View source for Emission spectrum

{{Short description|Frequencies of light emitted by atoms or chemical compounds}}
{{Use American English|date = February 2019}}
[[File:MHL.png|thumb|right|300 px|Emission spectrum of a ceramic metal halide lamp.]]
[[File:cmglee_Cambridge_Science_Festival_2016_sodium_lines.jpg|thumb|300px|A demonstration of the 589 nm D<sub>2</sub> (left) and 590 nm D<sub>1</sub> (right) emission sodium D lines using a wick with salt water in a flame]]

The '''emission spectrum''' of a [[chemical element]] or [[chemical compound]] is the [[Spectrum (physical sciences)|spectrum]] of [[frequencies]] of [[electromagnetic radiation]] emitted due to [[electron]]s making a [[atomic electron transition|transition]] from a high energy state to a lower energy state. The [[photon energy]] of the emitted [[photon]]s 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.

==Emission==
In [[physics]], emission is the process by which a higher energy quantum mechanical state of a particle becomes converted to a lower one through the emission of a [[photon]], resulting in the production of [[light]]. The frequency of light emitted is a function of the energy of the transition.

Since energy must be conserved, the energy difference between the two states equals the energy carried off by the photon. The energy states of the transitions can lead to emissions over a very large range of frequencies. For example, [[visible light]] is emitted by the coupling of electronic states in atoms and molecules (then the phenomenon is called [[fluorescence]] or [[phosphorescence]]). On the other hand, nuclear shell transitions can emit high energy [[gamma rays]], while nuclear spin transitions emit low energy [[radio waves]].

The [[Beam emittance|emittance]] of an object quantifies how much light is emitted by it. This may be related to other properties of the object through the [[Stefan–Boltzmann law]].
For most substances, the amount of emission varies with the [[temperature]] and the [[spectroscopy|spectroscopic composition]] of the object, leading to the appearance of [[color temperature]] and [[emission lines]]. Precise measurements at many wavelengths allow the identification of a substance via [[emission spectroscopy]].

Emission of radiation is typically described using semi-classical quantum mechanics: the particle's energy levels and spacings are determined from [[quantum mechanics]], and light is treated as an oscillating electric field that can drive a transition if it is in resonance with the system's natural frequency. The quantum mechanics problem is treated using time-dependent [[perturbation theory]] and leads to the general result known as [[Fermi's golden rule]]. The description has been superseded by [[quantum electrodynamics]], although the semi-classical version continues to be more useful in most practical computations.

==Origins==
When the [[electrons]] in the atom are excited, for example by being heated, the additional [[energy]] pushes the electrons to higher energy orbitals. When the electrons fall back down and leave the excited state, energy is re-emitted in the form of a [[photon]]. The wavelength (or equivalently, frequency) of the photon is determined by the difference in energy between the two states. These emitted photons form the element's spectrum.

The fact that only certain colors appear in an element's atomic emission spectrum means that only certain frequencies of light are emitted. Each of these frequencies are related to energy by the formula:
<math display="block">E_{\text{photon}} = h\nu,</math>
where <math>E_{\text{photon}}</math> is the energy of the photon, <math>\nu</math> is its [[frequency]], and <math>h</math> is [[Planck's constant]].
This concludes that only [[photon]]s with specific energies are emitted by the atom. The principle of the atomic emission spectrum explains the varied colors in [[neon sign]]s, as well as chemical [[flame test]] results (described below).

The frequencies of light that an atom can emit are dependent on states the electrons can be in. When excited, an electron moves to a higher energy level or orbital. When the electron falls back to its ground level the light is emitted.

[[File:Emission spectrum-H.svg|none|757px|thumb|Emission spectrum of [[hydrogen]]]]
The above picture shows the visible light [[Hydrogen spectral series|emission spectrum for hydrogen]]. If only a single atom of hydrogen were present, then only a single wavelength would be observed at a given instant. Several of the possible emissions are observed because the sample contains many hydrogen atoms that are in different initial energy states and reach different final energy states. These different combinations lead to simultaneous emissions at different wavelengths.

[[File:Emission spectrum-Fe.svg|none|757px|thumb|Emission spectrum of [[iron]]]]

===Radiation from molecules===
As well as the electronic transitions discussed above, the energy of a molecule can also change via [[Rotational transition|rotational]], [[Vibrational transition|vibrational]], and [[Vibronic transition|vibronic]] (combined vibrational and electronic) transitions. These energy transitions often lead to closely spaced groups of many different [[spectral lines]], known as [[spectral bands]]. Unresolved band spectra may appear as a spectral continuum.

