Photoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule. Not every photon which encounters an atom or ion will photoionize it; the probability of photoionization is related to the photoionization cross-section, which depends on the energy of the photon and the target being considered. For photon energies below the ionization threshold, the photoionization cross-section is near zero, but with the development of pulsed lasers it has become possible to create intense, coherent light where multi-photon ionization may occur. At higher intensities, non-perturbative phenomena such as barrier suppression ionization and rescattering ionization are observed. Several photons of energy below the ionization threshold may combine their energies to ionize an atom; this probability decreases with the number of photons required, but the development of intense, pulsed lasers still makes it possible. In the perturbative regime, the probability of absorbing N photons depends on the laser-light intensity I as IN.
For higher intensities, this dependence becomes invalid due to the occurring AC Stark effect. Resonance-enhanced multiphoton ionization is a technique applied to the spectroscopy of atoms and small molecules in which a tunable laser can be used to access an excited intermediate state. Above-threshold ionization is an extension of multi-photon ionization where more photons are absorbed than would be necessary to ionize the atom; the excess energy gives the released electron higher kinetic energy than the usual case of just-above threshold ionization. More The system will have multiple peaks in its photoelectron spectrum which are separated by the photon energies, this indicates that the emitted electron has more kinetic energy than in the normal ionization case; the electrons released from the target will have an integer number of photon-energies more kinetic energy. When either the laser intensity is further increased or a longer wavelength is applied as compared with the regime in which multi-photon ionization takes place, a quasi-stationary approach can be used and results in the distortion of the atomic potential in such a way that only a low and narrow barrier between a bound state and the continuum states remains.
The electron can tunnel through or for larger distortions overcome this barrier. These phenomena are called over-the-barrier ionization, respectively. Ion source
Luminescence is spontaneous emission of light by a substance not resulting from heat. It can be caused by electrical energy, subatomic motions or stress on a crystal; this distinguishes luminescence from incandescence, light emitted by a substance as a result of heating. Radioactivity was thought of as a form of "radio-luminescence", although it is today considered to be separate since it involves more than electromagnetic radiation; the dials, hands and signs of aviation and navigational instruments and markings are coated with luminescent materials in a process known as "luminising". The following are types of luminescence: Chemiluminescence, the emission of light as a result of a chemical reaction Bioluminescence, a result of biochemical reactions in a living organism Electrochemiluminescence, a result of an electrochemical reaction Lyoluminescence, a result of dissolving a solid in a liquid solvent Candoluminescence, is light emitted by certain materials at elevated temperatures, which differs from the blackbody emission expected at the temperature in question.
Crystalloluminescence, produced during crystallization Electroluminescence, a result of an electric current passed through a substance Cathodoluminescence, a result of a luminescent material being struck by electrons Mechanoluminescence, a result of a mechanical action on a solid Triboluminescence, generated when bonds in a material are broken when that material is scratched, crushed, or rubbed Fractoluminescence, generated when bonds in certain crystals are broken by fractures Piezoluminescence, produced by the action of pressure on certain solids Sonoluminescence, a result of imploding bubbles in a liquid when excited by sound Photoluminescence, a result of absorption of photons Fluorescence, photoluminescence as a result of singlet–singlet electronic relaxation Phosphorescence, photoluminescence as a result of triplet–singlet electronic relaxation Raman emission, photoluminescence as a result of inelastic light scattering, Radioluminescence, a result of bombardment by ionizing radiation Thermoluminescence, the re-emission of absorbed energy when a substance is heatedCryoluminescence, the emission of light when an object is cooled Light-emitting diodes emit light via electro-luminescence.
