Spectral line

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Spectral-lines-continuous.svg
Continuous spectrum
Spectral-lines-absorption.svg
Absorption lines (discrete spectrum)
Absorption lines for air, under indirect illumination, with the direct light source not visible, so that the gas is not directly between source and detector. Here, Fraunhofer lines in sunlight and Rayleigh scattering of this sunlight is the "source." This is the spectrum of a blue sky somewhat close to the horizon, pointing east at around 3 or 4 pm (i.e., Sun toward the west
) on a clear day. Spectrum of blue sky.svg
Absorption lines for air, under indirect illumination, with the direct light source not visible, so that the gas is not directly between source and detector. Here, Fraunhofer lines in sunlight and Rayleigh scattering of this sunlight is the "source." This is the spectrum of a blue sky somewhat close to the horizon, pointing east at around 3 or 4 pm (i.e., Sun toward the west ) on a clear day.

A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to identify atoms and molecules. These "fingerprints" can be compared to the previously collected "fingerprints" of atoms [1] and molecules, [2] and are thus used to identify the atomic and molecular components of stars and planets, which would otherwise be impossible.

Contents

Types of line spectra

Continuous spectrum of an incandescent lamp (mid) and discrete spectrum lines of a fluorescent lamp (bottom) Simple spectroscope.jpg
Continuous spectrum of an incandescent lamp (mid) and discrete spectrum lines of a fluorescent lamp (bottom)

Spectral lines are the result of interaction between a quantum system (usually atoms, but sometimes molecules or atomic nuclei) and a single photon. When a photon has about the right amount of energy (which is connected to its frequency) [3] to allow a change in the energy state of the system (in the case of an atom this is usually an electron changing orbitals), the photon is absorbed. Then it will be spontaneously re-emitted, either in the same frequency as the original or in a cascade, where the sum of the energies of the photons emitted will be equal to the energy of the one absorbed (assuming the system returns to its original state).[ citation needed ]

A spectral line may be observed either as an emission line or an absorption line. Which type of line is observed depends on the type of material and its temperature relative to another emission source. An absorption line is produced when photons from a hot, broad spectrum source pass through a cold material. The intensity of light, over a narrow frequency range, is reduced due to absorption by the material and re-emission in random directions. By contrast, a bright emission line is produced when photons from a hot material are detected in the presence of a broad spectrum from a cold source. The intensity of light, over a narrow frequency range, is increased due to emission by the material.

Spectral lines are highly atom-specific, and can be used to identify the chemical composition of any medium capable of letting light pass through it. Several elements were discovered by spectroscopic means, including helium, thallium, and caesium. Spectral lines also depend on the physical conditions of the gas, so they are widely used to determine the chemical composition of stars and other celestial bodies that cannot be analyzed by other means, as well as their physical conditions.

Mechanisms other than atom-photon interaction can produce spectral lines. Depending on the exact physical interaction (with molecules, single particles, etc.), the frequency of the involved photons will vary widely, and lines can be observed across the electromagnetic spectrum, from radio waves to gamma rays.

Nomenclature

Strong spectral lines in the visible part of the spectrum often have a unique Fraunhofer line designation, such as K for a line at 393.366 nm emerging from singly-ionized Ca +, though some of the Fraunhofer "lines" are blends of multiple lines from several different species. In other cases, the lines are designated according to the level of ionization by adding a Roman numeral to the designation of the chemical element. Neutral atoms are denoted with the Roman numeral I, singly ionized atoms with II, and so on, so that, for example, Fe IX represents eight times ionized iron.

More detailed designations usually include the line wavelength and may include a multiplet number (for atomic lines) or band designation (for molecular lines). Many spectral lines of atomic hydrogen also have designations within their respective series, such as the Lyman series or Balmer series. Originally all spectral lines were classified into series: the Principal series, Sharp series , and Diffuse series . These series exist across atoms of all elements, and the patterns for all atoms are well-predicted by the Rydberg-Ritz formula. These series were later associated with suborbitals.

Line broadening and shift

There are a number of effects which control spectral line shape. A spectral line extends over a range of frequencies, not a single frequency (i.e., it has a nonzero linewidth). In addition, its center may be shifted from its nominal central wavelength. There are several reasons for this broadening and shift. These reasons may be divided into two general categories – broadening due to local conditions and broadening due to extended conditions. Broadening due to local conditions is due to effects which hold in a small region around the emitting element, usually small enough to assure local thermodynamic equilibrium. Broadening due to extended conditions may result from changes to the spectral distribution of the radiation as it traverses its path to the observer. It also may result from the combining of radiation from a number of regions which are far from each other.