==Emission spectroscopy==
Light consists of electromagnetic radiation of different wavelengths. Therefore, when the elements or their compounds are heated either on a flame or by an electric arc they emit energy in the form of light. Analysis of this light, with the help of a [[spectroscope]] gives us a discontinuous spectrum. A spectroscope or a spectrometer is an instrument which is used for separating the components of light, which have different wavelengths. The spectrum appears in a series of lines called the line spectrum. This line spectrum is called an atomic spectrum when it originates from an atom in elemental form. Each element has a different atomic spectrum. The production of line spectra by the atoms of an element indicate that an atom can radiate only a certain amount of energy. This leads to the conclusion that bound electrons cannot have just any amount of energy but only a certain amount of energy.

The emission spectrum can be used to determine the composition of a material, since it is different for each [[Chemical element|element]] of the [[periodic table]]. One example is [[astronomical spectroscopy]]: identifying the composition of [[star]]s by analysing the received light.
The emission spectrum characteristics of some elements are plainly visible to the naked eye when these elements are heated. For example, when platinum wire is dipped into a sodium nitrate solution and then inserted into a flame, the sodium atoms emit an amber yellow color. Similarly, when indium is inserted into a flame, the flame becomes blue. These definite characteristics allow elements to be identified by their atomic emission spectrum. Not all emitted lights are perceptible to the naked eye, as the spectrum also includes ultraviolet rays and infrared radiation.
An emission spectrum is formed when an excited gas is viewed directly through a spectroscope.

[[Image:AtomicLineSpEm.svg|thumb|Schematic diagram of [[spontaneous emission]]]]
'''Emission spectroscopy''' is a [[spectroscopy|spectroscopic]] technique which examines the wavelengths of [[photon]]s emitted by atoms or molecules during their transition from an [[excited state]] to a lower energy state. Each element emits a characteristic set of discrete wavelengths according to its [[electronic structure]], and by observing these wavelengths the elemental composition of the sample can be determined. Emission spectroscopy developed in the late 19th century and efforts in theoretical explanation of atomic emission spectra eventually led to [[quantum mechanics]].

There are many ways in which atoms can be brought to an excited state. Interaction with electromagnetic radiation is used in [[fluorescence spectroscopy]], protons or other heavier particles in [[Particle-Induced X-ray Emission]] and electrons or X-ray photons in [[Energy-dispersive X-ray spectroscopy]] or [[X-ray fluorescence]]. The simplest method is to heat the sample to a high temperature, after which the excitations are produced by collisions between the sample atoms. This method is used in [[flame emission spectroscopy]], and it was also the method used by [[Anders Jonas Ångström]] when he discovered the phenomenon of discrete emission lines in the 1850s.<ref>{{Cite web|url=http://www.synlube.com/Spectroscopy.htm|title=Spectroscopy Oil Analysis|last=Incorporated|first=SynLube|website=www.synlube.com|language=en|access-date=2017-02-24}}</ref>

Although the emission lines are caused by a transition between quantized energy states and may at first look very sharp, they do have a finite width, i.e. they are composed of more than one wavelength of light. This [[spectral line broadening]] has many different causes.

Emission spectroscopy is often referred to as '''optical emission spectroscopy''' because of the light nature of what is being emitted. {{Further|Atomic emission spectroscopy}}

