Editing Absorption spectroscopy (section)

==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)