Broadening due to local effects

Natural broadening

The lifetime of excited states results in natural broadening, also known as lifetime broadening. The uncertainty principle relates the lifetime of an excited state (due to spontaneous radiative decay or the Auger process) with the uncertainty of its energy. A short lifetime will have a large energy uncertainty and a broad emission. This broadening effect results in an unshifted Lorentzian profile. The natural broadening can be experimentally altered only to the extent that decay rates can be artificially suppressed or enhanced. [4]

Thermal Doppler broadening

The atoms in a gas which are emitting radiation will have a distribution of velocities. Each photon emitted will be "red"- or "blue"-shifted by the Doppler effect depending on the velocity of the atom relative to the observer. The higher the temperature of the gas, the wider the distribution of velocities in the gas. Since the spectral line is a combination of all of the emitted radiation, the higher the temperature of the gas, the broader the spectral line emitted from that gas. This broadening effect is described by a Gaussian profile and there is no associated shift.

Pressure broadening

The presence of nearby particles will affect the radiation emitted by an individual particle. There are two limiting cases by which this occurs:

  • Impact pressure broadening or collisional broadening: The collision of other particles with the light emitting particle interrupts the emission process, and by shortening the characteristic time for the process, increases the uncertainty in the energy emitted (as occurs in natural broadening). [5] The duration of the collision is much shorter than the lifetime of the emission process. This effect depends on both the density and the temperature of the gas. The broadening effect is described by a Lorentzian profile and there may be an associated shift.
  • Quasistatic pressure broadening: The presence of other particles shifts the energy levels in the emitting particle,[ clarification needed ] thereby altering the frequency of the emitted radiation. The duration of the influence is much longer than the lifetime of the emission process. This effect depends on the density of the gas, but is rather insensitive to temperature. The form of the line profile is determined by the functional form of the perturbing force with respect to distance from the perturbing particle. There may also be a shift in the line center. The general expression for the lineshape resulting from quasistatic pressure broadening is a 4-parameter generalization of the Gaussian distribution known as a stable distribution. [6]

Pressure broadening may also be classified by the nature of the perturbing force as follows:

  • Linear Stark broadening occurs via the linear Stark effect, which results from the interaction of an emitter with an electric field of a charged particle at a distance , causing a shift in energy that is linear in the field strength.
  • Resonance broadening occurs when the perturbing particle is of the same type as the emitting particle, which introduces the possibility of an energy exchange process.
  • Quadratic Stark broadening occurs via the quadratic Stark effect, which results from the interaction of an emitter with an electric field, causing a shift in energy that is quadratic in the field strength.
  • Van der Waals broadening occurs when the emitting particle is being perturbed by Van der Waals forces. For the quasistatic case, a Van der Waals profile [note 1] is often useful in describing the profile. The energy shift as a function of distance[ definition needed ] is given in the wings by e.g. the Lennard-Jones potential.

Inhomogeneous broadening

Inhomogeneous broadening is a general term for broadening because some emitting particles are in a different local environment from others, and therefore emit at a different frequency. This term is used especially for solids, where surfaces, grain boundaries, and stoichiometry variations can create a variety of local environments for a given atom to occupy. In liquids, the effects of inhomogeneous broadening is sometimes reduced by a process called motional narrowing .

Broadening due to non-local effects

Certain types of broadening are the result of conditions over a large region of space rather than simply upon conditions that are local to the emitting particle.

Opacity broadening

Electromagnetic radiation emitted at a particular point in space can be reabsorbed as it travels through space. This absorption depends on wavelength. The line is broadened because the photons at the line center have a greater reabsorption probability than the photons at the line wings. Indeed, the reabsorption near the line center may be so great as to cause a self reversal in which the intensity at the center of the line is less than in the wings. This process is also sometimes called self-absorption.

Macroscopic Doppler broadening

Radiation emitted by a moving source is subject to Doppler shift due to a finite line-of-sight velocity projection. If different parts of the emitting body have different velocities (along the line of sight), the resulting line will be broadened, with the line width proportional to the width of the velocity distribution. For example, radiation emitted from a distant rotating body, such as a star, will be broadened due to the line-of-sight variations in velocity on opposite sides of the star. The greater the rate of rotation, the broader the line. Another example is an imploding plasma shell in a Z-pinch.

