are investigating the possibility of using bioluminescent systems for street and decorative lighting, before the development of the safety lamp for use in coal mines, dried fish skins were used in Britain and Europe as a weak source of light. This experimental form of illumination avoided the necessity of using candles which risked sparking explosions of firedamp, another safe source of illumination in mines was bottles containing fireflies. In 1920, the American zoologist E. Newton Harvey published a monograph, The Nature of Animal Light, Harvey notes that Aristotle mentions light produced by dead fish and flesh, and that both Aristotle and Pliny the Elder mention light from damp wood. He also records that Robert Boyle experimented on these light sources, tuckey, in his posthumous 1818 Narrative of the Expedition to the Zaire, described catching the animals responsible for luminescence. He mentions pellucids, crustaceans, and cancers, under the microscope he described the luminous property to be in the brain, resembling a most brilliant amethyst about the size of a large pins head. Charles Darwin noticed bioluminescence in the sea, describing it in his Journal, While sailing in these latitudes on one very dark night, the sea presented a wonderful and most beautiful spectacle. There was a breeze, and every part of the surface. The vessel drove before her bows two billows of liquid phosphorus, and in her wake she was followed by a milky train. Darwin also observed a luminous jelly-fish of the genus Dianaea and noted that When the waves scintillate with bright green sparks, but there can be no doubt that very many other pelagic animals, when alive, are phosphorescent
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