Light
Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word refers to visible light, the visible spectrum, 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, between the infrared and the ultraviolet. This wavelength means a frequency range of 430–750 terahertz; the main source of light on Earth is the Sun. Sunlight provides the energy that green plants use to create sugars in the form of starches, which release energy into the living things that digest them; this process of photosynthesis provides all the energy used by living things. Another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems, electric lighting has replaced firelight; some species of animals generate their own light, a process called bioluminescence.
For example, fireflies use light to locate mates, vampire squids use it to hide themselves from prey. The primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, polarization, while its speed in a vacuum, 299,792,458 metres per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation, is experimentally found to always move at this speed in a vacuum. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays and radio waves are light. Like all types of EM radiation, visible light propagates as waves. However, the energy imparted by the waves is absorbed at single locations the way particles are absorbed; the absorbed energy of the EM waves is called a photon, represents the quanta of light. When a wave of light is transformed and absorbed as a photon, the energy of the wave collapses to a single location, this location is where the photon "arrives."
This is. This dual wave-like and particle-like nature of light is known as the wave–particle duality; the study of light, known as optics, is an important research area in modern physics. EM radiation, or EMR, is classified by wavelength into radio waves, infrared, the visible spectrum that we perceive as light, ultraviolet, X-rays, gamma rays; the behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, 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. EMR in the visible light region consists of quanta that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which leads to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humans because its photons no longer have enough individual energy to cause a lasting molecular change in the visual molecule retinal in the human retina, which change triggers the sensation of vision.
There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. 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, how these animals detect it. Above the range of visible light, ultraviolet light becomes invisible to humans because it is absorbed by the cornea below 360 nm and the internal lens below 400 nm. Furthermore, the rods and cones located in the retina of the human eye cannot detect the 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, in much the same chemical way that humans detect visible light. Various sources define visible light as narrowly as 420–680 nm to as broadly as 380–800 nm. Under ideal laboratory conditions, people can see infrared up to at least 1050 nm.
Plant growth is affected by the color spectrum of light, a process known as photomorphogenesis. The speed of light in a vacuum is defined to be 299,792,458 m/s; the fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at this same speed in vacuum. Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit. However, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s. Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849.
Fizeau dire
Photovoltaics
Photovoltaics is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics and electrochemistry. A photovoltaic system employs solar panels, each comprising a number of solar cells, which generate electrical power. PV installations may be ground-mounted, rooftop mounted or wall mounted; the mount may use a solar tracker to follow the sun across the sky. Solar PV has specific advantages as an energy source: once installed, its operation generates no pollution and no greenhouse gas emissions, it shows simple scalability in respect of power needs and silicon has large availability in the Earth’s crust. PV systems have the major disadvantage that the power output works best with direct sunlight, so about 10-25% is lost if a tracking system is not used. Dust and other obstructions in the atmosphere diminish the power output. Another important issue is the concentration of the production in the hours corresponding to main insolation, which do not match the peaks in demand in human activity cycles.
Unless current societal patterns of consumption and electrical networks adjust to this scenario, electricity still needs to be stored for use or made up by other power sources hydrocarbons. Photovoltaic systems have long been used in specialized applications, stand-alone and grid-connected PV systems have been in use since the 1990s, they were first mass-produced in 2000, when German environmentalists and the Eurosolar organization got government funding for a ten thousand roof program. Advances in technology and increased manufacturing scale have in any case reduced the cost, increased the reliability, increased the efficiency of photovoltaic installations. Net metering and financial incentives, such as preferential feed-in tariffs for solar-generated electricity, have supported solar PV installations in many countries. More than 100 countries now use solar PV. After hydro and wind powers, PV is the third renewable energy source in terms of global capacity. At the end of 2016, worldwide installed PV capacity increased to more than 300 gigawatts, covering two percent of global electricity demand.