Radiative broadening

Radiative broadening of the spectral absorption profile occurs because the on-resonance absorption in the center of the profile is saturated at much lower intensities than the off-resonant wings. Therefore, as intensity rises, absorption in the wings rises faster than absorption in the center, leading to a broadening of the profile. Radiative broadening occurs even at very low light intensities.

Combined effects

Each of these mechanisms can act in isolation or in combination with others. Assuming each effect is independent, the observed line profile is a convolution of the line profiles of each mechanism. For example, a combination of the thermal Doppler broadening and the impact pressure broadening yields a Voigt profile.

However, the different line broadening mechanisms are not always independent. For example, the collisional effects and the motional Doppler shifts can act in a coherent manner, resulting under some conditions even in a collisional narrowing, known as the Dicke effect.

Spectral lines of chemical elements

Bands

The phrase "spectral lines", when not qualified, usually refers to lines having wavelengths in the visible band of the full electromagnetic spectrum. Many spectral lines occur at wavelengths outside this range. At shorter wavelengths, which correspond to higher energies, ultraviolet spectral lines include the Lyman series of hydrogen. At the much shorter wavelengths of X-rays, the lines are known as characteristic X-rays because they remain largely unchanged for a given chemical element, independent of their chemical environment. Longer wavelengths correspond to lower energies, where the infrared spectral lines include the Paschen series of hydrogen. At even longer wavelengths, the radio spectrum includes the 21-cm line used to detect neutral hydrogen throughout the cosmos.

Visible light

For each element, the following table shows the spectral lines which appear in the visible spectrum at about 400-700 nm.

ElementZSymbolSpectral lines
hydrogen 1H Hydrogen spectrum visible.png
helium 2He Helium spectrum visible.png
lithium 3Li Lithium spectrum visible.png
beryllium 4Be Beryllium spectrum visible.png
boron 5B Boron spectrum visible.png
carbon 6C Carbon spectrum visible.png
nitrogen 7N Nitrogen spectrum visible.png
oxygen 8O Oxygen spectrum visible.png
fluorine 9F Fluorine spectrum visible.png
neon 10Ne Neon spectrum visible.png
sodium 11Na Sodium spectrum visible.png
magnesium 12Mg Magnesium spectrum visible.png
aluminium 13Al Aluminium spectrum visible.png
silicon 14Si Silicon spectrum visible.png
phosphorus 15P Phosphorus spectrum visible.png
sulfur 16S Sulfur spectrum visible.png
chlorine 17Cl Chlorine spectrum visible.png
argon 18Ar Argon spectrum visible.png
potassium 19K Potassium spectrum visible.png
calcium 20Ca Calcium spectrum visible.png
scandium 21Sc Scandium spectrum visible.png
titanium 22Ti Titanium spectrum visible.png
vanadium 23V Vanadium spectrum visible.png
chromium 24Cr Chromium spectrum visible.png
manganese 25Mn Manganese spectrum visible.png
iron 26Fe Iron spectrum visible.png
cobalt 27Co Cobalt spectrum visible.png
nickel 28Ni Nickel spectrum visible.png
copper 29Cu Copper spectrum visible.png
zinc 30Zn Zinc spectrum visible.png
gallium 31Ga Gallium spectrum visible.png
germanium 32Ge Germanium spectrum visible.png
arsenic 33As Arsenic spectrum visible.png
selenium 34Se Selenium spectrum visible.png
bromine 35Br Bromine spectrum visible.png
krypton 36Kr Krypton spectrum visible.png
rubidium 37Rb Rubidium spectrum visible.png
strontium 38Sr Strontium spectrum visible.png
yttrium 39Y Yttrium spectrum visible.png
zirconium 40Zr Zirconium spectrum visible.png
niobium 41Nb Niobium spectrum visible.png
molybdenum 42Mo Molybdenum spectrum visible.png
technetium 43Tc Technetium spectrum visible.png
ruthenium 44Ru Ruthenium spectrum visible.png
rhodium 45Rh Rhodium spectrum visible.png
palladium 46Pd Palladium spectrum visible.png
silver 47Ag Silver spectrum visible.png
cadmium 48Cd Cadmium spectrum visible.png
indium 49In Indium spectrum visible.png
tin 50Sn Tin spectrum visible.png
antimony 51Sb Antimony spectrum visible.png
tellurium 52Te Tellurium spectrum visible.png
iodine 53I Iodine spectrum visible.png
xenon 54Xe Xenon spectrum visible.png
caesium 55Cs Caesium spectrum visible.png
barium 56Ba Barium spectrum visible.png
lanthanum 57La Lanthanum spectrum visible.png
cerium 58Ce Cerium spectrum visible.png
praseodymium 59Pr Praseodymium spectrum visible.png
neodymium 60Nd Neodymium spectrum visible.png
promethium 61Pm Promethium spectrum visible.png
samarium 62Sm Samarium spectrum visible.png
europium 63Eu Europium spectrum visible.png
gadolinium 64Gd Gadolinium spectrum visible.png
terbium 65Tb Terbium spectrum visible.png
dysprosium 66Dy Dysprosium spectrum visible.png
holmium 67Ho Holmium spectrum visible.png
erbium 68Er Erbium spectrum visible.png
thulium 69Tm Thulium spectrum visible.png
ytterbium 70Yb Ytterbium spectrum visible.png
lutetium 71Lu Lutetium spectrum visible.png
hafnium 72Hf Hafnium spectrum visible.png
tantalum 73Ta Tantalum spectrum visible.png
tungsten 74W Tungsten spectrum visible.png
rhenium 75Re Rhenium spectrum visible.png
osmium 76Os Osmium spectrum visible.png
iridium 77Ir Iridium spectrum visible.png
platinum 78Pt Platinum spectrum visible.png
gold 79Au Gold spectrum visible.png
thallium 81Tl Thallium spectrum visible.png
lead 82Pb Lead spectrum visible.png
bismuth 83Bi Bismuth spectrum visible.png
polonium 84Po Polonium spectrum visible.png
radon 86Rn Radon spectrum visible.png
radium 88Ra Radium spectrum visible.png
actinium 89Ac Actinium spectrum visible.png
thorium 90Th Thorium spectrum visible.png
protactinium 91Pa Protactinium spectrum visible.png
uranium 92U Uranium spectrum visible.png
neptunium 93Np Neptunium spectrum visible.png
plutonium 94Pu Plutonium spectrum visible.png
americium 95Am Americium spectrum visible.png
curium 96Cm Curium spectrum visible.png
berkelium 97Bk Berkelium spectrum visible.png
californium 98Cf Californium spectrum visible.png
einsteinium 99Es Einsteinium spectrum visible.png