Phosphors, materials that emit light when irradiated by higher-energy electromagnetic radiation or particle radiation Phosphor thermometry, measuring temperature using phosphorescence Thermoluminescence dating Thermoluminescent dosimeter Non-disruptive observation of processes within a cell. Luminescence occurs in some minerals when they are exposed to low-powered sources of ultraviolet or infrared electromagnetic radiation, at atmospheric pressure and atmospheric temperatures; this property of these minerals can be used during the process of mineral identification at rock outcrops in the field, or in the laboratory. List of light sources Fluorophores.org A database of luminescent dyes
Dye-sensitized solar cell
A dye-sensitized solar cell is a low-cost solar cell belonging to the group of thin film solar cells. It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photoelectrochemical system; the modern version of a dye solar cell known as the Grätzel cell, was co-invented in 1988 by Brian O'Regan and Michael Grätzel at UC Berkeley and this work was developed by the aforementioned scientists at the École Polytechnique Fédérale de Lausanne until the publication of the first high efficiency DSSC in 1991. Michael Grätzel has been awarded the 2010 Millennium Technology Prize for this invention; the DSSC has a number of attractive features. In practice it has proven difficult to eliminate a number of expensive materials, notably platinum and ruthenium, the liquid electrolyte presents a serious challenge to making a cell suitable for use in all weather. Although its conversion efficiency is less than the best thin-film cells, in theory its price/performance ratio should be good enough to allow them to compete with fossil fuel electrical generation by achieving grid parity.
Commercial applications, which were held up due to chemical stability problems, are forecast in the European Union Photovoltaic Roadmap to contribute to renewable electricity generation by 2020. In a traditional solid-state semiconductor, a solar cell is made from two doped crystals, one doped with n-type impurities, which add additional free conduction band electrons, the other doped with p-type impurities, which add additional electron holes; when placed in contact, some of the electrons in the n-type portion flow into the p-type to "fill in" the missing electrons known as electron holes. Enough electrons will flow across the boundary to equalize the Fermi levels of the two materials; the result is a region at the interface, the p-n junction, where charge carriers are depleted and/or accumulated on each side of the interface. In silicon, this transfer of electrons produces a potential barrier of about 0.6 to 0.7 V. When placed in the sun, photons of the sunlight can excite electrons on the p-type side of the semiconductor, a process known as photoexcitation.
In silicon, sunlight can provide enough energy to push an electron out of the lower-energy valence band into the higher-energy conduction band. As the name implies, electrons in the conduction band are free to move about the silicon; when a load is placed across the cell as a whole, these electrons will flow out of the p-type side into the n-type side, lose energy while moving through the external circuit, flow back into the p-type material where they can once again re-combine with the valence-band hole they left behind. In this way, sunlight creates an electric current. In any semiconductor, the band gap means that only photons with that amount of energy, or more, will contribute to producing a current. In the case of silicon, the majority of visible light from red to violet has sufficient energy to make this happen. Higher energy photons, those at the blue and violet end of the spectrum, have more than enough energy to cross the band gap. Another issue is that in order to have a reasonable chance of capturing a photon, the n-type layer has to be thick.
This increases the chance that a freshly ejected electron will meet up with a created hole in the material before reaching the p-n junction. These effects produce an upper limit on the efficiency of silicon solar cells around 12 to 15% for common modules and up to 25% for the best laboratory cells. By far the biggest problem with the conventional approach is cost. There have been a number of different approaches to reduce this cost over the last decade, notably the thin-film approaches, but to date they have seen limited application due to a variety of practical problems. Another line of research has been to improve efficiency through the multi-junction approach, although these cells are high cost and suitable only for large commercial deployments. In general terms the types of cells suitable for rooftop deployment have not changed in efficiency, although costs have dropped somewhat due to increased supply. In the late 1960s it was discovered that illuminated organic dyes can generate electricity at oxide electrodes in electrochemical cells.