China, followed by Japan and the United States, is the fastest growing market, while Germany remains the world's largest producer, with solar PV providing seven percent of annual domestic electricity consumption. With current technology, photovoltaics recoups the energy needed to manufacture them in 1.5 years in Southern Europe and 2.5 years in Northern Europe. The term "photovoltaic" comes from the Greek φῶς meaning "light", from "volt", the unit of electro-motive force, the volt, which in turn comes from the last name of the Italian physicist Alessandro Volta, inventor of the battery; the term "photo-voltaic" has been in use in English since 1849. Photovoltaics are best known as a method for generating electric power by using solar cells to convert energy from the sun into a flow of electrons by the photovoltaic effect. Solar cells produce direct current electricity from sunlight which can be used to power equipment or to recharge a battery; the first practical application of photovoltaics was to power orbiting satellites and other spacecraft, but today the majority of photovoltaic modules are used for grid connected power generation.
In this case an inverter is required to convert the DC to AC. There is a smaller market for off-grid power for remote dwellings, recreational vehicles, electric cars, roadside emergency telephones, remote sensing, cathodic protection of pipelines. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material. Copper solar cables connect modules and sub-fields; because of the growing demand for renewable energy sources, the manufacturing of solar cells and photovoltaic arrays has advanced in recent years. Solar photovoltaic power generation has long been seen as a clean energy technology which draws upon the planet’s most plentiful and distributed renewable energy source – the sun. Cells require protection from the environment and are packaged in solar panels. Photovoltaic power capacity is measured as maximum power output under standardized test conditions in "Wp"; the actual power output at a particular point in time may be less than or greater than this standardized, or "rated", depending on geographical location, time of day, weather conditions, other factors.
Solar photovoltaic array capacity factors are under 25%, lower than many other industrial sources of electricity. For best performance, terrestrial PV systems aim to maximize the time. Solar trackers achieve this by moving PV panels to follow the sun; the increase can be by as much as 50 % in summer. Static mounted. Panels are set to latitude tilt, an angle equal to the latitude, but performance can be improved by adjusting the angle for summer or winter; as with other semiconductor devices, temperatures above room temperature reduce the performance of photovoltaics. A number of solar panels may be mounted vertically above each other in a tower, if the zenith distance of the Sun is greater than zero, the tower can be turned horizontally as a whole and each panels additionally around a horizontal axis. In such a tower the panels can follow the Sun exactly; such a device may be described as a ladder mounted on a turnable disk. Each step of that ladder is the middle axis of a rectangular solar panel.
In case the zenith distance of the Sun reaches zero, the "ladder" may be rotated
Quantum cascade laser
Quantum cascade lasers are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jerome Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, Alfred Cho at Bell Laboratories in 1994. Unlike typical interband semiconductor lasers that emit electromagnetic radiation through the recombination of electron–hole pairs across the material band gap, QCLs are unipolar and laser emission is achieved through the use of intersubband transitions in a repeated stack of semiconductor multiple quantum well heterostructures, an idea first proposed in the paper "Possibility of amplification of electromagnetic waves in a semiconductor with a superlattice" by R. F. Kazarinov and R. A. Suris in 1971. Within a bulk semiconductor crystal, electrons may occupy states in one of two continuous energy bands - the valence band, populated with low energy electrons and the conduction band, sparsely populated with high energy electrons.
The two energy bands are separated by an energy band gap in which there are no permitted states available for electrons to occupy. Conventional semiconductor laser diodes generate light by a single photon being emitted when a high energy electron in the conduction band recombines with a hole in the valence band; the energy of the photon and hence the emission wavelength of laser diodes is therefore determined by the band gap of the material system used. A QCL however does not use bulk semiconductor materials in its optically active region. Instead it consists of a periodic series of thin layers of varying material composition forming a superlattice; the superlattice introduces a varying electric potential across the length of the device, meaning that there is a varying probability of electrons occupying different positions over the length of the device. This is referred to as one-dimensional multiple quantum well confinement and leads to the splitting of the band of permitted energies into a number of discrete electronic subbands.
