Spectroscopy

{{Short description|Study involving matter and electromagnetic radiation}} [[File:Light dispersion of a mercury-vapor lamp with a flint glass prism IPNr°0125.jpg|thumb|A prism separates white light by dispersing it into its component colors, which can then be studied using spectroscopy.]]

'''Spectroscopy''' is the field of study that measures and interprets electromagnetic spectra as it interacts with matter.<ref>{{cite book | chapter=Introduction | title=Atomic Emission Spectrometry: AES - Spark, Arc, Laser Excitation | first1=Heinz-Gerd | last1=Joosten | first2=Alfred | last2=Golloch | first3=Jörg | last3=Flock | first4=Susan | last4=Killewald | publisher=Walter de Gruyter GmbH & Co KG | year=2020 | page=1 | doi=10.1515/9783110529692-001 | doi-broken-date=15 January 2026 | isbn=978-3-11-052969-2 | chapter-url=https://books.google.com/books?id=dhTzDwAAQBAJ&pg=PA1 }}</ref> In narrower contexts, spectroscopy is the precise study of color as generalized from radiated visible light to all bands of the electromagnetic spectrum.

Spectroscopy, primarily in the electromagnetic spectrum, is a fundamental exploratory tool in the fields of astronomy, chemistry, materials science, and physics, allowing the composition, physical and electronic structure of matter to be investigated at the atomic, molecular and macro scale, and over astronomical distances.

Historically, spectroscopy originated as the study of the wavelength dependence of the absorption by gas phase matter of visible light dispersed by a prism. Current applications of spectroscopy include biomedical spectroscopy in the areas of tissue analysis and medical imaging. Matter waves and acoustic waves can be considered forms of radiative energy, and recently gravitational waves have been associated with a spectral signature in the context of the Laser Interferometer Gravitational-Wave Observatory (LIGO).<ref>{{Citation |last=Bartusiak |first=Marcia |title=Einstein's Unfinished Symphony: The Story of a Gamble, Two Black Holes, and a New Age of Astronomy |date=2017-06-27 |url=https://www.degruyter.com/document/doi/10.12987/9780300228120/html |work=Einstein's Unfinished Symphony |access-date=2023-05-22 |publisher=Yale University Press |language=en |doi=10.12987/9780300228120 |oclc=1039140043 |isbn=978-0-300-22812-0|s2cid=246149887 |url-access=subscription }} [https://books.google.com/books?id=6mMlDwAAQBAJ Google Books]</ref>

== Introduction == Spectroscopy is a branch of science concerned with the spectra of electromagnetic radiation as a function of its wavelength or frequency, as measured by spectrographic equipment and other techniques, in order to obtain information concerning the structure and properties of matter.<ref>{{Cite book | url=https://books.google.com/books?id=U5MdAQAAIAAJ | title=The Oxford American College Dictionary | date=2002 | publisher=G.P. Putnam's Sons | isbn=978-0-399-14415-8 | oclc=48965005}}</ref> Spectral measurement devices are referred to as spectrometers, spectrophotometers, spectrographs or spectral analyzers. Most spectroscopic analysis in the laboratory starts with a sample to be analyzed. A light source is sent through a monochromator to spatially separate the colors before passing a selected frequency band through the sample, then the output is captured by a photodiode.<ref name=Hammes_2005>{{cite book | title=Spectroscopy for the Biological Sciences | first=Gordon G. | last=Hammes | publisher=John Wiley & Sons | year=2005 | isbn=978-0-471-73354-6 | pages=36–37 | url=https://books.google.com/books?id=glECXyfF4dcC&pg=PA37 }}</ref> For astronomical purposes, the telescope must be equipped with the light dispersion device.<ref>{{cite book | title=Fundamental Astronomy | editor1-first=Hannu | editor1-last=Karttunen | editor2-first=Pekka | editor2-last=Kröger | editor3-first=Heikki | editor3-last=Oja | editor4-first=Markku | editor4-last=Poutanen | editor5-first=Karl Johan | editor5-last=Donner | edition=6th | publisher=Springer | year=2016 | isbn=978-3-662-53045-0 | page=73 | url=https://books.google.com/books?id=ndd2DQAAQBAJ&pg=PA73 }}</ref> There are various versions of this basic setup that may be employed.