In an effort to understand and simulate the primary processes in photosynthesis the phenomenon was studied at the University of California at Berkeley with chlorophyll extracted from spinach. On the basis of such experiments electric power generation via the dye sensitization solar cell principle was demonstrated and discussed in 1972; the instability of the dye solar cell was identified as a main challenge. Its efficiency could, during the following two decades, be improved by optimizing the porosity of the electrode prepared from fine oxide powder, but the instability remained a problem. A modern DSSC is composed of a porous layer of titanium dioxide nanoparticles, covered with a molecular dye that absorbs sunlight, like the chlorophyll in green leaves; the titanium dioxide is immersed under an electrolyte soluti
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium; when the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited state, population inversion is achieved. In this condition, the mechanism of stimulated emission can take place and the medium can act as a laser or an optical amplifier; the pump power must be higher than the lasing threshold of the laser. The pump energy is provided in the form of light or electric current, but more exotic sources have been used, such as chemical or nuclear reactions. A laser pumped with an arc lamp or a flashlamp is pumped through the lateral wall of the lasing medium, in the form of a crystal rod containing a metallic impurity or a glass tube containing a liquid dye, in a condition known as "side-pumping." To use the lamp's energy most efficiently, the lamps and lasing medium are contained in a reflective cavity that will redirect most of the lamp's energy into the rod or dye cell.
In the most common configuration, the gain medium is in the form of a rod located at one focus of a mirrored cavity, consisting of an elliptical cross-section perpendicular to the rod's axis. The flashlamp is a tube located at the other focus of the ellipse; the mirror's coating is chosen to reflect wavelengths that are shorter than the lasing output while absorbing or transmitting wavelengths that are the same or longer, to minimize thermal lensing. In other cases an absorber for the longer wavelengths is used; the lamp is surrounded by a cylindrical jacket called a flow tube. This flow tube is made of a glass that will absorb unsuitable wavelengths, such as ultraviolet, or provide a path for cooling water which absorbs infrared; the jacket is given a dielectric coating that reflects unsuitable wavelengths of light back into the lamp. This light is absorbed and some of it is re-emitted at suitable wavelengths; the flow tube serves to protect the rod in the event of a violent lamp failure. Smaller ellipses create fewer reflections.
For a single flashlamp, if the lamp and rod are equal diameter, an ellipse, twice as wide as it is high is the most efficient at imaging the light into the rod. The rod and the lamp are long to minimize the effect of losses at the end faces and to provide a sufficient length of gain medium. Longer flashlamps are more efficient at transferring electrical energy into light, due to higher impedance. However, if the rod is too long in relation to its diameter a condition called "prelasing" can occur, depleting the rod's energy before it can properly build up. Rod ends are antireflection coated or cut at Brewster's angle to minimize this effect. Flat mirrors are often used at the ends of the pump cavity to reduce loss. Variations on this design use more complex mirrors composed of overlapping elliptical shapes, to allow multiple flashlamps to pump a single rod; this allows greater power, but are less efficient because not all of the light is imaged into the rod, leading to increased thermal losses.
These losses can be minimized by using a close-coupled cavity. This approach may allow more symmetric increasing beam quality, however. Another configuration uses a rod and a flashlamp in a cavity made of a diffuse reflecting material, such as spectralon or powdered barium sulfate; these cavities are circular or oblong, as focusing the light is not a primary objective. This doesn't couple the light as well into the lasing medium, since the light makes many reflections before reaching the rod, but requires less maintenance than metalized reflectors; the increased number of reflections is compensated for by the diffuse medium's higher reflectivity: 99% compared to 97% for a gold mirror. This approach is more compatible with multiple lamps. Parasitic modes occur when reflections are generated in directions other than along the length of the rod, which can use up energy that would otherwise be available to the beam; this can be a particular problem. Cylindrical laser rods support whispering gallery modes due to total internal reflection between the rod and the cooling water, which reflect continuously around the circumference of the rod.
Light pipe modes can reflect down the length of the rod in a zig-zag path. If the rod has an antireflection coating, or is immersed in a fluid that matches its refractive index, it can reduce these parasitic reflections. If the barrel of the rod is rough ground, or grooved, internal reflections can be dispersed. Pumping with a single lamp tends to focus most of the energy on one side, worsening the beam profile, it is common for rods to have a frosted barrel, to diffuse the light, providing a more distribution of light throughout the rod. This allows more energy absorption throughout the gain medium for a better transverse mode. A frosted flow tube or diffuse reflector, while leading to lowered transfer efficiency, helps increase this effect, improving the gain. Laser host materials are chosen to have a low absorption. Therefore, any light at frequencies not absorbed by the doping will go back into the lamp and reheat the plasma, shortening lamp life. Flashlamps were the earliest energy source for lasers.