By suitable design of the layer thicknesses it is possible to engineer a population inversion between two subbands in the system, required in order to achieve laser emission. Because the position of the energy levels in the system is determined by the layer thicknesses and not the material, it is possible to tune the emission wavelength of QCLs over a wide range in the same material system. Additionally, in semiconductor laser diodes and holes are annihilated after recombining across the band gap and can play no further part in photon generation. However, in a unipolar QCL, once an electron has undergone an intersubband transition and emitted a photon in one period of the superlattice, it can tunnel into the next period of the structure where another photon can be emitted; this process of a single electron causing the emission of multiple photons as it traverses through the QCL structure gives rise to the name cascade and makes a quantum efficiency of greater than unity possible which leads to higher output powers than semiconductor laser diodes.
QCLs are based upon a three-level system. Assuming the formation of the wavefunctions is a fast process compared to the scattering between states, the time independent solutions to the Schrödinger equation may be applied and the system can be modelled using rate equations; each subband contains a number of electrons n i which scatter between levels with a lifetime τ i f, where i and f are the initial and final subband indices. Assuming that no other subbands are populated, the rate equations for the three level lasers are given by: d n 3 d t = I i n + n 1 τ 13 + n 2 τ 23 − n 3 τ 31 − n 3 τ 32 d n 2 d t = n 3 τ 32 + n 1 τ 12 − n 2 τ 21 − n 2 τ 23 d n 1 d t = n 2 τ 21 + n 3 τ 31 − n 1 τ 13 − n 1 τ 12 − I o u t {\displaystyle =+--{\frac {n_
Electric field
An electric field surrounds an electric charge, exerts force on other charges in the field, attracting or repelling them. Electric field is sometimes abbreviated as E-field. Mathematically the electric field is a vector field that associates to each point in space the force per unit of charge exerted on an infinitesimal positive test charge at rest at that point; the SI unit for electric field strength is volt per meter. Newtons per coulomb is used as a unit of electric field strengh. Electric fields are created by time-varying magnetic fields. Electric fields are important in many areas of physics, are exploited electrical technology. On an atomic scale, the electric field is responsible for the attractive force between the atomic nucleus and electrons that holds atoms together, the forces between atoms that cause chemical bonding. Electric fields and magnetic fields are both manifestations of the electromagnetic force, one of the four fundamental forces of nature. From Coulomb's law a particle with electric charge q 1 at position x 1 exerts a force on a particle with charge q 0 at position x 0 of F = 1 4 π ε 0 q 1 q 0 2 r ^ 1, 0 where r 1, 0 is the unit vector in the direction from point x 1 to point x 0, ε0 is the electric constant in C2 m−2 N−1When the charges q 0 and q 1 have the same sign this force is positive, directed away from the other charge, indicating the particles repel each other.
When the charges have unlike signs the force is negative, indicating the particles attract. To make it easy to calculate the Coulomb force on any charge at position x 0 this expression can be divided by q 0, leaving an expression that only depends on the other charge E = F q 0 = 1 4 π ε 0 q 1 2 r ^ 1, 0 This is the electric field at point x 0 due to the point charge q 1. Since this formula gives the electric field magnitude and direction at any point x 0 in space it defines a vector field. From the above formula it can be seen that the electric field due to a point charge is everywhere directed away from the charge if it is positive, toward the charge if it is negative, its magnitude decreases with the inverse square of the distance from the charge. If there are multiple charges, the resultant Coulomb force on a charge can be found by summing the vectors of the forces due to each charge; this shows the electric field obeys the superposition principle: the total electric field at a point due to a collection of charges is just equal to the vector sum of the electric fields at that point due to the individual charges.
E = E 1 + E 2 + E 3 + ⋯ = 1 4 π ε 0 q 1 2 r ^ 1 + 1 4 π ε 0 q 2 ( x 2 −
Optical amplifier
An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed. Optical amplifiers are important in optical laser physics, they are used as optical repeaters in the long distance fiberoptic cables which carry much of the world's telecommunication links. There are several different physical mechanisms that can be used to amplify a light signal, which correspond to the major types of optical amplifiers. In doped fiber amplifiers and bulk lasers, stimulated emission in the amplifier's gain medium causes amplification of incoming light. In semiconductor optical amplifiers, electron-hole recombination occurs. In Raman amplifiers, Raman scattering of incoming light with phonons in the lattice of the gain medium produces photons coherent with the incoming photons. Parametric amplifiers use parametric amplification.