[[File:High resolution solar spectrum (noao-sun).jpg|right|thumb|High resolution spectrum of the Sun, showing the discrete line pattern created by elements in the stellar atmosphere]] Spectroscopy began with Isaac Newton splitting light with a prism; a key moment in the development of modern optics.<ref>{{cite web | url=https://www.aaas.org/isaac-newton-and-problem-color | date=19 November 2012 | access-date=2025-11-23 | title=Isaac Newton and the problem of color | first=Steven A. | last=Edwards | publisher=American Association for the Advancement of Science }}</ref> Therefore, it was originally the study of visible light that we call color. Following the contributions of James Clerk Maxwell, this study later came to include the entire electromagnetic spectrum.<ref>{{cite web | title=1861: James Clerk Maxwell's greatest year | date=18 April 2011 | publisher=King's College London | url=http://www.kcl.ac.uk/newsevents/news/newsrecords/2011/04Apr/JamesClerkMaxwell.aspx | access-date=28 March 2013 | archive-url=https://web.archive.org/web/20130622095747/http://www.kcl.ac.uk/newsevents/news/newsrecords/2011/04Apr/JamesClerkMaxwell.aspx | archive-date=22 June 2013 }}</ref> Although color is involved in spectroscopy, it is not equivalent to the absorption and reflection of certain electromagnetic waves that give objects or elements a sense of color to our eyes. Rather, spectroscopy involves the splitting of light by a prism, diffraction grating, or similar instrument, to display a particular discrete line pattern called a "spectrum", which is unique for each different type of element or molecule. Most elements are first put into a gaseous state to allow the spectra to be examined, although today other methods can be used for different phases of matter. Each element that is diffracted by a prism-like instrument displays either an absorption spectrum or an emission spectrum depending upon whether the element is being cooled or heated.<ref name="auto">{{cite web | publisher=PASCO | url=https://www.pasco.com/products/guides/what-is-spectroscopy | title=What is Spectroscopy? | archive-url=https://web.archive.org/web/20230614114048/https://www.pasco.com/products/guides/what-is-spectroscopy | archive-date=2023-06-14 | url-status=dead }}</ref>

Until recently all spectroscopy involved the study of line spectra and most spectroscopy still does.<ref>{{cite journal | last=Sutton | first=M. A. | title=Sir John Herschel and the Development of Spectroscopy in Britain | journal=The British Journal for the History of Science | volume=7 | issue=1 | publisher=Cambridge University Press | year=1974 | pages=42–60 | jstor=4025175 | doi=10.1017/S0007087400012851 }}</ref> Vibrational spectroscopy is the branch of spectroscopy that studies the spectra, which are caused by vibrations of molecules.<ref>{{cite book | last=Lazić | first=Dejan | chapter=Introduction to Raman Microscopy/Spectroscopy | title=Application of Molecular Methods and Raman Microscopy/Spectroscopy in Agricultural Sciences and Food Technology | editor1-first=Biljana Vucelić | editor-last=Radović | editor2-first=Dejan | editor2-last=Lazić | editor3-first=Miomir | editor3-last=Nikšić | location=London | publisher=Ubiquity Press | year=2019 | pages=143–50 | jstor=j.ctvmd85qp.12 | doi=10.5334/bbj.i| isbn=978-1-911529-52-1 | doi-access=free }}</ref> However, the latest developments in spectroscopy can sometimes dispense with the dispersion technique. In biochemical spectroscopy, information can be gathered about biological tissue by absorption and light scattering techniques. Light scattering spectroscopy is a type of reflectance spectroscopy that determines tissue structures by examining elastic scattering.<ref name="link.aps.org">{{Cite journal |last1=Perelman |first1=L. T. |last2=Backman |first2=V. |last3=Wallace |first3=M. |last4=Zonios |first4=G. |last5=Manoharan |first5=R. |last6=Nusrat |first6=A. |last7=Shields |first7=S. |last8=Seiler |first8=M. |last9=Lima |first9=C. |last10=Hamano |first10=T. |last11=Itzkan |first11=I. |last12=Van Dam |first12=J. |last13=Crawford |first13=J. M. |last14=Feld |first14=M. S. |date=1998-01-19 |title=Observation of Periodic Fine Structure in Reflectance from Biological Tissue: A New Technique for Measuring Nuclear Size Distribution |url=https://link.aps.org/doi/10.1103/PhysRevLett.80.627 |journal=Physical Review Letters |volume=80 |issue=3 |pages=627–630 |doi=10.1103/PhysRevLett.80.627|bibcode=1998PhRvL..80..627P |url-access=subscription }}</ref> In such a case, it is the tissue that acts as a diffraction or dispersion mechanism.

