Editing Absorption spectroscopy

{{Short description|Spectroscopic techniques that measure the absorption of radiation}}
[[File:Spectroscopy overview.svg|thumb|400px|An overview of [[electromagnetic radiation]] absorption. This example discusses the general principle using [[visible spectrum|visible light]]. A white beam [[light source|source]] – emitting light of multiple [[wavelength]]s – is focused on a sample (the [[complementary color]] pairs are indicated by the yellow dotted lines). Upon striking the sample, [[photon]]s that match the energy gap of the [[molecule]]s present (green light in this example) are ''absorbed'' in order to excite the molecule. Other photons transmit unaffected and, if the radiation is in the visible region (400–700 nm), the sample color is the complementary color of the absorbed light. By comparing the [[attenuation]] of the transmitted light with the incident, an absorption spectrum can be obtained.|alt=]]
[[File:Sodium in atmosphere of exoplanet HD 209458.jpg|thumb|400px|The first direct detection and chemical analysis of the [[atmosphere]] of an [[exoplanet]], in 2001. [[Sodium]] in the atmosphere filters the [[starlight]] of [[HD 209458]] as the [[giant planet]] passes in front of the star.]]

'''Absorption spectroscopy''' is [[spectroscopy]] that involves techniques that measure the [[absorption (electromagnetic radiation)|absorption]] of [[electromagnetic 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 spectrum]]. Absorption spectroscopy is performed across the [[electromagnetic spectrum]].

Absorption spectroscopy is employed as an [[analytical chemistry]] tool to determine the presence of a particular substance in a sample and, in many cases, to quantify the amount of the substance present. [[Infrared spectroscopy|Infrared]] and [[ultraviolet–visible spectroscopy]] are particularly common in analytical applications. Absorption spectroscopy is also employed in studies of molecular and atomic physics, astronomical spectroscopy and remote sensing.

There is a wide range of experimental approaches for measuring absorption spectra. The most common arrangement is to direct a generated beam of radiation at a sample and detect the intensity of the radiation that passes through it. The transmitted energy can be used to calculate the absorption. The source, sample arrangement and detection technique vary significantly depending on the frequency range and the purpose of the experiment.

Following are the major types of absorption spectroscopy:<ref>{{cite book |title=Fundamentals and Techniques of Biophysics and Molecular biology |last=Kumar |first=Pranav |publisher=Pathfinder publication |year=2018 |isbn=978-93-80473-15-4 |location=New Delhi |page=33}}</ref>
{| class="wikitable"
|+
!Sr. No
!Electromagnetic radiation
!Spectroscopic type
|-
|1
|X-ray
|[[X-ray absorption spectroscopy]]
|-
|2
|Ultraviolet–visible
|UV–vis absorption spectroscopy
|-
|3
|Infrared
|IR absorption spectroscopy
|-
|4
|Microwave
|Microwave absorption spectroscopy
|-
|5
|Radio wave
|[[Electron spin resonance spectroscopy]]
[[Nuclear magnetic resonance spectroscopy]]
|}

==Absorption spectrum==
[[File:Fraunhofer lines.svg|thumb|upright=2|Solar spectrum with [[Fraunhofer lines]] as it appears visually]]

A material's absorption spectrum is the fraction of incident radiation absorbed by the material over a range of frequencies of electromagnetic radiation. The absorption spectrum is primarily determined<ref>Modern Spectroscopy (Paperback) by J. Michael Hollas {{ISBN|978-0-470-84416-8}}</ref><ref>Symmetry and Spectroscopy: An Introduction to Vibrational and Electronic Spectroscopy (Paperback) by Daniel C. Harris, Michael D. Bertolucci {{ISBN|978-0-486-66144-5}}</ref><ref>Spectra of Atoms and Molecules by Peter F. Bernath {{ISBN|978-0-19-517759-6}}</ref> by the [[atom]]ic and [[molecule|molecular]] composition of the material. Radiation is more likely to be absorbed at frequencies that match the energy difference between two [[quantum state|quantum mechanical states]] of the molecules . The absorption that occurs due to a transition between two states is referred to as an [[absorption line]] and a spectrum is typically composed of many lines.

