Editing Spectral line

{{Short description|Distinctive narrow spectral feature of chemical species}}
{{multiple image
| direction         = vertical
| width             = 320
| image1            = Spectral-lines-continuous.svg
| alt1              = Continuous spectrum
| caption1          = Continuous spectrum
| image2            = Spectral-lines-emission.svg
| alt2              = Emission lines
| caption2          = [[Emission spectrum|Emission lines]] ([[discrete spectrum]])
| image3            = Spectral-lines-absorption.svg
| alt3              = Absorption lines
| caption3          = Absorption spectrum with [[absorption lines]]
}}

A '''spectral line''' is a weaker or stronger region in an otherwise uniform and [[continuous spectrum]]. It may result from [[emission (electromagnetic radiation)|emission]] or [[absorption (electromagnetic radiation)|absorption]] of [[light]] in a narrow [[frequency]] range, compared with the nearby frequencies. Spectral lines are often used to identify [[atom]]s and [[molecule]]s. These "fingerprints" can be compared to the previously collected ones of atoms<ref>{{Citation| publisher = National Institute of Standards and Technology| last1 = Kramida| first1 = Alexander| last2 = Ralchenko| first2 = Yuri| title = NIST Atomic Spectra Database, NIST Standard Reference Database 78| journal = <!-- Deny Citation Bot-->| access-date = 2021-06-27| date = 1999| url = http://www.nist.gov/pml/data/asd.cfm}}</ref> and molecules,<ref name="RothmanGordon2013">{{cite journal |last1=Rothman|first1=L.S.|last2=Gordon|first2=I.E.|last3=Babikov|first3=Y.|last4=Barbe|first4=A.|last5=Chris Benner|first5=D.|last6=Bernath|first6=P.F.|last7=Birk|first7=M.|last8=Bizzocchi|first8=L.|last9=Boudon|first9=V.|last10=Brown|first10=L.R.|last11=Campargue|first11=A.|last12=Chance|first12=K.|last13=Cohen|first13=E.A.|last14=Coudert|first14=L.H.|last15=Devi|first15=V.M.|last16=Drouin|first16=B.J.|last17=Fayt|first17=A.|last18=Flaud|first18=J.-M.|last19=Gamache|first19=R.R.|last20=Harrison|first20=J.J.|last21=Hartmann|first21=J.-M.|last22=Hill|first22=C.|last23=Hodges|first23=J.T.|last24=Jacquemart|first24=D.|last25=Jolly|first25=A.|last26=Lamouroux|first26=J.|last27=Le Roy|first27=R.J.|last28=Li|first28=G.|last29=Long|first29=D.A. |last30=Lyulin|first30=O.M.|last31=Mackie|first31=C.J.|last32=Massie|first32=S.T.|last33=Mikhailenko|first33=S.|last34=Müller|first34=H.S.P.|last35=Naumenko|first35=O.V.|last36=Nikitin|first36=A.V.|last37=Orphal|first37=J.|last38=Perevalov|first38=V.|last39=Perrin|first39=A.|last40=Polovtseva|first40=E.R.|last41=Richard|first41=C.|last42=Smith|first42=M.A.H.|last43=Starikova|first43=E.|last44=Sung|first44=K.|last45=Tashkun|first45=S.|last46=Tennyson|first46=J.|last47=Toon|first47=G.C.|last48=Tyuterev|first48=Vl.G.|last49=Wagner|first49=G. |title=The HITRAN2012 molecular spectroscopic database |journal=Journal of Quantitative Spectroscopy and Radiative Transfer |volume=130 |year=2013 |pages=4–50 |issn=0022-4073 |doi=10.1016/j.jqsrt.2013.07.002 |display-authors=29 |bibcode=2013JQSRT.130....4R|url=https://scholarworks.wm.edu/aspubs/114}}</ref> and are thus used to identify the atomic and molecular components of [[star]]s and [[planet]]s, which would otherwise be impossible.