Spectroscopic studies were central to the development of quantum mechanics. The first useful quantum atomic models, including Bohr model, the Schrödinger equation, and Matrix mechanics, reproduced the spectral lines of hydrogen. These equated discrete quantum jumps of the bound electron in a hydrogen atom to the discrete hydrogen spectrum. Max Planck's explanation of blackbody radiation involved spectroscopy because he was comparing the wavelength of light using a photometer to the temperature of a Black Body.<ref>{{cite book | last=Kumar | first=Manjit | chapter=Chap. 1 | title=Quantum: Einstein, Bohr, and the great debate about the nature of reality | edition=1st American | year=2008 | publisher=W. W. Norton & Co. | isbn=978-1-84831-103-9 | url=https://dokumen.pub/quantum-einstein-bohr-and-the-great-debate-about-the-nature-of-reality-9781848311039-1848311036.html | access-date=2025-11-23 }}</ref> Spectroscopy is used in physical and analytical chemistry because atoms and molecules have unique spectra. As a result, these spectra can be used to detect, identify and quantify information about the atoms and molecules.

Spectroscopy is used in astronomy and remote sensing on Earth. Most research telescopes have spectrographs. The measured spectra are used to determine the chemical composition and physical properties of astronomical objects, such as their temperature, elemental abundances, velocity, rotation, magnetic field, and more.<ref>{{cite web | title=Spectra and What They Can Tell Us | publisher=NASA Goddard Space Flight Center | url=https://imagine.gsfc.nasa.gov/science/toolbox/spectra1.html | date=August 2013 | website=Imagine the Universe! | access-date=2025-11-23 }}</ref> An important use for spectroscopy is in biochemistry. Molecular samples may be analyzed for species identification and energy content.<ref>{{cite web | title=Basic Spectroscopy | date=16 October 2008 | first1=Santi | last1=Nonell | first2=Cristiano | last2=Viappiani | url=http://photobiology.info/Nonell_Viappiani.html | website=Photobiological Sciences Online | access-date=2025-11-23 }}</ref>

== Theory == The underlying premise of spectroscopy is that light is made of different wavelengths and that each wavelength corresponds to a different frequency. The importance of spectroscopy is centered around the fact that every element in the periodic table has a unique light spectrum described by the frequencies of light it emits or absorbs consistently appearing in the same part of the electromagnetic spectrum when that light is diffracted.<ref>{{cite web | title=Spectroscopy 101 – Types of Spectra and Spectroscopy | publisher=NASA | date=September 9, 2025 | editor-first=Stephen | editor-last=Sabia | url=https://science.nasa.gov/mission/webb/science-overview/science-explainers/spectroscopy-101-types-of-spectra-and-spectroscopy/ | access-date=2025-12-02 }}</ref> This opened up an entire field of study with anything that contains atoms. Spectroscopy is the key to understanding the atomic properties of all matter. As such spectroscopy opened up many new sub-fields of science yet undiscovered. The idea that each atomic element has its unique spectral signature enabled spectroscopy to be used in a broad number of fields each with a specific goal achieved by different spectroscopic procedures. The National Institute of Standards and Technology maintains a public Atomic Spectra Database that is continually updated with precise measurements.<ref>{{cite journal | last1=Kramida | first1=Alexander | last2=Ralchenko | first2=Yuri | title=Atomic Spectra Database | journal=NIST Standard Reference Database 78 | version=5.12 | date=November 2024 | publisher=National Institute of Standards and Technology | doi=10.18434/T4W30F | url=https://www.nist.gov/pml/atomic-spectra-database | access-date=2025-11-24 }}</ref>

With an absorption spectrophotometer, the level of absorption of a light source is determined by the Beer-Lambert Law: <math display="block">\ln\left(\frac{I_0}{I}\right)=\varepsilon \ell c</math> where <math>I_0</math> is the light intensity before passing through the sample, <math>I</math> is the output intensity, <math>\varepsilon</math> is the extinction coefficient, <math>\ell</math> is the path length through the sample, and <math>c</math> is the concentration of the sample. The extinction coefficient depends on the wavelength selected and the molecule being sampled.<ref name=Hammes_2005/>

