Editing Absorption spectroscopy (section)

==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.