Editing Spectroscopy (section)

== Classification of methods ==
[[File:A Taste of ESPRESSO.jpg|thumb|A huge diffraction grating at the heart of the ultra-precise [[ESPRESSO]] spectrograph.<ref>{{cite web|title=A Taste of ESPRESSO|url=http://www.eso.org/public/images/potw1537a/|access-date=15 September 2015}}</ref>]]
Spectroscopy is a sufficiently broad field that many sub-disciplines exist, each with numerous implementations of specific spectroscopic techniques. The various implementations and techniques can be classified in several ways.

=== Type of radiative energy ===
The types of spectroscopy are distinguished by the type of radiative energy involved in the interaction. In many applications, the spectrum is determined by measuring changes in the intensity or frequency of this energy. The types of radiative energy studied include:
* [[Electromagnetic radiation]] was the first source of energy used for spectroscopic studies. Techniques that employ electromagnetic radiation are typically classified by the wavelength region of the spectrum and include [[microwave spectroscopy|microwave]], [[terahertz spectroscopy and technology|terahertz]], [[infrared spectroscopy|infrared]], [[near-infrared spectroscopy|near-infrared]], [[Ultraviolet–visible spectroscopy|ultraviolet-visible]], [[x-ray spectroscopy|x-ray]], and [[gamma spectroscopy|gamma]] spectroscopy.
* Particles, because of their [[Matter wave|de Broglie waves]], can also be a source of radiative energy. Both [[electron spectroscopy|electron]] and [[neutron spectroscopy]] are commonly used. For a particle, its [[kinetic energy]] determines its wavelength.
* [[Spectrogram|Acoustic spectroscopy]] involves radiated pressure waves.
* [[Dynamic mechanical analysis]] can be employed to impart radiating energy, similar to acoustic waves, to solid materials.

=== Nature of the interaction ===
The types of spectroscopy also can be distinguished by the nature of the interaction between the energy and the material. These interactions include:<ref name="Crouch-Skoog-Holler2007">{{cite book |author=Crouch |first1=Stanley R. |url=https://books.google.com/books?id=GrOsQgAACAAJ |title=Principles of instrumental analysis |last2=Skoog |first2=Douglas A. |last3=Holler |first3=F. J. |publisher=Thomson Brooks/Cole |year=2007 |isbn=9780495012016 |location=Australia}}</ref>
* [[Absorption spectroscopy]]: Absorption occurs when energy from the radiative source is absorbed by the material. Absorption is often determined by measuring the fraction of energy transmitted through the material, with absorption decreasing the transmitted portion.
* [[Emission spectroscopy]]: Emission indicates that radiative energy is released by the material. A material's [[blackbody spectrum]] is a spontaneous emission spectrum determined by its temperature. This feature can be measured in the infrared by instruments such as the atmospheric emitted radiance interferometer.<ref>{{cite journal | last1=Mariani | first1=Z. | last2=Strong | first2=K. | last3=Wolff | first3=M. | last4=Rowe | first4=P. | year=2012 | title=Infrared measurements in the Arctic using two Atmospheric Emitted Radiance Interferometers | journal= Atmospheric Measurement Techniques| volume=5 | issue=2 | pages=329–344 | doi=10.5194/amt-5-329-2012|  last5=Walden | first5=V. | last6=Fogal | first6=P. F. | last7=Duck | first7=T. | last8=Lesins | first8=G. | last9=Turner | first9=D. S. | last10=Cox | first10=C. | last11=Eloranta | first11=E. | last12=Drummond | first12=J. R. | last13=Roy | first13=C.| last14=Turner | first14=D. D. | last15=Hudak | first15=D. | last16=Lindenmaier | first16=I. A. | bibcode=2012AMT.....5..329M| doi-access=free }}</ref> Emission can also be induced by other sources of energy such as [[flame spectroscopy|flames]], [[Spark (fire)|sparks]], [[electric arc]]s or electromagnetic radiation in the case of [[fluorescence spectroscopy|fluorescence]].
* [[Elastic scattering]] and [[reflectivity|reflection]] spectroscopy determine how incident radiation is reflected or scattered by a material. [[Crystallography]] employs the scattering of high energy radiation, such as x-rays and electrons, to examine the arrangement of atoms in proteins and solid crystals.
* [[Impedance spectroscopy]]: Impedance is the ability of a medium to impede or slow the transmittance of energy. For [[optics|optical]] applications, this is characterized by the [[index of refraction]].
* [[Inelastic scattering]] phenomena involve an exchange of energy between the radiation and the matter that shifts the wavelength of the scattered radiation. These include [[Raman scattering|Raman]] and [[Compton scattering]].
* [[Coherent spectroscopy|Coherent]] or resonance spectroscopy are techniques where the radiative energy couples two quantum states of the material in a [[coherence (physics)|coherent]] interaction that is sustained by the radiating field. The coherence can be disrupted by other interactions, such as particle collisions and energy transfer, and so often require high intensity radiation to be sustained. [[Nuclear magnetic resonance spectroscopy|Nuclear magnetic resonance (NMR) spectroscopy]] is a widely used resonance method, and [[ultrafast laser spectroscopy]] is also possible in the infrared and visible spectral regions.
* [[Nuclear spectroscopy]] are methods that use the properties of specific [[Atomic nucleus|nuclei]] to probe the [[local structure]] in matter, mainly [[condensed matter]], [[molecule]]s in liquids or frozen liquids and bio-molecules.
* [[Quantum logic spectroscopy]] is a general technique used in [[ion traps]] that enables precision spectroscopy of ions with internal structures that preclude [[laser cooling]], state manipulation, and detection. [[Quantum logic]] operations enable a controllable ion to exchange information with a co-trapped ion that has a complex or unknown electronic structure.