The frequencies at which absorption lines occur, as well as their relative intensities, primarily depend on the [[electronic structure|electronic]] and [[molecular structure]] of the sample. The frequencies will also depend on the interactions between molecules in the sample, the [[crystal|crystal structure]] in solids, and on several environmental factors (e.g., [[temperature]], [[pressure]], [[electric field]], [[magnetic field]]). The lines will also have a [[spectral linewidth|width]] and [[spectral line#Line broadening and shift|shape]] that are primarily determined by the [[spectral density]] or the [[density of states]] of the system.

===Theory===
Absorption lines are typically classified by the nature of the quantum mechanical change induced in the molecule or atom. [[Rotational spectroscopy|Rotational lines]], for instance, occur when the rotational state of a molecule is changed. Rotational lines are typically found in the microwave spectral region. [[Vibrational spectroscopy|Vibrational lines]] correspond to changes in the vibrational state of the molecule and are typically found in the infrared region. Electronic lines correspond to a change in the electronic state of an atom or molecule and are typically found in the visible and ultraviolet region. X-ray absorptions are associated with the excitation of [[electronic structure#Shells and subshells|inner shell]] electrons in atoms. These changes can also be combined (e.g. [[rotational–vibrational coupling|rotation–vibration transitions]]), leading to new absorption lines at the combined energy of the two changes.

The energy associated with the quantum mechanical change primarily determines the frequency of the absorption line but the frequency can be shifted by several types of interactions. Electric and magnetic fields can cause a shift. Interactions with neighboring molecules can cause shifts. For instance, absorption lines of the gas phase molecule can shift significantly when that molecule is in a liquid or solid phase and interacting more strongly with neighboring molecules.

The width and shape of absorption lines are determined by the instrument used for the observation, the material absorbing the radiation and the physical environment of that material. It is common for lines to have the shape of a [[Gaussian distribution|Gaussian]] or [[Lorentzian distribution|Lorentzian]] distribution. It is also common for a line to be described solely by its intensity and [[spectral linewidth|width]] instead of the entire shape being characterized.

The integrated intensity—obtained by [[integral|integrating]] the area under the absorption line—is proportional to the amount of the absorbing substance present. The intensity is also related to the temperature of the substance and the quantum mechanical interaction between the radiation and the absorber. This interaction is quantified by the [[transition moment]] and depends on the particular lower state the transition starts from, and the upper state it is connected to.

The width of absorption lines may be determined by the [[spectrometer]] used to record it. A spectrometer has an inherent limit on how narrow a line it can [[spectral resolution|resolve]] and so the observed width may be at this limit. If the width is larger than the resolution limit, then it is primarily determined by the environment of the absorber. A liquid or solid absorber, in which neighboring molecules strongly interact with one another, tends to have broader absorption lines than a gas. Increasing the temperature or pressure of the absorbing material will also tend to increase the line width. It is also common for several neighboring transitions to be close enough to one another that their lines overlap and the resulting overall line is therefore broader yet.

===Relation to transmission spectrum===
Absorption and transmission spectra represent equivalent information and one can be calculated from the other through a mathematical transformation. A transmission spectrum will have its maximum intensities at wavelengths where the absorption is weakest because more light is transmitted through the sample. An absorption spectrum will have its maximum intensities at wavelengths where the absorption is strongest.

===Relation to emission spectrum===
[[File:Emission spectrum-Fe.svg|thumb|400px|The emission spectrum of [[iron]]]]

[[Emission (electromagnetic radiation)|Emission]] is a process by which a substance releases energy in the form of electromagnetic radiation. Emission can occur at any frequency at which absorption can occur, and this allows the absorption lines to be determined from an emission spectrum. The [[emission spectrum]] will typically have a quite different intensity pattern from the absorption spectrum, though, so the two are not equivalent. The absorption spectrum can be calculated from the emission spectrum using [[Einstein coefficients]].