Resonances by the frequency were first characterized in mechanical systems such as pendulums, which have a frequency of motion noted famously by Galileo.<ref>{{cite book | first=Isaac | last=Asimov | author-link=Isaac Asimov | title=Understanding Physics | series=Dorset Press Reprints Series | volume=1 | page=108 | orig-date=1966 | year=1988 | publisher=Barnes & Noble Publishing | isbn=978-0-88029-251-1 | url=https://books.google.com/books?id=pSKvaLV6zkcC&pg=PA108 }}</ref> In quantum mechanical systems, the analogous resonance is a coupling of two quantum mechanical stationary states of a system, such as two atomic orbitals, via an oscillatory source of energy such as a photon. The coupling of the two states is strongest when the source energy matches the energy difference between the two states.{{cn|date=November 2025|reason=Sources don't describe a quantum jump as a resonance between states.}} That is, a photon at the right energy is more likely to cause an electron to jump between two orbitals, a process called electron excitation. The energy {{mvar|E}} of a photon is related to its frequency {{mvar|ν}} by {{math|''E'' {{=}} ''hν''}} where {{mvar|h}} is the Planck constant,<ref>{{cite web | title=Energy, Frequency, and Wavelength | work=3.091: Introduction to Solid State Chemistry | first=Maddie | last=Sutula | date=Fall 2018 | publisher=Massachusetts Institute of Technology | url=https://ocw.mit.edu/courses/3-091-introduction-to-solid-state-chemistry-fall-2018/50e1280c59c49bcc1dda8fc57834e150_MIT3_091F18_REC4.pdf | access-date=2025-11-27 }}</ref> and so a spectrum of the system response vs. photon frequency will peak at the resonant frequency or energy.

Any part of the electromagnetic spectrum may be used to analyze a sample from the infrared to the ultraviolet telling scientists different properties about the very same sample, a discovery that led to a broadening of the field of spectroscopy. For instance in chemical analysis, the most common types of spectroscopy include atomic spectroscopy, infrared spectroscopy, ultraviolet and visible spectroscopy, Raman spectroscopy and nuclear magnetic resonance.<ref>{{cite web | last=Saul | first=Louise | date=January 9, 2019 | url=https://www.azooptics.com/Article.aspx?ArticleID=1382 | title=The Different Types of Spectroscopy for Chemical Analysis | publisher=AZO Optics | access-date=2021-11-10 }}</ref> In nuclear magnetic resonance (NMR), the theory behind it is that frequency is analogous to resonance and its corresponding resonant frequency.