== Types of line spectra ==
[[Image:Simple spectroscope.jpg|thumb|right|Continuous spectrum of an [[Incandescent light bulb|incandescent lamp]] (mid) and discrete spectrum lines of a [[Compact fluorescent lamp|fluorescent lamp]] (bottom)]]

Spectral lines are the result of interaction between a [[Quantum mechanics|quantum system]] (usually [[atom]]s, but sometimes [[molecule]]s or [[atomic nucleus|atomic nuclei]]) and a single [[photon]]. When a photon has about the right amount of [[photon energy|energy]] (which is connected to its frequency)<ref>[[Einstein, Albert]] (1905). "[[Annus Mirabilis papers#Photoelectric effect|On a Heuristic Viewpoint Concerning the Production and Transformation of Light]]".</ref> to allow a change in the energy state of the system (in the case of an atom this is usually an [[electron]] changing [[Electron configuration|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 [[number density|density]] of the material, so they are widely used to determine the physical conditions of [[star]]s 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 wave]]s to [[gamma ray]]s.

==Nomenclature{{anchor|nomencature_anchor}}==
Strong spectral lines in the [[visible spectrum|visible part of the electromagnetic spectrum]] often have a unique [[Fraunhofer line]] designation, such as '''K''' for a line at 393.366&nbsp;nm emerging from singly-ionized calcium atom, '''Ca<sup>+</sup>''', though some of the Fraunhofer "lines" are blends of multiple lines from several different [[Chemical species|species]].

In other cases, the lines are designated according to the level of [[ion]]ization 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:

'''Cu II''' {{Em dash}} copper ion with +1 charge, '''Cu<sup>1+</sup>'''

'''Fe III''' {{Em dash}} iron ion with +2 charge, '''Fe<sup>2+</sup>'''

More detailed designations usually include the line [[wavelength]] and may include a [[multiplet]] number (for atomic lines) or [[Molecular spectra or band spectra|band designation]] (for molecular lines). Many spectral lines of atomic [[hydrogen]] also have designations within their respective [[Hydrogen spectral series|series]], such as the [[Lyman series]] or [[Balmer series]]. Originally all spectral lines were classified into series: the ''[[Principal series (spectroscopy)|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 combination principle|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 tiny [[spectral band]] with a nonzero range of frequencies, not a single frequency (i.e., a nonzero [[spectral width]]). 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 emission|spontaneous radiative decay]] or the [[Auger effect|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.<ref>{{Cite book| publisher = Wiley| isbn = 978-0-471-12533-4| last1 = Krainov| first1 = Vladimir| last2 = Reiss| first2 = Howard| last3 = Smirnov| first3 = Boris| title = Radiative Processes in Atomic Physics| date = 1997| doi = 10.1002/3527605606}}</ref>
A short lifetime will have a large energy uncertainty and a broad emission. This broadening effect results in an unshifted [[Lorentzian function|Lorentzian profile]]. The natural broadening can be experimentally altered only to the extent that decay rates can be artificially suppressed or enhanced.<ref>For example, in the following article, decay was suppressed via a microwave cavity, thus reducing the natural broadening: {{cite journal|last=Gabrielse|first=Gerald|author2=H. Dehmelt |title=Observation of Inhibited Spontaneous Emission|journal=Physical Review Letters|volume=55|pages=67–70|year=1985|doi=10.1103/PhysRevLett.55.67|pmid=10031682|issue=1|bibcode=1985PhRvL..55...67G}}</ref>

==== Thermal Doppler broadening ====
{{Main|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 function|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).<ref>{{cite web|url=http://www.fas.harvard.edu/~scdiroff/lds/QuantumRelativity/CollisionalBroadening/CollisionalBroadening.html |title=Collisional Broadening |publisher=Fas.harvard.edu |archive-url=https://web.archive.org/web/20150924042746/http://www.fas.harvard.edu/~scdiroff/lds/QuantumRelativity/CollisionalBroadening/CollisionalBroadening.html |access-date=2015-09-24|archive-date=2015-09-24 }}</ref><!--<ref>{{cite web|url=http://www.fas.harvard.edu/~scdiroff/lds/QuantumRelativity/CollisionalBroadening/CollisionalBroadening.html |title=Collisional Broadening |publisher=Fas.harvard.edu |access-date=2015-05-02}}</ref>--> 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 function|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 (see [[spectral band]]), 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]].<ref>{{cite journal | first = G. | last = Peach | year = 1981 | title = Theory of the pressure broadening and shift of spectral lines | journal = Advances in Physics | volume = 30 | issue = 3 | pages = 367–474 | url = http://journalsonline.tandf.co.uk/openurl.asp?genre=article&eissn=1460-6976&volume=30&issue=3&spage=367 | archive-url = https://archive.today/20130114060003/http://journalsonline.tandf.co.uk/openurl.asp?genre=article&eissn=1460-6976&volume=30&issue=3&spage=367 | url-status = dead | archive-date = 2013-01-14 | doi = 10.1080/00018738100101467 | bibcode = 1981AdPhy..30..367P | access-date = 2005-12-09 | url-access = subscription }}</ref>