===Relation to scattering and reflection spectra===
The scattering and reflection spectra of a material are influenced by both its [[refractive index]] and its absorption spectrum. In an optical context, the absorption spectrum is typically quantified by the [[refractive index#Dispersion and absorption|extinction coefficient]], and the extinction and index coefficients are quantitatively related through the [[Kramers–Kronig relations]]. Therefore, the absorption spectrum can be derived from a scattering or reflection spectrum. This typically requires simplifying assumptions or models, and so the derived absorption spectrum is an approximation.

==Applications==
[[File:Identification of Ices in the Solar System.jpg|thumb|300px|The infrared absorption spectrum of NASA laboratory [[sulfur dioxide]] ice is compared with the infrared absorption spectra of ices on [[Jupiter]]'s moon, '''[[Io (moon)|Io]]''' credit NASA, Bernard Schmitt, and [[UKIRT]].]]

Absorption spectroscopy is useful in chemical analysis<ref>James D. Ingle Jr. and Stanley R. Crouch, ''Spectrochemical Analysis'', Prentice Hall, 1988, {{ISBN|0-13-826876-2}}</ref> because of its specificity and its quantitative nature. The specificity of absorption spectra allows compounds to be distinguished from one another in a mixture, making absorption spectroscopy useful in wide variety of applications. For instance, [[Infrared gas analyzer]]s can be used to identify the presence of pollutants in the air, distinguishing the pollutant from nitrogen, oxygen, water, and other expected constituents.<ref>{{cite web |title=Gaseous Pollutants – Fourier Transform Infrared Spectroscopy |url=http://www.epa.gov/apti/course422/ce4b4.html |access-date=2009-09-30 |url-status=dead |archive-url=https://web.archive.org/web/20121023233506/http://www.epa.gov/apti/course422/ce4b4.html |archive-date=2012-10-23}}</ref>

The specificity also allows unknown samples to be identified by comparing a measured spectrum with a library of reference spectra. In many cases, it is possible to determine qualitative information about a sample even if it is not in a library. Infrared spectra, for instance, have characteristics absorption bands that indicate if carbon-hydrogen or carbon-oxygen bonds are present.

An absorption spectrum can be quantitatively related to the amount of material present using the [[Beer–Lambert law]]. Determining the absolute concentration of a compound requires knowledge of the compound's [[absorption coefficient]]. The absorption coefficient for some compounds is available from reference sources, and it can also be determined by measuring the spectrum of a calibration standard with a known concentration of the target.

===Remote sensing===
One of the unique advantages of spectroscopy as an analytical technique is that measurements can be made without bringing the instrument and sample into contact. Radiation that travels between a sample and an instrument will contain the spectral information, so the measurement can be made [[remote sensing|remotely]]. Remote spectral sensing is valuable in many situations. For example, measurements can be made in toxic or hazardous environments without placing an operator or instrument at risk. Also, sample material does not have to be brought into contact with the instrument—preventing possible cross contamination.

Remote spectral measurements present several challenges compared to laboratory measurements. The space in between the sample of interest and the instrument may also have spectral absorptions. These absorptions can mask or confound the absorption spectrum of the sample. These background interferences may also vary over time. The source of radiation in remote measurements is often an environmental source, such as sunlight or the thermal radiation from a warm object, and this makes it necessary to distinguish spectral absorption from changes in the source spectrum.

To simplify these challenges, [[differential optical absorption spectroscopy]] has gained some popularity, as it focusses on differential absorption features and omits broad-band absorption such as aerosol extinction and extinction due to rayleigh scattering. This method is applied to ground-based, airborne, and satellite-based measurements. Some ground-based methods provide the possibility to retrieve tropospheric and stratospheric trace gas profiles.

