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Continuous spectrum
Continuous spectrum
Absorption lines
Absorption spectrum with 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[clarification needed]) on a clear day.
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[clarification needed]) 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 ones 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.

Types of line spectra

Continuous spectrum of an incandescent lamp (mid) and discrete spectrum lines of a fluorescent lamp (bottom)
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 the energy will be spontaneously re-emitted, either as one photon at the same frequency as the original one 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).

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 cooler 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, perhaps in the presence of a broad spectrum from a cooler source. The intensity of light, over a narrow frequency range, is increased due to emission by the hot material.

Spectral lines are highly atom-specific, and can be used to identify the chemical composition of any medium. Several elements, including helium, thallium, and caesium, were discovered by spectroscopic means. Spectral lines also depend on the temperature and density of the material, so they are widely used to determine the physical conditions of stars and other celestial bodies that cannot be analyzed by other means.

Depending on the material and its physical conditions, the energy of the involved photons can vary widely, with the spectral lines observed across the electromagnetic spectrum, from radio waves to gamma rays.


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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. Some authors use the term "radiative broadening" to refer specifically to the part of natural broadening caused by the spontaneous radiative decay.[4] 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.[5]

Thermal Doppler broadening

Main article: 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:

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

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

Opacity broadening is an example of a non-local broadening mechanism. 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 (this effect usually referred to as rotational broadening). The greater the rate of rotation, the broader the line. Another example is an imploding plasma shell in a Z-pinch.

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

See also: Hydrogen spectral series


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.

Element Z Symbol Spectral lines
hydrogen 1 H
Hydrogen spectrum visible.png
helium 2 He
Helium spectrum visible.png
lithium 3 Li
Lithium spectrum visible.png
beryllium 4 Be
Beryllium spectrum visible.png
boron 5 B
Boron spectrum visible.png
carbon 6 C
Carbon spectrum visible.png
nitrogen 7 N
Nitrogen spectrum visible.png
oxygen 8 O
Oxygen spectrum visible.png
fluorine 9 F
Fluorine spectrum visible.png
neon 10 Ne
Neon spectrum visible.png
sodium 11 Na
Sodium spectrum visible.png
magnesium 12 Mg
Magnesium spectrum visible.png
aluminium 13 Al
Aluminium spectrum visible.png
silicon 14 Si
Silicon spectrum visible.png
phosphorus 15 P
Phosphorus spectrum visible.png
sulfur 16 S
Sulfur spectrum visible.png
chlorine 17 Cl
Chlorine spectrum visible.png
argon 18 Ar
Argon spectrum visible.png
potassium 19 K
Potassium spectrum visible.png
calcium 20 Ca
Calcium spectrum visible.png
scandium 21 Sc
Scandium spectrum visible.png
titanium 22 Ti
Titanium spectrum visible.png
vanadium 23 V
Vanadium spectrum visible.png
chromium 24 Cr
Chromium spectrum visible.png
manganese 25 Mn
Manganese spectrum visible.png
iron 26 Fe
Iron spectrum visible.png
cobalt 27 Co
Cobalt spectrum visible.png
nickel 28 Ni
Nickel spectrum visible.png
copper 29 Cu
Copper spectrum visible.png
zinc 30 Zn
Zinc spectrum visible.png
gallium 31 Ga
Gallium spectrum visible.png
germanium 32 Ge
Germanium spectrum visible.png
arsenic 33 As
Arsenic spectrum visible.png
selenium 34 Se
Selenium spectrum visible.png
bromine 35 Br
Bromine spectrum visible.png
krypton 36 Kr
Krypton spectrum visible.png
rubidium 37 Rb
Rubidium spectrum visible.png
strontium 38 Sr
Strontium spectrum visible.png
yttrium 39 Y
Yttrium spectrum visible.png
zirconium 40 Zr
Zirconium spectrum visible.png
niobium 41 Nb
Niobium spectrum visible.png
molybdenum 42 Mo
Molybdenum spectrum visible.png
technetium 43 Tc
Technetium spectrum visible.png
ruthenium 44 Ru
Ruthenium spectrum visible.png
rhodium 45 Rh
Rhodium spectrum visible.png
palladium 46 Pd
Palladium spectrum visible.png
silver 47 Ag
Silver spectrum visible.png
cadmium 48 Cd
Cadmium spectrum visible.png
indium 49 In
Indium spectrum visible.png
tin 50 Sn
Tin spectrum visible.png
antimony 51 Sb
Antimony spectrum visible.png
tellurium 52 Te
Tellurium spectrum visible.png
iodine 53 I
Iodine spectrum visible.png
xenon 54 Xe
Xenon spectrum visible.png
caesium 55 Cs
Caesium spectrum visible.png
barium 56 Ba
Barium spectrum visible.png
lanthanum 57 La
Lanthanum spectrum visible.png
cerium 58 Ce
Cerium spectrum visible.png
praseodymium 59 Pr
Praseodymium spectrum visible.png
neodymium 60 Nd
Neodymium spectrum visible.png
promethium 61 Pm
Promethium spectrum visible.png
samarium 62 Sm
Samarium spectrum visible.png
europium 63 Eu
Europium spectrum visible.png
gadolinium 64 Gd
Gadolinium spectrum visible.png
terbium 65 Tb
Terbium spectrum visible.png
dysprosium 66 Dy
Dysprosium spectrum visible.png
holmium 67 Ho
Holmium spectrum visible.png
erbium 68 Er
Erbium spectrum visible.png
thulium 69 Tm
Thulium spectrum visible.png
ytterbium 70 Yb
Ytterbium spectrum visible.png
lutetium 71 Lu
Lutetium spectrum visible.png
hafnium 72 Hf
Hafnium spectrum visible.png
tantalum 73 Ta
Tantalum spectrum visible.png
tungsten 74 W
Tungsten spectrum visible.png
rhenium 75 Re
Rhenium spectrum visible.png
osmium 76 Os
Osmium spectrum visible.png
iridium 77 Ir
Iridium spectrum visible.png
platinum 78 Pt
Platinum spectrum visible.png
gold 79 Au
Gold spectrum visible.png
mercury 80 Hg
Mercury spectrum visible.png
thallium 81 Tl
Thallium spectrum visible.png
lead 82 Pb
Lead spectrum visible.png
bismuth 83 Bi
Bismuth spectrum visible.png
polonium 84 Po
Polonium spectrum visible.png
astatine 85 At
radon 86 Rn
Radon spectrum visible.png
francium 87 Fr
radium 88 Ra
Radium spectrum visible.png
actinium 89 Ac
Actinium spectrum visible.png
thorium 90 Th
Thorium spectrum visible.png
protactinium 91 Pa
Protactinium spectrum visible.png
uranium 92 U
Uranium spectrum visible.png
neptunium 93 Np
Neptunium spectrum visible.png
plutonium 94 Pu
Plutonium spectrum visible.png
americium 95 Am
Americium spectrum visible.png
curium 96 Cm
Curium spectrum visible.png
berkelium 97 Bk
Berkelium spectrum visible.png
californium 98 Cf
Californium spectrum visible.png
einsteinium 99 Es
Einsteinium spectrum visible.png
fermium–oganesson 101–118 Fm–Og

See also


  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,


  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. ^ Krainov, Vladimir; Reiss, Howard; Smirnov, Boris (1997). Radiative Processes in Atomic Physics. Wiley. doi:10.1002/3527605606. ISBN 978-0-471-12533-4.
  5. ^ 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.
  6. ^ "Collisional Broadening". Archived from the original on 2015-09-24. Retrieved 2015-09-24.
  7. ^ 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. Retrieved 2005-12-09.

Further reading