== 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 | publisher=The European Southern Observatory | date=14 September 2015 | 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,<ref>{{cite journal | title=Microwave Spectroscopy | first=C. H. | last=Townes | journal=American Scientist | volume=40 | issue=2 | date=April 1952 | pages=270–290 | publisher=Sigma Xi, The Scientific Research Honor Society | jstor=27826432 }}</ref> terahertz,<ref>{{cite journal | title=Terahertz spectroscopy and imaging – Modern techniques and applications | first1=P. U. | last1=Jepsen | first2=D. G. | last2=Cooke | first3=M. | last3=Koch | journal=Laser & Photonics Reviews | volume=5 | issue=1 | date=January 2011 | pages=124–166 | doi=10.1002/lpor.201000011 }}</ref> infrared,<ref>{{cite journal | title=Infrared Spectroscopy | first1=Lily M. | last1=Ng | first2=Reiko | last2=Simmons | journal=Analytical Chemistry | volume=71 | issue=12 | date=May 20, 1999 | pages=343–350 | publisher=American Chemical Society | doi=10.1021/a1999908r | pmid=10384791 }}</ref> near-infrared,<ref>{{cite journal | title=Principles, Techniques, and Limitations of Near Infrared Spectroscopy | first1=Marco | last1=Ferrari | first2=Leonardo | last2=Mottola | first3=Valentina | last3=Quaresima | journal=Canadian Journal of Applied Physiology | volume=29 | issue=4 | date=August 2004 | pages=463–487 | doi=10.1139/h04-031 | pmid=15328595 }}</ref> ultraviolet-visible,<ref>{{cite journal | title=UV-Vis spectroscopy | last1=Picollo | first1=Marcello | last2=Aceto | first2=Maurizio | last3=Vitorino | first3=Tatiana | journal=Physical Sciences Reviews | volume=4 | issue=4 | year=2019 | article-number=20180008 | doi=10.1515/psr-2018-0008 }}</ref> X-ray,<ref name=Hippert_et_al_eds_2005/> and gamma spectroscopy.<ref>{{cite book | chapter=Gamma-Ray Spectroscopy | last1=Dunn | first1=W. L. | last2=McGregor | first2=D. S. | last3=Shultis | first3=J. K. | year=2021 | editor1-last=Fleck | editor1-first=I. | editor2-last=Titov | editor2-first=M. | editor3-last=Grupen | editor3-first=C. | editor4-last=Buvat | editor4-first=I. | title=Handbook of Particle Detection and Imaging | pages=515–582 | publisher=Springer, Cham. | doi=10.1007/978-3-319-93785-4_17 | isbn=978-3-319-93784-7 }}</ref> * Particles, because of their de Broglie waves, can be a source of radiative energy.<ref name=Anderson_2025>{{cite book | title=Physics and Modern Life: A Panoramic Overview of the Fundamental Science and Its Human Impact | first=Michael | last=Anderson | publisher=Springer Nature | year=2025 | pages=473–474 | isbn=978-3-031-77825-4 | url=https://books.google.com/books?id=gaVjEQAAQBAJ&pg=PA473 }}</ref> Both electron<ref>{{cite book | title=Electron Scattering and Related Spectroscopies | first1=Maurizio | last1=De Crescenzi | first2=M. Novella | last2=Piancastelli | publisher=World Scientific | year=1996 | isbn=9789810223007 | pages=1–5 | url=https://books.google.com/books?id=HzEN5tPPzuwC&pg=PA11 }}</ref> and neutron spectroscopy are used.<ref name=Hippert_et_al_eds_2005>{{cite book | title=Neutron and X-ray Spectroscopy | editor1-first=Françoise | editor1-last=Hippert | editor2-first=Erik | editor2-last=Geissler | editor3-first=Jean Louis | editor3-last=Hodeau | editor4-first=Eddy | editor4-last=Lelièvre-Berna | editor5-first=Jean-René | editor5-last=Regnard | publisher=Springer Science & Business Media | year=2005 | isbn=978-1-4020-3336-0 | url=https://books.google.com/books?id=C03N3xu8UWUC&pg=PR23 }}</ref> For a particle, its kinetic energy determines its wavelength.<ref name=Anderson_2025/> * Acoustic spectroscopy involves radiated pressure waves.<ref>{{cite encyclopedia | title=Acoustic Spectrometer, Acoustic Spectrometry of Solids | encyclopedia=Encyclopedic Dictionary of Condensed Matter Physics | first=Charles P. | last=Poole Jr. | publisher=Academic Press | year=2004 | isbn=978-0-470-23300-9 | pages=13–15 | url=https://books.google.com/books?id=CXwrqM2hU0EC&pg=PA14 }}</ref> * Dynamic mechanical analysis can be employed to impart radiating energy, similar to acoustic waves, to solid materials.<ref>{{cite book | chapter=Dynamic Mechanical Analysis | title=Characterization and Analysis of Polymers | publisher=John Wiley & Sons | year=2008 | isbn=978-0-470-23300-9 | page=649 | chapter-url=https://books.google.com/books?id=IWqmp9oMToIC&pg=PA649 }}</ref>