=== Type of material ===
Spectroscopic studies are designed so that the [[radiant energy]] interacts with specific types of matter.

==== Atoms ====
[[File:Welsbach_Figure_IV_spektroskopische-lytischen-chemie-21.pdf|thumb|Atomic spectra comparison table, from "Spektroskopische Methoden der analytischen Chemie" (1922).]]
[[Atomic spectroscopy]] was the first application of spectroscopy. [[Atomic absorption spectroscopy]] and [[atomic emission spectroscopy]] involve visible and ultraviolet light. These absorptions and emissions, often referred to as atomic spectral lines, are due to [[Atomic structure#Energy levels|electronic transitions]] of outer shell electrons as they rise and fall from one electron orbit to another. Atoms also have distinct x-ray spectra that are attributable to the excitation of inner shell electrons to excited states.

Atoms of different elements have distinct spectra and therefore atomic spectroscopy allows for the identification and quantitation of a sample's elemental composition. After inventing the spectroscope, [[Robert Bunsen]] and [[Gustav Kirchhoff]] discovered new elements by observing their emission spectra. Atomic absorption lines are observed in the solar spectrum and referred to as [[Fraunhofer lines]] after their discoverer. A comprehensive explanation of the [[Hydrogen spectral series|hydrogen spectrum]] was an early success of quantum mechanics and explained the [[Lamb shift]] observed in the hydrogen spectrum, which further led to the development of [[quantum electrodynamics]].

Modern implementations of atomic spectroscopy for studying visible and ultraviolet transitions include [[flame emission spectroscopy]], [[inductively coupled plasma atomic emission spectroscopy]], [[Glow discharge#Use in analytical chemistry|glow discharge spectroscopy]], [[microwave induced plasma]] spectroscopy, and spark or arc emission spectroscopy. Techniques for studying x-ray spectra include [[X-ray spectroscopy]] and [[X-ray fluorescence]].

==== Molecules ====
The combination of atoms into molecules leads to the creation of unique types of energetic states and therefore unique spectra of the transitions between these states. Molecular spectra can be obtained due to electron spin states ([[electron paramagnetic resonance]]), [[rotational spectroscopy|molecular rotations]], [[molecular vibration]], and electronic states. Rotations are collective motions of the atomic nuclei and typically lead to spectra in the microwave and millimetre-wave spectral regions. Rotational spectroscopy and microwave spectroscopy are synonymous. Vibrations are relative motions of the atomic nuclei and are studied by both infrared and [[Raman spectroscopy]]. Electronic excitations are studied using visible and ultraviolet spectroscopy as well as [[fluorescence spectroscopy]].<ref name="Crouch-Skoog-Holler2007" /><ref>{{Cite book |last=Kroto |first=H. W. |url=https://books.google.com/books?id=Nu8NAQAAIAAJ |title=Molecular Rotation Spectra |date=1975 |publisher=Wiley |isbn=9780471508533 |oclc=793428}}</ref><ref>Philip R. Bunker and Per Jensen (1998),''Molecular Symmetry and Spectroscopy'', 2nd ed.  NRC Research Press, Ottawa [https://volumesdirect.com/products/molecular-symmetry-and-spectroscopy?_pos=1&_sid=ed0cc0319&_ss=r] {{ISBN|9780660196282}}</ref><ref>{{Cite book |last1=Papoušek |first1=Dušan |last2=Aliev |first2=Mamed Ragimovich |url=https://books.google.com/books?id=fb7vAAAAMAAJ |title=Molecular Vibrational-rotational Spectra: Theory and Applications of High Resolution Infrared, Microwave, and Raman Spectroscopy of Polyatomic Molecules |date=1982 |publisher=Elsevier Scientific Publishing Company |isbn=9780444997371 |location=Amsterdam |oclc=7278301}}</ref><ref>{{Cite book |last1=Wilson |first1=Edgar B. |url=https://books.google.com/books?id=CPkvsDrPiv0C |title=Molecular Vibrations: The Theory of Infrared and Raman Vibrational Spectra |last2=Decius |first2=John C. |last3=Cross |first3=Paul C. |date=1980-03-01 |publisher=Courier Corporation |isbn=9780486639413 |oclc=1023249001}}</ref>

Studies in molecular spectroscopy led to the development of the first [[maser]] and contributed to the subsequent development of the [[laser]].

==== Crystals and extended materials ====
The combination of atoms or molecules into crystals or other extended forms leads to the creation of additional energetic states. These states are numerous and therefore have a high density of states. This high density often makes the spectra weaker and less distinct, i.e., broader. For instance, blackbody radiation is due to the thermal motions of atoms and molecules within a material. Acoustic and mechanical responses are due to collective motions as well.
Pure crystals, though, can have distinct spectral transitions, and the crystal arrangement also has an effect on the observed molecular spectra. The regular [[lattice structure]] of crystals also scatters x-rays, electrons or neutrons allowing for crystallographic studies.

==== Nuclei ====
Nuclei also have distinct energy states that are widely separated and lead to [[gamma ray]] spectra. Distinct nuclear spin states can have their energy separated by a magnetic field, and this allows for [[nuclear magnetic resonance spectroscopy]].