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 <math>r</math>, causing a shift in energy that is linear in the field strength. <math>(\Delta E \sim 1/r^2)</math>
* ''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. <math>(\Delta E \sim 1/r^3)</math>
* ''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. <math>(\Delta E \sim 1/r^4)</math>
* ''Van der Waals broadening'' occurs when the emitting particle is being perturbed by [[Van der Waals force]]s. For the quasistatic case, a [[Lévy distribution|Van der Waals profile]]<ref group="note">"Van der Waals profile" appears as lowercase in almost all sources, such as: [https://books.google.com/books?id=Wve2AAAAIAAJ&q=%22Van+der+Waals+profile%22&dq=%22Van+der+Waals+profile%22&hl=en 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 [https://books.google.com/books?id=2XpVAAAAMAAJ&q=%22Van+der+Waals+profile%22&dq=%22Van+der+Waals+profile%22&hl=en Journal of technical physics], Volume 36, by Instytut Podstawowych Problemów Techniki (Polska Akademia Nauk), publisher: Państwowe Wydawn. Naukowe., 1995,<!-- and many more --></ref> is often useful in describing the profile. The energy shift as a function of distance between the interacting particles is given in the wings by e.g. the [[Lennard-Jones potential]]. <math>(\Delta E \sim 1/r^6)</math>

==== 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 (physics)|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}}
[[File:Spectrum of blue sky.svg|thumb|right|upright=1.4|Absorption lines for air, under indirect illumination, 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, looking east with the sun to the west at around 3–4 pm on a clear day.]]

===Bands===
The phrase "spectral lines", when not qualified, usually refers to lines having wavelengths in the [[visible spectrum|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-ray]]s, the lines are known as [[characteristic X-ray]]s 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 [[Hydrogen spectral series#Paschen series (Bohr series, n′ = 3)|Paschen series]] of hydrogen. At even longer wavelengths, the [[radio spectrum]] includes the [[Hydrogen line|21-cm line]] used to detect neutral hydrogen throughout the [[Hydrogen line#In cosmology|cosmos]].

== See also ==
* [[Absorption spectrum]]
* [[Atomic spectral line]]
* [[Bohr model]]
* [[Electron configuration]]
* [[Emission spectrum]]
* [[Fourier transform]]
* [[Fraunhofer line]]
* [[Gas-discharge lamp#Color|Table of emission spectra of gas discharge lamps]]
* [[Hydrogen line]] (21-cm line)
* [[Hydrogen spectral series]]
* [[Spectral band]]
* [[Spectroscopy]]
* [[Splatalogue]]

== Notes ==
{{Reflist|group="note"}}

== References ==
{{Reflist}}

== Further reading ==
* {{cite book
 | first = Hans R. | last = Griem | year = 1997
 | title = Principles of Plasma Spectroscopy
 | publisher = University Press | location = Cambridge | isbn = 0-521-45504-9 }}
* {{cite book
 | first = Hans R. | last = Griem | year = 1974
 | title = Spectral Line Broadening by Plasmas
 | publisher = [[Academic Press]] | location = New York | isbn = 0-12-302850-7 }}
* {{cite book
 | first = Hans R. | last = Griem | year = 1964
 | title = Plasma Spectroscopy
 | publisher = McGraw-Hill book Company | location = New York|url=https://books.google.com/books?id=7gcvAAAAIAAJ }}
{{Authority control}}

[[Category:Spectroscopy]]
[[Category:Spectrum (physical sciences)]]