=== Nature of the interaction === The types of spectroscopy 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 | title=Principles of instrumental analysis | last1=Crouch | first1=Stanley R. | last2=Skoog | first2=Douglas A. | last3=Holler | first3=F. J. | publisher=Thomson Brooks/Cole | year=2007 | isbn=978-0-495-01201-6 | location=Australia | url=https://books.google.com/books?id=GrOsQgAACAAJ }}</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 be induced by other sources of energy such as flames, sparks, electric arcs or electromagnetic radiation in the case of fluorescence. * Elastic scattering and 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, where impedance is the ability of a medium to impede or slow the transmittance of energy.<ref>{{cite journal | last=Macdonald | first=J. R. | title=Impedance spectroscopy | journal=Annals of Biomedical Engineering | volume=20 | pages=289–305 | year=1992 | issue=3 | doi=10.1007/BF02368532 | pmid=1443825 }}</ref> For optical applications, this is characterized by the index of refraction. * Inelastic scattering phenomena involve an exchange of energy between X-ray radiation and the matter that shifts the wavelength of the scattered radiation. These include Raman and Compton scattering.<ref>{{cite book | last1=Alexandropoulos | first1=N. G. | last2=Theodoridou | first2=I. | year=1988 | chapter=X-Ray Inelastic Scattering Spectroscopy and Its Applications in Solid State Physics | editor1-last=Ferreira | editor1-first=J. G. | editor2-last=Ramos | editor2-first=M. T. | title=X-Ray Spectroscopy in Atomic and Solid State Physics | series=NATO ASI Series | volume=187 | pages=279–299 | publisher=Springer | location=Boston, MA. | doi=10.1007/978-1-4613-0731-0_13 | isbn=978-1-4612-8054-5 }}</ref> * Coherent or resonance spectroscopy are techniques where the radiative energy couples two quantum states of the material in a 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 requires high intensity radiation to be sustained. Nuclear magnetic resonance (NMR) spectroscopy is a widely used resonance method, and ultrafast laser spectroscopy is possible in the infrared and visible spectral regions. * Nuclear spectroscopy are methods that use the properties of specific nuclei to probe the local structure in matter, mainly condensed matter, molecules 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.<ref>{{cite journal | title=Spectroscopy Using Quantum Logic | first1=P. O. | last1=Schmidt | first2=T. | last2=Rosenband | first3=C. | last3=Langer | first4=W. M. | last4=Itano | first5=J. C. | last5=Bergquist | first6=D. J. | last6=Wineland | journal=Science | date=July 29, 2005 | volume=309 | issue=5735 | pages=749–752 | doi=10.1126/science.1114375 }}</ref> 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. These studies can be divided into three broad categories:<ref>{{cite book | title=Essentials of Chemical Biology: Structures and Dynamics of Biological Macromolecules In Vitro and In Vivo | first1=Andrew D. | last1=Miller | first2=Julian A. | last2=Tanner | edition=2nd | publisher=John Wiley & Sons | year=2024 | page=128 | isbn=978-1-119-43797-0 | url=https://books.google.com/books?id=Zu3pEAAAQBAJ&pg=PA128 }}</ref> electronic spectroscopy, which measures the transition of electrons between different energy states through absorption or emission of visible or ultraviolet energy; vibronic spectroscopy of molecules induced by the absorption of infrared energy; and rotational spectroscopy of molecules caused by microwave energy.<ref>{{cite journal | title=Taking the Pulse of Molecular Rotational Spectroscopy | first=Brooks H. | last=Pate | journal=Science | date=August 19, 2011 | volume=333 | issue=6045 | pages=947–948 | doi=10.1126/science.1207994 | pmid=21852481 }}</ref> The last two can be combined into rotational–vibrational spectroscopy of a gas.