==History==
{{Further|History of spectroscopy}}

In 1756 Thomas Melvill  observed the emission of distinct patterns of colour when [[salt (chemistry)|salts]] were added to [[Alcohol (chemistry)|alcohol]] flames.<ref>{{cite journal |last1=Melvill |first1=Thomas |title=Observations on light and colours |journal=Essays and Observations, Physical and Literary. Read Before a Society in Edinburgh |date=1756 |volume=2 |pages=12–90 |url=https://babel.hathitrust.org/cgi/pt?id=hvd.hxp3ik;view=1up;seq=30}} ; see pp. 33–36.</ref>  By 1785 [[James Gregory (astronomer and mathematician)|James Gregory]] discovered the principles of diffraction grating and  American astronomer [[David Rittenhouse]] made the first engineered [[diffraction grating]].<ref>See: 
* Frauhofer. Jos. (1821) [https://books.google.com/books?id=k-EAAAAAYAAJ&pg=RA1-PA3 "Neue Modifikation des Lichtes durch gegenseitige Einwirkung und Beugung der Strahlen, und Gesetze derselben"] (New modification of light by the mutual influence and the diffraction of [light] rays, and the laws thereof), ''Denkschriften der Königlichen Akademie der Wissenschaften zu München'' (Memoirs of the Royal Academy of Science in Munich), '''8''': 3–76.
* Fraunhofer, Jos. (1823) [http://gallica.bnf.fr/ark:/12148/bpt6k15083w/f353.image "Kurzer Bericht von den Resultaten neuerer Versuche über die Gesetze des Lichtes, und die Theorie derselben"] (Short account of the results of new experiments on the laws of light, and the theory thereof) ''Annalen der Physik'', '''74'''(8): 337–378.</ref><ref>{{cite journal |author=Parker AR |title=A geological history of reflecting optics |journal=Journal of the Royal Society, Interface |volume=2 |issue=2 |pages=1–17 |date=March 2005 |pmid=16849159 |pmc=1578258 |doi=10.1098/rsif.2004.0026}}</ref> In 1821 [[Joseph von Fraunhofer]] solidified this significant experimental leap of replacing a prism as the source of wavelength [[dispersion (optics)|dispersion]] improving the [[spectral resolution]] and allowing for the dispersed wavelengths to be quantified.<ref name="OpenStaxAstronomy-5.3">OpenStax Astronomy, "Spectroscopy in Astronomy". OpenStax CNX. Sep 29, 2016 http://cnx.org/contents/1f92a120-370a-4547-b14e-a3df3ce6f083@3 {{Open access}}</ref>

In 1835, [[Charles Wheatstone]] reported that different metals could be distinguished by bright lines in the emission spectra of their [[Atomic emission spectroscopy#Spark and arc atomic emission spectroscopy|sparks]], thereby introducing an alternative to flame spectroscopy.<ref name="Bowers">{{cite book
 | title = Sir Charles Wheatstone FRS: 1802-1875
 | edition = 2nd
 | author = Brian Bowers
 | publisher = IET
 | date = 2001
 | isbn = 978-0-85296-103-2
 | pages = 207–208
 | url = https://books.google.com/books?id=m65tKWiI-MkC&q=Wheatstone+spectrum+analysis+metals&pg=PA208
 }}</ref><ref>{{cite book |last1=Wheatstone |title=Report of the Fifth Meeting of the British Association for the Advancement of Science; Held at Dublin in 1835. Notices and Abstracts of Communications to the British Association for the Advancement of Science, at the Dublin Meeting, August 1835. |date=1836 |publisher=John Murray |location=London, England |pages=11–12 |chapter-url=https://books.google.com/books?id=fLtZAAAAcAAJ&pg=RA1-PA11 |chapter=On the prismatic decomposition of electrical light}}</ref>
In 1849, [[Léon Foucault|J. B. L. Foucault]] experimentally demonstrated that [[absorption (electromagnetic radiation)|absorption]] and emission lines at the same wavelength are both due to the same material, with the difference between the two originating from the temperature of the light source.<ref name="Brand 60">Brand, pp. 60-62</ref><ref>See:
* {{cite journal |last1=Foucault |first1=L. |title=Lumière électrique |journal=Société Philomatique de Paris. Extraits des Procès-Verbaux de Séances. |date=1849 |volume=13 |pages=16–20 |url=https://www.biodiversitylibrary.org/item/98244#page/106/mode/1up |trans-title=Electric light |language=French}}
* {{cite journal |last1=Foucault |first1=L. |title=Lumière électrique |journal=L'Institut, Journal Universel des Sciences |date=7 February 1849 |volume=17 |issue=788 |pages=44–46 |url=https://books.google.com/books?id=HggFrG2xd50C&pg=RA1-PA44 |trans-title=Electric light |language=French}}</ref>
In 1853, the [[Sweden|Swedish]] physicist [[Anders Jonas Ångström]] presented observations and theories about gas spectra.<ref>See:
* {{cite journal |last1=Ångström |first1=A.J. |title=Optiska undersökningar |journal=Kongliga Vetenskaps-Akademiens Handlingar [Proceedings of the Royal Academy of Science] |date=1852 |volume=40 |pages=333–360 |url=https://babel.hathitrust.org/cgi/pt?id=njp.32101078256672;view=1up;seq=339 |trans-title=Optical investigations |language=Swedish}} 
* {{cite journal |last1=Ångström |first1=A.J. |title=Optische Untersuchungen |journal=Annalen der Physik und Chemie |date=1855a |volume=94 |pages=141–165 |url=https://babel.hathitrust.org/cgi/pt?id=uc1.b4433632;view=1up;seq=161 |trans-title=Optical investigations |language=German}}
* {{cite journal |last1=Ångström |first1=A.J. |title=Optical researches |journal=Philosophical Magazine |date=1855b |volume=9 |pages=327–342 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015024087606;view=1up;seq=353 |series=4th series |doi=10.1080/14786445508641880}}</ref> Ångström postulated that an incandescent gas emits luminous rays of the same wavelength as those it can absorb. At the same time [[Sir George Stokes, 1st Baronet|George Stokes]] and [[William Thomson, 1st Baron Kelvin|William Thomson (Kelvin)]] were discussing similar postulates.<ref name="Brand 60"/> Ångström also measured the emission spectrum from hydrogen later labeled the [[Balmer lines]].<ref name="Wagner letter">{{cite journal
| last1 = Wagner
| first1 = H. J.
| date = 2005
| title = Early Spectroscopy and the Balmer Lines of Hydrogen
| journal = Journal of Chemical Education
| volume = 82
| issue = 3
| pages = 380
| doi = 10.1021/ed082p380.1
|bibcode = 2005JChEd..82..380W | doi-access = free
}}</ref><ref>(Ångström, 1852), p. 352; (Ångström, 1855b), p. 337.</ref>
In 1854 and 1855, [[David Alter]] published observations on the spectra of metals and gases, including an independent observation of the [[Balmer lines]] of hydrogen.<ref name="Retcofsky letter">{{cite journal
| last1 = Retcofsky
| first1 = H. L.
| date = 2003
| title = Spectrum Analysis Discoverer?
| journal = Journal of Chemical Education
| volume = 80
| issue = 9
| pages = 1003
| doi = 10.1021/ed080p1003.1
|bibcode = 2003JChEd..80.1003R | doi-access = free
}}</ref><ref>See:
* {{cite journal |last1=Alter |first1=David |title=On certain physical properties of light, produced by the combustion of different metals, in the electric spark, refracted by a prism |journal=The American Journal of Science and Arts |date=1854 |volume=18 |pages=55–57 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015036921982;view=1up;seq=69 |series=2nd series}}
* {{cite journal |last1=Alter |first1=D. |title=On certain physical properties of the light of the electric spark, within certain gases, as seen through a prism |journal=The American Journal of Science and Arts |date=1855 |volume=19 |pages=213–214 |url=https://babel.hathitrust.org/cgi/pt?id=hvd.32044102903614;view=1up;seq=227 |series=2nd series}} Alter's observations of hydrogen's optical spectrum appear on p. 213.</ref>