They are used for high pulsed energies in both dye lasers. They produce a broad spectrum of light, causing most of the energy to be wasted as heat in the gain medium. Flashlamps tend to have a short lifetime; the first laser consisted of a helical flashlamp surrounding a ruby rod. Quartz flashla
Photochemistry is the branch of chemistry concerned with the chemical effects of light. This term is used to describe a chemical reaction caused by absorption of ultraviolet, visible light or infrared radiation. In nature, photochemistry is of immense importance as it is the basis of photosynthesis and the formation of vitamin D with sunlight. Photochemical reactions proceed differently than temperature-driven reactions. Photochemical paths access high energy intermediates that cannot be generated thermally, thereby overcoming large activation barriers in a short period of time, allowing reactions otherwise inaccessible by thermal processes. Photochemistry is destructive, as illustrated by the photodegradation of plastics. Photoexcitation is the first step in a photochemical process where the reactant is elevated to a state of higher energy, an excited state; the first law of photochemistry, known as the Grotthuss–Draper law, states that light must be absorbed by a chemical substance in order for a photochemical reaction to take place.
According to the second law of photochemistry, known as the Stark-Einstein law, for each photon of light absorbed by a chemical system, no more than one molecule is activated for a photochemical reaction, as defined by the quantum yield. When a molecule or atom in the ground state absorbs light, one electron is excited to a higher orbital level; this electron maintains its spin according to the spin selection rule. The excitation to a higher singlet state can be from HOMO to LUMO or to a higher orbital, so that singlet excitation states S1, S2, S3… at different energies are possible. Kasha's rule stipulates that higher singlet states would relax by radiationless decay or internal conversion to S1. Thus, S1 is but not always, the only relevant singlet excited state; this excited state S1 can further relax to S0 by IC, but by an allowed radiative transition from S1 to S0 that emits a photon. Alternatively, it is possible for the excited state S1 to undergo spin inversion and to generate a triplet excited state T1 having two unpaired electrons with the same spin.
This violation of the spin selection rule is possible by intersystem crossing of the vibrational and electronic levels of S1 and T1. According to Hund's rule of maximum multiplicity, this T1 state would be somewhat more stable than S1; this triplet state can relax to the ground state S0 by radiationless IC or by a radiation pathway called phosphorescence. This process implies a change of electronic spin, forbidden by spin selection rules, making phosphorescence much slower than fluorescence. Thus, triplet states have longer lifetimes than singlet states; these transitions are summarized in a state energy diagram or Jablonski diagram, the paradigm of molecular photochemistry. These excited species, either S1 or T1, have a half empty low-energy orbital, are more oxidizing than the ground state, but at the same time, they have an electron in a high energy orbital, are thus more reducing. In general, excited species are prone to participate in electron transfer processes. Photochemical reactions require a light source that emits wavelengths corresponding to an electronic transition in the reactant.
In the early experiments, sunlight was the light source. Mercury-vapor lamps are more common in the laboratory. Low pressure mercury vapor lamps emit at 254 nm. For polychromatic sources, wavelength ranges can be selected using filters. Alternatively, laser beams are monochromatic and LEDs have a narrowband that can be efficiently used, as well as Rayonet lamps, to get monochromatic beams; the emitted light must of course reach the targeted functional group without being blocked by the reactor, medium, or other functional groups present. For many applications, quartz is used for the reactors as well as to contain the lamp. Pyrex absorbs at wavelengths shorter than 275 nm; the solvent is an important experimental parameter. Solvents are potential reactants and for this reason, chlorinated solvents are avoided because the C-Cl bond can lead to chlorination of the substrate. Absorbing solvents prevent photons from reaching the substrate. Hydrocarbon solvents absorb only at short wavelengths and are thus preferred for photochemical experiments requiring high energy photons.