==== Atoms ==== 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 electronic transitions of outer shell electrons as they rise and fall from one electron orbit to another. Atoms 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 Robert Bunsen and Gustav Kirchhoff invented the spectroscope, Bunsen discovered cesium and rubidium by observing their emission spectra.<ref>{{cite book | title=The Lost Elements: The Periodic Table's Shadow Side | first1=Marco | last1=Fontani | first2=Mariagrazia | last2=Costa | first3=Mary Virginia | last3=Orna | publisher=Oxford University Press | year=2015 | isbn=978-0-19-938334-4 | pages=24–25 | url=https://books.google.com/books?id=Ck9jBAAAQBAJ&pg=PA24 }}</ref> Atomic absorption lines are observed in the solar spectrum and referred to as Fraunhofer lines after their discoverer.<ref>{{cite journal | title=Fraunhofer and his spectral lines | first=Myles W. | last=Jackson | journal=Annalen der Physik | volume=526 | issue=7–8 | date=August 2014 | pages=A65–A69 | doi=10.1002/andp.201400807 }}</ref> A comprehensive explanation of the hydrogen spectrum was an early success of quantum mechanics<ref>{{cite book | title=The Historical Development of Quantum Theory, Volume 5, Part 2 | first1=Jagdish | last1=Mehra | first2=Helmut | last2=Rechenberg | publisher=Springer Science & Business Media | year=2001 | isbn=978-0-387-95180-5 | pages=459–463 | url=https://books.google.com/books?id=-pL56OcVubgC&pg=PA462 }}</ref> and explained the Lamb shift observed in the hydrogen spectrum,<ref>{{cite book | chapter=Perturbation Theory in Quantum Mechanics | first1=Luigi E. | last1=Picasso | first2=Luciano | last2=Bracci | first3=Emilio | last3=D'Emilio | title=Mathematics of Complexity and Dynamical Systems | editor-first=Robert A. | editor-last=Meyers | publisher=Springer Science & Business Media | year=2011 | isbn=978-1-4614-1805-4 | page=1352 | chapter-url=https://books.google.com/books?id=iXEmLUcXAPcC&pg=PA1352 }}</ref> 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,<ref>{{cite book | title=Basic Concepts Of Analytical Chemistry | first=S. M. | last=Khopkar | publisher=New Age International | year=1998 | isbn=978-81-224-1159-1 | pages=284–292 | url=https://books.google.com/books?id=e8Ju_n8DN1sC&pg=PA284 }}</ref> glow discharge spectroscopy,<ref>{{cite journal | title=Glow discharge mass spectrometry | first1=W. W. | last1=Harrison | first2=K. R. | last2=Hess | first3=R. K. | last3=Marcus | first4=F. L. | last4=King | journal=Analytical Chemistry | date=1986 | volume=58 | issue=2 | pages=341A–356A | publisher=American Chemical Society | doi=10.1021/ac00293a002 }}</ref> microwave induced plasma spectroscopy,<ref>{{cite journal | title=Microwave-induced plasma–optical emission spectrometry – fundamental aspects and applications in metal speciation analysis | first1=B. | last1=Rosenkranz | first2=J. | last2=Bettmer | journal=TrAC Trends in Analytical Chemistry | volume=19 | issue=2–3 | date=February–March 2000 | pages=138–156 | doi=10.1016/S0165-9936(99)00189-2 }}</ref> and spark or arc emission spectroscopy.<ref>{{cite journal | first1=B. | last1=Vayner | first2=D. C. | last2=Ferguson | first3=J. T. | last3=Galofaro | title=Emission Spectra of Arc Plasmas | journal=IEEE Transactions on Plasma Science | volume=36 | issue=5 | pages=2219–2227 | date=October 2008 | doi=10.1109/TPS.2008.2001424 }}</ref> Techniques for studying X-ray spectra include X-ray spectroscopy<ref name=Hippert_et_al_eds_2005/> and X-ray fluorescence.<ref>{{cite book | chapter=Elemental Analysis by X-Ray Fluorescence Spectroscopy | first1=A. D. | last1=Karathanasis | first2=B. F. | last2=Hajek | editor1-first=D. L. | editor1-last=Sparks | editor2-first=A. L. | editor2-last=Page | editor3-first=P. A. | editor3-last=Helmke | editor4-first=R. H. | editor4-last=Loeppert | editor5-first=P. N. | editor5-last=Soltanpour | editor6-first=M. A. | editor6-last=Tabatabai | editor7-first=C. T. | editor7-last=Johnston | editor8-first=M. E. | editor8-last=Sumner | title=Methods of Soil Analysis: Part 3 Chemical Methods | series=SSSA Book Series | date=January 1996 | isbn=978-0-89118-825-4 | doi=10.2136/sssabookser5.3.c7 }}</ref>

==== 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), 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=978-0-471-50853-3 |oclc=793428}}</ref><ref>{{cite book | first1=Philip R. | last1=Bunker | first2=Per | last2=Jensen | year=1998 | title=Molecular Symmetry and Spectroscopy | edition=2nd | publisher=NRC Research Press | location=Ottawa | url=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=978-0-444-99737-1 |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=978-0-486-63941-3 |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.{{cn|date=December 2025}} 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.{{cn|date=December 2025}} Pure crystals, though, can have distinct spectral transitions, and the crystal arrangement has an effect on the observed molecular spectra. The regular lattice structure of crystals scatters X-rays,<ref>{{cite journal | title=X Ray crystallography | first1=M. S. | last1=Smyth | first2=J. H. J. | last2=Martin | journal=Molecular Pathology | date=February 2000 | volume=53 | issue=1 | pages=8–14 | doi=10.1136/mp.53.1.8 | doi-access=free | pmc=1186895 | pmid=10884915 }}</ref> electrons,<ref>{{cite journal | title=Electron crystallography | first=D. L. | last=Dorset | journal=Acta Crystallographica Section B | year=1996 | volume=B52 | issue=5 | pages=753–769 | doi=10.1107/S0108768196005599 | pmid=8900031 }}</ref> or neutrons,<ref>{{cite journal | title=Neutron crystallography: opportunities, challenges, and limitations | first1=Matthew P. | last1=Blakeley | first2=Paul | last2=Langan | first3=Nobuo | last3=Niimura | first4=Alberto | last4=Podjarny | journal=Current Opinion in Structural Biology | volume=18 | issue=5 | date=October 2008 | pages=593–600 | doi=10.1016/j.sbi.2008.06.009 | pmid=18656544 }}</ref> allowing for crystallographic studies.