Any laser active gain medium can be pumped to produce gain for light at the wavelength of a laser made with the same material as its gain medium. Such amplifiers are used to produce high power laser systems. Special types such as regenerative amplifiers and chirped-pulse amplifiers are used to amplify ultrashort pulses. Solid-state amplifiers are optical amplifiers that uses a wide range of doped solid-state materials and different geometries to amplify optical signals; the variety of materials allows the amplification of different wavelength while the shape of the medium can distinguish between more suitable for energy of average power scaling. Beside their use in fundamental research from gravitational wave detection to high energy physics at NIF they can be found in many today’s ultra short pulsed lasers. Doped fiber amplifiers are optical amplifiers that use a doped optical fiber as a gain medium to amplify an optical signal, they are related to fiber lasers. The signal to be amplified and a pump laser are multiplexed into the doped fiber, the signal is amplified through interaction with the doping ions.
The most common example is the Erbium Doped Fiber Amplifier, where the core of a silica fiber is doped with trivalent erbium ions and can be efficiently pumped with a laser at a wavelength of 980 nm or 1,480 nm, exhibits gain in the 1,550 nm region. An erbium-doped waveguide amplifier is an optical amplifier that uses a waveguide to boost an optical signal. Amplification is achieved by stimulated emission of photons from dopant ions in the doped fiber; the pump laser excites ions into a higher energy from where they can decay via stimulated emission of a photon at the signal wavelength back to a lower energy level. The excited ions can decay spontaneously or through nonradiative processes involving interactions with phonons of the glass matrix; these last two decay mechanisms compete with stimulated emission reducing the efficiency of light amplification. The amplification window of an optical amplifier is the range of optical wavelengths for which the amplifier yields a usable gain; the amplification window is determined by the spectroscopic properties of the dopant ions, the glass structure of the optical fiber, the wavelength and power of the pump laser.
Although the electronic transitions of an isolated ion are well defined, broadening of the energy levels occurs when the ions are incorporated into the glass of the optical fiber and thus the amplification window is broadened. This broadening is both inhomogeneous. Homogeneous broadening arises from the interactions with phonons of the glass, while inhomogeneous broadening is caused by differences in the glass sites where different ions are hosted. Different sites expose ions to different local electric fields, which shifts the energy levels via the Stark effect. In addition, the Stark effect removes the degeneracy of energy states having the same total angular momentum. Thus, for example, the trivalent erbium ion has a ground state with J = 15/2, in the presence of an electric field splits into J + 1/2 = 8 sublevels with different energies; the first excited state has J = 13/2 and therefore a Stark manifold with 7 sublevels. Transitions from the J = 13/2 excited state to the J= 15/2 ground state are responsible for the gain at 1.5 µm wavelength.
The gain spectrum of the EDFA has several peaks. The net result is a broad spectrum; the broad gain-bandwidth of fiber amplifiers make them useful in wavelength-division multiplexed communications systems as a single amplifier can be utilized to amplify all signals being carried on a fiber and whose wavelengths fall within the gain window. A high-powered beam of light is mixed with the input signal using a wavelength selective coupler; the input signal and the excitation light must be at different wavelengths. The mixed light is guided into a section of fiber with erbium ions included in the core; this high-powered light beam excites the erbium ions to their higher-energy state. When the photons belonging to the signal at a different wavelength from the pump light meet the excited erbium atoms, the erbium atoms give up some of their energy to the signal and return to their lower-energy state. A significant point is that the erbium gives up its energy in the form of additional photons which are in the same phase and direction as the signal being amplified.