===Astronomy===
[[File:Cumulative-absorption-spectrum-hubble-telescope.jpg|thumb|upright=2|Absorption spectrum observed by the [[Hubble Space Telescope]]]]

[[Astronomical spectroscopy]] is a particularly significant type of remote spectral sensing. In this case, the objects and samples of interest are so distant from earth that electromagnetic radiation is the only means available to measure them. Astronomical spectra contain both absorption and emission spectral information. Absorption spectroscopy has been particularly important for understanding [[interstellar cloud]]s and determining that some of them contain [[molecular cloud|molecules]]. Absorption spectroscopy is also employed in the study of [[exoplanet|extrasolar planets]]. Detection of extrasolar planets by [[methods of detecting exoplanets#Transit photometry|transit photometry]] also measures their absorption spectrum and allows for the determination of the planet's atmospheric composition,<ref name="Khd">{{cite journal |last1=Khalafinejad |first1=S. |last2=Essen |first2=C. von |last3=Hoeijmakers |first3=H. J. |last4=Zhou |first4=G. |last5=Klocová |first5=T. |last6=Schmitt |first6=J. H. M. M. |last7=Dreizler |first7=S. |last8=Lopez-Morales |first8=M. |last9=Husser |first9=T.-O. |date=2017-02-01 |title=Exoplanetary atmospheric sodium revealed by orbital motion |journal=Astronomy & Astrophysics |language=en |volume=598 |pages=A131 |doi=10.1051/0004-6361/201629473 |issn=0004-6361 |arxiv=1610.01610 |bibcode=2017A&A...598A.131K |s2cid=55263138}}</ref> temperature, pressure, and [[scale height]], and hence allows also for the determination of the planet's mass.<ref>{{cite journal |last=de Wit |first=Julien |author2=Seager, S. |title=Constraining Exoplanet Mass from Transmission Spectroscopy |journal=Science |date=19 December 2013 |volume=342 |issue=6165 |pages=1473–1477 |doi=10.1126/science.1245450 |arxiv=1401.6181 |bibcode=2013Sci...342.1473D |pmid=24357312 |s2cid=206552152}}</ref>

===Atomic and molecular physics===
Theoretical models, principally [[quantum mechanics|quantum mechanical]] models, allow for the absorption spectra of atoms and molecules to be related to other physical properties such as [[electronic structure]], [[atomic mass|atomic]] or [[molecular mass]], and [[molecular geometry]]. Therefore, measurements of the absorption spectrum are used to determine these other properties. [[Microwave spectroscopy]], for example, allows for the determination of bond lengths and angles with high precision.

In addition, spectral measurements can be used to determine the accuracy of theoretical predictions. For example, the [[Lamb shift]] measured in the [[hydrogen atom|hydrogen]] atomic absorption spectrum was not expected to exist at the time it was measured. Its discovery spurred and guided the development of [[quantum electrodynamics]], and measurements of the Lamb shift are now used to determine the [[fine-structure constant]].

==Experimental methods==

===Basic approach===
The most straightforward approach to absorption spectroscopy is to generate radiation with a source, measure a reference spectrum of that radiation with a [[photodetector|detector]] and then re-measure the sample spectrum after placing the material of interest in between the source and detector. The two measured spectra can then be combined to determine the material's absorption spectrum. The sample spectrum alone is not sufficient to determine the absorption spectrum because it will be affected by the experimental conditions—the spectrum of the source, the absorption spectra of other materials between the source and detector, and the wavelength dependent characteristics of the detector. The reference spectrum will be affected in the same way, though, by these experimental conditions and therefore the combination yields the absorption spectrum of the material alone.

A wide variety of radiation sources are employed in order to cover the electromagnetic spectrum. For spectroscopy, it is generally desirable for a source to cover a broad swath of wavelengths in order to measure a broad region of the absorption spectrum. Some sources inherently emit a broad spectrum. Examples of these include [[globar]]s or other [[black body]] sources in the infrared, [[mercury lamp]]s in the visible and ultraviolet, and [[X-ray tube]]s. One recently developed, novel source of broad spectrum radiation is [[synchrotron radiation]], which covers all of these spectral regions. Other radiation sources generate a narrow spectrum, but the emission wavelength can be tuned to cover a spectral range. Examples of these include [[klystron]]s in the microwave region and [[laser]]s across the infrared, visible, and ultraviolet region (though not all lasers have tunable wavelengths).