See also

Notes

  1. "Van der Waals profile" appears as lowercase in almost all sources, such as: Statistical mechanics of the liquid surface by Clive Anthony Croxton, 1980, A Wiley-Interscience publication, ISBN   0-471-27663-4, ISBN   978-0-471-27663-0; and in Journal of technical physics, Volume 36, by Instytut Podstawowych Problemów Techniki (Polska Akademia Nauk), publisher: Państwowe Wydawn. Naukowe., 1995,

Related Research Articles

The electromagnetic spectrum is the range of frequencies of electromagnetic radiation and their respective wavelengths and photon energies.

Spectroscopy Study involving matter and electromagnetic radiation

Spectroscopy is the study of the interaction between matter and electromagnetic radiation as a function of the wavelength or frequency of the radiation. In simpler terms, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum; indeed, historically, spectroscopy originated as the study of the wavelength dependence of the absorption by gas phase matter of visible light dispersed by a prism. Matter waves and acoustic waves can also be considered forms of radiative energy, and recently gravitational waves have been associated with a spectral signature in the context of the Laser Interferometer Gravitational-Wave Observatory (LIGO).

Interstellar medium Matter and radiation in the space between the star systems in a galaxy

In astronomy, the interstellar medium (ISM) is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.

X-ray fluorescence Physical phenomenom

X-ray fluorescence (XRF) is the emission of characteristic "secondary" X-rays from a material that has been excited by being bombarded with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis and chemical analysis, particularly in the investigation of metals, glass, ceramics and building materials, and for research in geochemistry, forensic science, archaeology and art objects such as paintings

Thermal radiation Electromagnetic radiation generated by the thermal motion of particles

Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. Particle motion results in charge-acceleration or dipole oscillation which produces electromagnetic radiation.