By 1859, [[Gustav Kirchhoff]] and [[Robert Bunsen]] noticed that several [[Fraunhofer lines]] (lines in the solar spectrum) coincide with characteristic emission lines identified in the spectra of heated elements.<ref>See:

* Gustav Kirchhoff (1859) [https://books.google.com/books?id=CMgAAAAAYAAJ&pg=PA662 "Ueber die Fraunhofer'schen Linien"] (On Fraunhofer's lines), ''Monatsbericht der Königlichen Preussische Akademie der Wissenschaften zu Berlin'' (Monthly report of the Royal Prussian Academy of Sciences in Berlin), 662–665.
* Gustav Kirchhoff (1859) [https://books.google.com/books?id=uksDAAAAYAAJ&pg=RA1-PA251 "Ueber das Sonnenspektrum"] (On the sun's spectrum), ''Verhandlungen des naturhistorisch-medizinischen Vereins zu Heidelberg'' (Proceedings of the Natural History / Medical Association in Heidelberg), '''1''' (7) :  251–255.</ref><ref>{{cite journal|author=G. Kirchhoff|date=1860|title=Ueber die Fraunhofer'schen Linien|url=https://zenodo.org/record/1423666|journal=Annalen der Physik|volume=185|issue=1|pages=148–150|bibcode=1860AnP...185..148K|doi=10.1002/andp.18601850115}}</ref> It was correctly deduced that dark lines in the solar spectrum are caused by absorption by chemical elements in the [[solar atmosphere]].<ref>{{cite journal|author=G. Kirchhoff|date=1860|title=Ueber das Verhältniss zwischen dem Emissionsvermögen und dem Absorptionsvermögen der Körper für Wärme und Licht|url=https://zenodo.org/record/1423668|journal=Annalen der Physik|volume=185|issue=2|pages=275–301|bibcode=1860AnP...185..275K|doi=10.1002/andp.18601850205|doi-access=free}}</ref>

==Experimental technique in flame emission spectroscopy==
The solution containing the relevant substance to be analysed is drawn into the burner and dispersed into the flame as a fine spray. The solvent evaporates first, leaving finely divided [[solid]] particles which move to the hottest region of the flame where gaseous [[atoms]] and [[ions]] are produced through the dissociation of molecules. Here [[electrons]] are excited as described above, and the spontaneously emit photon to decay to lower energy states. It is common for a [[monochromator]] to be used to allow for easy detection.