So the signal is amplified along
Photonic crystal
A photonic crystal is a periodic optical nanostructure that affects the motion of photons in much the same way that ionic lattices affect electrons in solids. Photonic crystals occur in nature in the form of structural coloration and animal reflectors, and, in different forms, promise to be useful in a range of applications. In 1887 the English physicist Lord Rayleigh experimented with periodic multi-layer dielectric stacks, showing they had a photonic band-gap in one dimension. Research interest grew with work in 1987 by Eli Yablonovitch and Sajeev John on periodic optical structures with more than one dimension—now called photonic crystals. Photonic crystals can be fabricated for two, or three dimensions. One-dimensional photonic crystals can be stuck together. Two-dimensional ones can be made by drilling holes in a suitable substrate. Fabrication methods for three-dimensional ones include drilling under different angles, stacking multiple 2-D layers on top of each other, direct laser writing, or, for example, instigating self-assembly of spheres in a matrix and dissolving the spheres.
Photonic crystals can, in principle, find uses. Existing applications include thin-film optics with coatings for lenses. Two-dimensional photonic-crystal fibers are used in nonlinear devices and to guide exotic wavelengths. Three-dimensional crystals may one day be used in optical computers. Three-dimensional photonic crystals could lead to more efficient photovoltaic cells as a source of power for electronics, thus cutting down the need for an electrical input for power. Photonic crystals are composed of periodic dielectric, metallo-dielectric—or superconductor microstructures or nanostructures that affect electromagnetic wave propagation in the same way that the periodic potential in a semiconductor crystal affects electron motion by defining allowed and forbidden electronic energy bands. Photonic crystals contain repeating regions of high and low dielectric constant. Photons either propagate depending on their wavelength. Wavelengths that propagate are called modes, groups of allowed modes form bands.
Disallowed bands of wavelengths are called photonic band gaps. This gives rise to distinct optical phenomena, such as inhibition of spontaneous emission, high-reflecting omni-directional mirrors, low-loss-waveguiding. Intuitively, the bandgap of photonic crystals can be understood to arise from the destructive interference of multiple reflections of light propagating in the crystal at the interfaces of the high- and low- dielectric constant regions, akin to the bandgaps of electrons in solids; the periodicity of the photonic crystal structure must be around half the wavelength of the electromagnetic waves to be diffracted. This is ~350 nm to ~650 nm for photonic crystals that operate in the visible part of the spectrum—or less, depending on average index of refraction; the repeating regions of high and low dielectric constant must, therefore, be fabricated at this scale, difficult. Photonic crystals have been studied in one form or another since 1887, but no one used the term photonic crystal until over 100 years later—after Eli Yablonovitch and Sajeev John published two milestone papers on photonic crystals in 1987.
The early history is well documented in the form of a story when it was identified as one of the landmark developments in physics by the American Physical Society. Before 1987, one-dimensional photonic crystals in the form of periodic multi-layer dielectric stacks were studied extensively. Lord Rayleigh started their study in 1887, by showing that such systems have a one-dimensional photonic band-gap, a spectral range of large reflectivity, known as a stop-band. Today, such structures are used in a diverse range of applications—from reflective coatings to enhancing LED efficiency to reflective mirrors in certain laser cavities. A detailed theoretical study of one-dimensional optical structures was performed by Vladimir P. Bykov, the first to investigate the effect of a photonic band-gap on the spontaneous emission from atoms and molecules embedded within the photonic structure. Bykov speculated as to what could happen if two- or three-dimensional periodic optical structures were used; the concept of three-dimensional photonic crystals was discussed by Ohtaka in 1979, who developed a formalism for the calculation of the photonic band structure.