Emission spectrum Frequencies of light emitted by atoms or chemical compounds

The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an atom or molecule making a transition from a high energy state to a lower energy state. The photon energy of the emitted photon is equal to the energy difference between the two states. There are many possible electron transitions for each atom, and each transition has a specific energy difference. This collection of different transitions, leading to different radiated wavelengths, make up an emission spectrum. Each element's emission spectrum is unique. Therefore, spectroscopy can be used to identify elements in matter of unknown composition. Similarly, the emission spectra of molecules can be used in chemical analysis of substances.

Absorption spectroscopy Spectroscopic techniques that measure the absorption of radiation

Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a function of frequency, and this variation is the absorption spectrum. Absorption spectroscopy is performed across the electromagnetic spectrum.

Plasma diagnostics are a pool of methods, instruments, and experimental techniques used to measure properties of a plasma, such as plasma components' density, distribution function over energy (temperature), their spatial profiles and dynamics, which enable to derive plasma parameters.

Atomic spectroscopy is the study of the electromagnetic radiation absorbed and emitted by atoms. Since unique elements have characteristic (signature) spectra, atomic spectroscopy, specifically the electromagnetic spectrum or mass spectrum, is applied for determination of elemental compositions. It can be divided by atomization source or by the type of spectroscopy used. In the latter case, the main division is between optical and mass spectrometry. Mass spectrometry generally gives significantly better analytical performance, but is also significantly more complex. This complexity translates into higher purchase costs, higher operational costs, more operator training, and a greater number of components that can potentially fail. Because optical spectroscopy is often less expensive and has performance adequate for many tasks, it is far more common Atomic absorption spectrometers are one of the most commonly sold and used analytical devices.

Absorption band Range on the electromagnetic spectrum which are characteristic of a certain transition from initial to final state in a substance

According to quantum mechanics, atoms and molecules can only hold certain defined quantities of energy, or exist in specific states. When such quanta of electromagnetic radiation are emitted or absorbed by an atom or molecule, energy of the radiation changes the state of the atom or molecule from an initial state to a final state. An absorption band is a range of wavelengths, frequencies or energies in the electromagnetic spectrum which are characteristic of a particular transition from initial to final state in a substance.

Einstein coefficients

Einstein coefficients are mathematical quantities which are a measure of the probability of absorption or emission of light by an atom or molecule. The Einstein A coefficients are related to the rate of spontaneous emission of light, and the Einstein B coefficients are related to the absorption and stimulated emission of light.

In atomic physics, Doppler broadening is the broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules. Different velocities of the emitting particles result in different Doppler shifts, the cumulative effect of which is the line broadening. This resulting line profile is known as a Doppler profile. A particular case is the thermal Doppler broadening due to the thermal motion of the particles. Then, the broadening depends only on the frequency of the spectral line, the mass of the emitting particles, and their temperature, and therefore can be used for inferring the temperature of an emitting body.

Quantum mechanics is the study of very small things. It explains the behavior of matter and its interactions with energy on the scale of atomic and subatomic particles. By contrast, classical physics explains matter and energy only on a scale familiar to human experience, including the behavior of astronomical bodies such as the Moon. Classical physics is still used in much of modern science and technology. However, towards the end of the 19th century, scientists discovered phenomena in both the large (macro) and the small (micro) worlds that classical physics could not explain. The desire to resolve inconsistencies between observed phenomena and classical theory led to two major revolutions in physics that created a shift in the original scientific paradigm: the theory of relativity and the development of quantum mechanics. This article describes how physicists discovered the limitations of classical physics and developed the main concepts of the quantum theory that replaced it in the early decades of the 20th century. It describes these concepts in roughly the order in which they were first discovered. For a more complete history of the subject, see History of quantum mechanics.

Hydrogen spectral series Important atomic emission spectra

The emission spectrum of atomic hydrogen has been divided into a number of spectral series, with wavelengths given by the Rydberg formula. These observed spectral lines are due to the electron making transitions between two energy levels in an atom. The classification of the series by the Rydberg formula was important in the development of quantum mechanics. The spectral series are important in astronomical spectroscopy for detecting the presence of hydrogen and calculating red shifts.