On a simple level, flame emission spectroscopy can be observed using just a [[flame]] and samples of metal salts. This method of qualitative analysis is called a [[flame test]]. For example, [[sodium]] salts placed in the flame will glow yellow from sodium ions, while [[strontium]] (used in road flares) ions color it red. [[Copper]] wire will create a blue colored flame, however in the presence of [[chloride]] gives green (molecular contribution by CuCl). <!-- Our company used this in hazardous waste testing contracts for preliminary classification of barrel contents at McChord AFB and Tacoma Superfund site. There is an EPA reference somewhere in their public documentation. -->

==Emission coefficient==
'''Emission coefficient''' is a coefficient in the power output per unit time of an [[Electromagnetic radiation|electromagnetic]] source, a calculated value in [[physics]]. The emission coefficient of a gas varies with the [[wavelength]] of the light. It has units of ms<sup>−3</sup>sr<sup>−1</sup>.<ref>{{cite book|last=Carroll|first=Bradley W.|title=An Introduction to Modern Astrophysics|year=2007|publisher=[[Pearson Education]]|location=CA, USA|isbn=978-0-8053-0402-2|pages=256}}</ref> It is also used as a measure of [[natural environment|environmental]] emissions (by mass) per MWh of [[electricity generation|electricity generated]], see: [[Emission factor]].

===Scattering of light===
In [[Thomson scattering]] a charged particle emits radiation under incident light. The particle may be an ordinary atomic electron, so emission coefficients have practical applications.

If {{math|1='''''X''''' ''dV'' ''d''Ω ''dλ''}} is the energy scattered by a volume element {{math|''dV''}} into solid angle {{math|''d''Ω}} between wavelengths {{mvar|λ}} and {{math|''λ'' + ''dλ''}} per unit time then the Emission [[coefficient]] is {{math|1='''''X'''''}}.

The values of {{math|1='''''X'''''}} in Thomson scattering can be [[Thomson scattering|predicted]] from incident flux, the density of the charged particles and their Thomson differential cross section (area/solid angle).

===Spontaneous emission===
A warm body emitting [[photon]]s has a [[monochromatic]] emission coefficient relating to its temperature and total power radiation. This is sometimes called the second [[Einstein coefficient]], and can be deduced from [[quantum mechanics|quantum mechanical theory]].

==See also==
*[[Absorption spectroscopy]]
*[[Absorption spectrum]]
*[[Atomic spectral line]]
*[[Electromagnetic spectroscopy]]
*[[Gas-discharge lamp]], Table of emission spectra of gas discharge lamps
*[[Isomeric shift]]
*[[Isotopic shift]]
*[[Luminous coefficient]]
*[[Plasma (physics)|Plasma]] physics
*[[Rydberg formula]]
*[[Spectral theory]]
*The [[Diode]] equation includes the emission coefficient (which is not related to the one discussed here)
*[[Thermionic emission]]

==References==
{{reflist}}

==External links==
*[http://www.spectralcalc.com/ Emission spectra of atmospheric gases]
*[http://physics.nist.gov/PhysRefData/contents-atomic.html NIST Physical Reference Data—Atomic Spectroscopy Databases]
*[https://web.archive.org/web/20061110112125/http://student.fizika.org/~nnctc/spectra.htm Color Simulation of Element Emission Spectrum Based on NIST data]
*[https://web.archive.org/web/20081120193438/http://www.doctortang.com/AP%20Chemistry/Lab%2004%20Quantitative%20Spectroscopy%20of%20Hydrogen%20Emission%20Spectrum.pdf Hydrogen emission spectrum]

{{Branches of spectroscopy}}
{{Authority control}}

[[Category:Atomic physics]]
[[Category:Emission spectroscopy]]
[[Category:Electromagnetic radiation]]
[[Category:Physical quantities]]
[[Category:Analytical chemistry]]
[[Category:Scattering]]
[[Category:Nuclear physics]]