Solvents containing unsaturation absorb at longer wavelengths and can usefully filter out short wavelengths. For example and acetone "cut off" at wavelengths shorter than 215 and 330 nm, respectively. Continuous flow photochemistry offers multiple advantages over batch photochemistry. Photochemical reactions are driven by the number of photons that are able to activate molecules causing the desired reaction; the large surface area to volume ratio of a microreactor maximizes the illumination, at the same time allows for efficient cooling, which decreases the thermal side products. In the case of photochemical reactions, light provides the activation energy. Simplistically, light is one mechanism for providing the activation energy required for many reactions. If laser light is employed, it is possible to selectively excite a molecule so as to produce a desired electronic and vibrational state; the emission from a particular state may be selectively monitored, providing a measure of the population of that state
Photochromism is the reversible transformation of a chemical species between two forms by the absorption of electromagnetic radiation, where the two forms have different absorption spectra. Trivially, this can be described as a reversible change of colour upon exposure to light. One of the most famous reversible photochromic applications is color changing lenses for sunglasses, as found in eyeglasses; the largest limitation in using PC technology is that the materials cannot be made stable enough to withstand thousands of hours of outdoor exposure so long-term outdoor applications are not appropriate at this time. The switching speed of photochromic dyes is sensitive to the rigidity of the environment around the dye; as a result, they switch most in solution and slowest in the rigid environment like a polymer lens. In 2005 it was reported that attaching flexible polymers with low glass transition temperature to the dyes allow them to switch much more in a rigid lens; some spirooxazines with siloxane polymers attached switch at near solution-like speeds though they are in a rigid lens matrix.
Photochromic units have been employed extensively in supramolecular chemistry. Their ability to give a light-controlled reversible shape change means that they can be used to make or break molecular recognition motifs, or to cause a consequent shape change in their surroundings. Thus, photochromic units have been demonstrated as components of molecular switches; the coupling of photochromic units to enzymes or enzyme cofactors provides the ability to reversibly turn enzymes "on" and "off", by altering their shape or orientation in such a way that their functions are either "working" or "broken". The possibility of using photochromic compounds for data storage was first suggested in 1956 by Yehuda Hirshberg. Since that time, there have been many investigations by various academic and commercial groups in the area of 3D optical data storage which promises discs that can hold a terabyte of data. Issues with thermal back-reactions and destructive reading dogged these studies, but more more-stable systems have been developed.
Reversible photochromics are found in applications such as toys, cosmetics and industrial applications. If necessary, they can be made to change between desired colors by combination with a permanent pigment. Researchers at the Center for Exploitation of Solar Energy at the University of Copenhagen Department of Chemistry are studying, the Photochromic Dihydroazulene–Vinylheptafulvene System, for possible application to harvest solar energy and store it for significant amounts of time. Although storage lifetimes are attractive, for a real device it must of course be possible to trigger the back-reaction, which calls for further iterations in the future. Photochromism was discovered in the late 1880s, including work by Markwald, who studied the reversible change of color of 2,3,4,4-tetrachloronaphthalen-1-one in the solid state, he labeled this phenomenon "phototropy", this name was used until the 1950s when Yehuda Hirshberg, of the Weizmann Institute of Science in Israel proposed the term "photochromism".
Photochromism can take place in both organic and inorganic compounds, has its place in biological systems. Photochromism does not have a rigorous definition, but is used to describe compounds that undergo a reversible photochemical reaction where an absorption band in the visible part of the electromagnetic spectrum changes in strength or wavelength. In many cases, an absorbance band is present in only one form; the degree of change required for a photochemical reaction to be dubbed "photochromic" is that which appears dramatic by eye, but in essence there is no dividing line between photochromic reactions and other photochemistry. Therefore, while the trans-cis isomerization of azobenzene is considered a photochromic reaction, the analogous reaction of stilbene is not. Since photochromism is just a special case of a photochemical reaction any photochemical reaction type may be used to produce photochromism with appropriate molecular design; some of the most common processes involved in photochromism are pericyclic reactions, cis-trans isomerizations, intramolecular hydrogen transfer, intramolecular group transfers, dissociation processes and electron transfers.