Lyman continuum photons Photons emitted from stars at photon energies above the Lyman limit

Lyman continuum photons, shortened to Ly continuum photons or Lyc photons, are the photons emitted from stars at photon energies above the Lyman limit. Hydrogen is ionized by absorbing LyC. Working from Victor Schumann's discovery of ultraviolet light, from 1906 to 1914, Theodore Lyman observed that atomic hydrogen absorbs light only at specific frequencies and the Lyman series is thus named after him. All the wavelengths in the Lyman series are in the ultraviolet band. This quantized absorption behavior occurs only up to an energy limit, known as the ionization energy. In the case of neutral atomic hydrogen, the minimum ionization energy is equal to the Lyman limit, where the photon has enough energy to completely ionize the atom, resulting in a free proton and a free electron. Above this energy, all wavelengths of light may be absorbed. This forms a continuum in the energy spectrum; the spectrum is continuous rather than composed of many discrete lines, which are seen at lower energies.

Atomic emission spectroscopy Analytical method using radiation to identify chemical elements in a sample

Atomic emission spectroscopy (AES) is a method of chemical analysis that uses the intensity of light emitted from a flame, plasma, arc, or spark at a particular wavelength to determine the quantity of an element in a sample. The wavelength of the atomic spectral line in the emission spectrum gives the identity of the element while the intensity of the emitted light is proportional to the number of atoms of the element. The sample may be excited by various methods.

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, and photochromism.

In spectroscopy, the Dicke effect, also known as Dicke narrowing or sometimes collisional narrowing, named after Robert H. Dicke, refers to narrowing of the Doppler broadening of a spectral line due to collisions the emitting species experiences with other particles.

Coronal radiative losses

In astronomy and in astrophysics, for radiative losses of the solar corona, it is meant the energy flux radiated from the external atmosphere of the Sun, and, in particular, the processes of production of the radiation coming from the solar corona and transition region, where the plasma is optically-thin. On the contrary, in the chromosphere, where the temperature decreases from the photospheric value of 6000 K to the minimum of 4400 K, the optical depth is about 1, and the radiation is thermal.

Spectrometer Used to measure spectral components of light

A spectrometer is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the spectral components are somehow mixed. In visible light a spectrometer can separate white light and measure individual narrow bands of color, called a spectrum. A mass spectrometer measures the spectrum of the masses of the atoms or molecules present in a gas. The first spectrometers were used to split light into an array of separate colors. Spectrometers were developed in early studies of physics, astronomy, and chemistry. The capability of spectroscopy to determine chemical composition drove its advancement and continues to be one of its primary uses. Spectrometers are used in astronomy to analyze the chemical composition of stars and planets, and spectrometers gather data on the origin of the universe.

References

  1. Kramida, Alexander; Ralchenko, Yuri (1999), NIST Atomic Spectra Database, NIST Standard Reference Database 78, National Institute of Standards and Technology, retrieved 2021-06-27
  2. Rothman, L.S.; Gordon, I.E.; Babikov, Y.; Barbe, A.; Chris Benner, D.; Bernath, P.F.; Birk, M.; Bizzocchi, L.; Boudon, V.; Brown, L.R.; Campargue, A.; Chance, K.; Cohen, E.A.; Coudert, L.H.; Devi, V.M.; Drouin, B.J.; Fayt, A.; Flaud, J.-M.; Gamache, R.R.; Harrison, J.J.; Hartmann, J.-M.; Hill, C.; Hodges, J.T.; Jacquemart, D.; Jolly, A.; Lamouroux, J.; Le Roy, R.J.; Li, G.; Long, D.A.; et al. (2013). "The HITRAN2012 molecular spectroscopic database". Journal of Quantitative Spectroscopy and Radiative Transfer. 130: 4–50. Bibcode:2013JQSRT.130....4R. doi:10.1016/j.jqsrt.2013.07.002. ISSN   0022-4073.
  3. Einstein, Albert (1905). "On a Heuristic Viewpoint Concerning the Production and Transformation of Light".
  4. For example, in the following article, decay was suppressed via a microwave cavity, thus reducing the natural broadening: Gabrielse, Gerald; H. Dehmelt (1985). "Observation of Inhibited Spontaneous Emission". Physical Review Letters. 55 (1): 67–70. Bibcode:1985PhRvL..55...67G. doi:10.1103/PhysRevLett.55.67. PMID   10031682.
  5. "Collisional Broadening". Fas.harvard.edu. Archived from the original on 2015-09-24. Retrieved 2015-09-24.
  6. Peach, G. (1981). "Theory of the pressure broadening and shift of spectral lines". Advances in Physics. 30 (3): 367–474. Bibcode:1981AdPhy..30..367P. doi:10.1080/00018738100101467. Archived from the original on 2013-01-14.

Further reading