However, these ideas did not take off until after the publication of two milestone papers in 1987 by Yablonovitch and John. Both these papers concerned high-dimensional periodic optical structures. Yablonovitch's main goal was to engineer photonic density of states to control the spontaneous emission of materials embedded in the photonic crystal. John's idea was to use photonic crystals to affect control of light. After 1987, the number of research papers concerning photonic crystals began to grow exponentially. However, due to the difficulty of fabricating these structures at optical scales, early studies were either theoretical or in the microwave regime, where photonic crystals can be built on the more accessible centimetre scale. By 1991, Yablonovitch had demonstrated the first three-dimensional photonic band-gap in the microwave regime; the structure that Yablonovitch was able
Photoemission spectroscopy
Photoemission spectroscopy known as photoelectron spectroscopy, refers to energy measurement of electrons emitted from solids, gases or liquids by the photoelectric effect, in order to determine the binding energies of electrons in a substance. The term refers to various techniques, depending on whether the ionization energy is provided by an X-ray photon, an EUV photon, or an ultraviolet photon. Regardless of the incident photon beam, all photoelectron spectroscopy revolves around the general theme of surface analysis by measuring the ejected electrons. X-ray photoelectron spectroscopy was developed by Kai Siegbahn starting in 1957 and is used to study the energy levels of atomic core electrons in solids. Siegbahn referred to the technique as "electron spectroscopy for chemical analysis", since the core levels have small chemical shifts depending on the chemical environment of the atom, ionized, allowing chemical structure to be determined. Siegbahn was awarded the Nobel Prize in 1981 for this work.
XPS is sometimes referred to as PESIS, whereas the lower-energy radiation of UV light is referred to as PESOS because it cannot excite core electrons. In the ultraviolet and visible region, the method is referred to as photoelectron spectroscopy for the study of gases, photoemission spectroscopy for solid surfaces. Ultraviolet photoelectron spectroscopy is used to study valence energy levels and chemical bonding the bonding character of molecular orbitals; the method was developed for gas-phase molecules in 1961 by Feodor I. Vilesov and in 1962 by David W. Turner, other early workers included David C. Frost, J. H. D. Eland and K. Kimura. Richard Smalley modified the technique and used a UV laser to excite the sample, in order to measure the binding energy of electrons in gaseous molecular clusters. Two-photon photoelectron spectroscopy extends the technique to optically excited electronic states through the introduction of a pump-and-probe scheme. Extreme-ultraviolet photoelectron spectroscopy lies in between XPS and UPS.
It is used to assess the valence band structure. Compared to XPS, it gives better energy resolution, compared to UPS, the ejected electrons are faster, resulting in less space charge and mitigated final state effects; the physics behind the PES technique is an application of the photoelectric effect. The sample is exposed to a beam of XUV light inducing photoelectric ionization; the energies of the emitted photoelectrons are characteristic of their original electronic states, depend on vibrational state and rotational level. For solids, photoelectrons can escape only from a depth on the order of nanometers, so that it is the surface layer, analyzed; because of the high frequency of the light, the substantial charge and energy of emitted electrons, photoemission is one of the most sensitive and accurate techniques for measuring the energies and shapes of electronic states and molecular and atomic orbitals. Photoemission is among the most sensitive methods of detecting substances in trace concentrations, provided the sample is compatible with ultra-high vacuum and the analyte can be distinguished from background.
Typical PES instruments use helium gas sources of UV light, with photon energy up to 52 eV. The photoelectrons that escaped into the vacuum are collected, energy resolved retarded and counted, which results in a spectrum of electron intensity as a function of the measured kinetic energy; because binding energy values are more applied and understood, the kinetic energy values, which are source dependent, are converted into binding energy values, which are source independent. This is achieved by applying Einstein's relation E k = h ν − E B; the h ν term of this equation is due to the energy of the UV light. Photoemission spectra are measured using synchrotron radiation sources; the binding energies of the measured electrons are characteristic of the chemical structure and molecular bonding of the material. By adding a source monochromator and increasing the energy resolution of the electron analyzer, peaks appear with full width at half maximum less than 5–8 meV. Angle resolved photoemission spectroscopy AR-PES Inverse photoemission spectroscopy IPS Ultra high vacuum UHV X-ray photoelectron spectroscopy XPS Ultraviolet photoelectron spectroscopy UPS Two-photon photoelectron spectroscopy 2PPE Vibronic spectroscopy Reinert, Friedrich.
"Photoemission spectroscopy—from early days to recent applications". New Journal of Physics. 7: 97. Bibcode:2005NJPh....7...97R. Doi:10.1088/1367-2630/7/1/097. ISSN 1367-2630. Presentation on principle of ARPES