Another requirement of photochromism is two states of the molecule should be thermally stable under ambient conditions for a reasonable time. All the same, nitrospiropyran is considered photochromic. All photochromic molecules back-isomerize to their more stable form at some rate, this back-isomerization is accelerated by heating. There is therefore a close relationship between thermochromic compounds; the timescale of thermal back-isomerization is important for applications, may be molecularly engineered. Photochromic compounds considered to be "thermally stable" include some diarylethenes, which do not back isomerize after heating at 80 C for 3 months. Since photochromic chromophores are dyes, operate according to well-known reactions, their molecular engineering to fine-tune their properties can be achieved easily using known design models, quantum mechanics calculations, experimentation. In particular, the tuning of absorbance bands to particular parts of the spectrum and the engineering of thermal stability have received much attention.
Sometimes, in the dye industry, the term "irreversible photochromic" is used to describe materials that undergo a permanent color change upon exposure to ultraviolet or visible light radiati
Photoelectrochemical processes are processes in photoelectrochemistry. These processes apply to photochemistry, optically pumped lasers, sensitized solar cells and photochromism. Electron excitation is the movement of an electron to a higher energy state; this can either be done by photoexcitation, where the original electron absorbs the photon and gains all the photon's energy or by electrical excitation, where the original electron absorbs the energy of another, energetic electron. Within a semiconductor crystal lattice, thermal excitation is a process where lattice vibrations provide enough energy to move electrons to a higher energy band; when an excited electron falls back to a lower energy state again, it is called electron relaxation. 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, associated with an atom being excited to an excited state; the excited state, in general, is in relation to the ground state, where the excited state is at a higher energy level than 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 photon takes place in accordance with Planck's quantum theory. Photoexcitation plays role in photoisomerization. Photoexcitation is exploited in dye-sensitized solar cells, luminescence, optically pumped lasers, in some photochromic applications. 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" indicates a reversible process. Photoisomerizable molecules are put to practical use, for instance, in pigments for rewritable CDs, DVDs, 3D optical data storage solutions. In addition, recent interest in photoisomerizable molecules has been aimed at molecular devices, such as molecular switches, molecular motors, molecular electronics. Photoisomerization behavior can be categorized into several classes.
Two major classes are trans-cis conversion, open-closed ring transition. Examples of the former include azobenzene; this type of compounds has a double bond, rotation or inversion around the double bond affords isomerization between the two states. Examples of the latter include diarylethene; 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 the same 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. For example, a single electron can have a kinetic energy equal to the energy of the incident photon minus the electron binding energy of the state it left.
Photons with energies less than the electron binding energy may be absorbed or scattered but will not photoionize the atom or ion. For example, to ionize hydrogen, photons need an energy greater than 13.6 electronvolts, which corresponds to a wavelength of 91.2 nm. For photons with greater energy than this, the energy of the emitted photoelectron is given by: m v 2 2 = h ν − 13.6 e V where h is Planck's constant and ν is the frequency of the photon. This formula defines the photoelectric effect. Not every photon which encounters an atom or ion will photoionize it; the probability of photoionization is related to the photoionization cross-section, which depends on the energy of the photon and the target being considered. For photon energies below the ionization threshold, the photoionization cross-section is near zero, but with the development of pulsed lasers it has become possible to create intense, coherent light where multi-photon ionization may occur. At higher intensities, non-perturbative phenomena such as barrier suppression ionization and rescattering ionization are observed.
Several photons of energy below the ionization threshold may combine their energies to ionize an atom. This probability decreases with the number of photons required, but the development of intense, pulsed lasers still makes it possible. In the perturbative regime, the probability of absorbing N photons depends on the laser-light intensity I as IN. Above threshold ionization is an extension of multi-photon ionization where more photons are absorbed than would be necessary to ionize the atom; the excess energy gives the released electron higher kinetic energy than the usual case of just-above threshold ionization. More the system will have multiple peaks in its photoelectron spectrum which are separated by the photon energies, this indicates that the emitted electron has more kinetic energy than in the normal ionization case; the electrons released from the target will have an integer number of photon-energies more kinetic energy. In intensity regions between 1014 W/cm2 and 1018 W/cm2, each of MPI, ATI, barrier suppression ionization can occur ea