The detector employed to measure the radiation power will also depend on the wavelength range of interest. Most detectors are sensitive to a fairly broad spectral range and the sensor selected will often depend more on the sensitivity and noise requirements of a given measurement. Examples of detectors common in spectroscopy include [[heterodyne receiver]]s in the microwave, [[bolometer]]s in the millimeter-wave and infrared, [[mercury cadmium telluride]] and other cooled [[semiconductor]] detectors in the infrared, and [[photodiode]]s and [[photomultiplier|photomultiplier tubes]] in the visible and ultraviolet.

If both the source and the detector cover a broad spectral region, then it is also necessary to introduce a means of [[spectral resolution|resolving]] the wavelength of the radiation in order to determine the spectrum. Often a [[spectrometer#Spectrographs|spectrograph]] is used to spatially separate the wavelengths of radiation so that the power at each wavelength can be measured independently. It is also common to employ [[interferometry]] to determine the spectrum—[[infrared spectroscopy#FTIR|Fourier transform infrared]] spectroscopy is a widely used implementation of this technique.

Two other issues that must be considered in setting up an absorption spectroscopy experiment include the [[optics]] used to direct the radiation and the means of holding or containing the sample material (called a [[cuvette]] or cell). For most UV, visible, and NIR measurements the use of precision quartz cuvettes are necessary. In both cases, it is important to select materials that have relatively little absorption of their own in the wavelength range of interest. The absorption of other materials could interfere with or mask the absorption from the sample. For instance, in several wavelength ranges it is necessary to measure the sample under [[vacuum]] or in a [[noble gas]] environment because gases in the [[atmosphere of Earth#Composition|atmosphere]] have interfering absorption features.

===Specific approaches===
*[[Astronomical spectroscopy]]
*[[Cavity ring-down spectroscopy]] (CRDS)
*[[Laser absorption spectrometry]] (LAS)
*[[Mössbauer spectroscopy]]
*[[Photoacoustic spectroscopy]]
*[[Photoemission spectroscopy]]
*[[Photothermal optical microscopy]]
*[[Photothermal spectroscopy]]
*[[Diffuse reflectance spectroscopy|Reflectance spectroscopy]]
*[[Reflection-absorption infrared spectroscopy]] (RAIRS)
*[[Total absorption spectroscopy]] (TAS)
*[[Tunable diode laser absorption spectroscopy]] (TDLAS)
*[[X-ray absorption fine structure]] (XAFS)
*[[X-ray absorption near edge structure]] (XANES)

==See also==
{{Div col|colwidth=20em}}
*[[Absorption (optics)]]
*[[Densitometry]]
*[[HITRAN]]
*[[Infrared gas analyzer]]
*[[Infrared spectroscopy of metal carbonyls]]
*[[Lyman-alpha forest]]
*[[Optical density]]
*[[Photoemission spectroscopy]]
*[[Transparent materials]]
*[[Water absorption]]
*[[White cell (spectroscopy)]]
*[[X-ray absorption spectroscopy]]
{{Div col end}}

==References==
{{Reflist}}

==External links==
* [https://web.archive.org/web/20160604010024/http://csep10.phys.utk.edu/astr162/lect/sun/spectrum.html Solar absorption spectrum] ({{circa|1998}}) (archived)
* [https://webbtelescope.org/contents/articles/spectroscopy-101--types-of-spectra-and-spectroscopy WEBB Space Telescope, Part 3 of a series: Spectroscopy 101 – Types of Spectra and Spectroscopy]
* [https://www.spectralcalc.com/spectral_browser/db_intensity.php Plot Absorption Intensity for many molecules in HITRAN database]

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[[Category:Absorption spectroscopy| ]]
[[Category:Analytical chemistry]]
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[[Category:Electromagnetic radiation]]
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