==== Nuclei ==== Nuclei have distinct energy states that are widely separated and lead to gamma ray spectra.{{cn|date=December 2025}} Distinct nuclear spin states can have their energy separated by a magnetic field, and this allows for nuclear magnetic resonance spectroscopy.<ref>{{cite web | title=Understanding NMR Spectroscopy | first=James | last=Keeler | publisher=University of Cambridge | url=https://www-keeler.ch.cam.ac.uk/lectures/Irvine/ | access-date=2025-12-12 }}</ref>

== Other types == {{Prose|section|date=April 2016}} Other types of spectroscopy are distinguished by specific applications or implementations: * Acoustic resonance spectroscopy is based on sound waves primarily in the audible and ultrasonic regions.<ref>{{cite journal | last1=Ripoche | first1=J. | last2=Maze | first2=G. | last3=Izbicki | first3=JL | title=A new acoustic spectroscopy: Resonance spectroscopy by the MIIR | journal=Journal of Nondestructive Evaluation | volume=5 | pages=69–79 | year=1985 | issue=2 | doi=10.1007/BF00566957 }}</ref> * Auger electron spectroscopy is a method used to study surfaces of materials on a micro-scale. It is often used in connection with electron microscopy.<ref>{{cite book | last1=Gunawardane | first1=R. P. | last2=Arumainayagam | first2=C. R. | year=2006 | chapter=Auger Electron Spectroscopy | editor-last=Vij | editor-first=D. | title=Handbook of Applied Solid State Spectroscopy | pages=451–483 | publisher=Springer | location=Boston, MA. | doi=10.1007/0-387-37590-2_10 | isbn=978-0-387-32497-5 }}</ref> * Cavity ring-down spectroscopy enables measurement of absolute optical extinction by samples that scatter and absorb light.<ref>{{cite book | chapter=An Introduction to Cavity Ring-Down Spectroscopy | first1=Kevin K. | last1=Lehmann | first2=Giel | last2=Berden | first3=Richard | last3=Engeln | title=Cavity Ring-Down Spectroscopy: Techniques and Applications | editor1-first=Giel | editor1-last=Berden | editor2-first=Richard | editor2-last=Engeln | publisher=John Wiley & Sons | year=2009 | isbn=978-1-4443-0824-2 | pages=1–3 | chapter-url=https://books.google.com/books?id=5jQM88VYwzQC&pg=PA1 }}</ref> * Circular dichroism spectroscopy measures the differential absorption of left- and right-handed circularly polarized light.<ref>{{cite book | last1=Hoffmann | first1=S. V. | last2=Fano | first2=M. | last3=van de Weert | first3=M. | year=2016 | chapter=Circular Dichroism Spectroscopy for Structural Characterization of Proteins | editor1-last=Müllertz | editor1-first=A. | editor2-last=Perrie | editor2-first=Y. | editor3-last=Rades | editor3-first=T. | title=Analytical Techniques in the Pharmaceutical Sciences | series=Advances in Delivery Science and Technology | pages=223–251 | publisher=Springer | location=New York, NY. | doi=10.1007/978-1-4939-4029-5_6 | isbn=978-1-4939-4027-1 }}</ref> * Coherent anti-Stokes Raman spectroscopy is a recent technique that has high sensitivity and powerful applications for ''in vivo'' spectroscopy and imaging.<ref>{{cite journal | last1=Evans | first1=C. L. | last2=Xie | first2=X. S. | date=2008 | title=Coherent Anti-Stokes Raman Scattering Microscopy: Chemical Imaging for Biology and Medicine | journal=Annual Review of Analytical Chemistry | volume=1 | pages=883–909 | doi=10.1146/annurev.anchem.1.031207.112754 | pmid=20636101 | bibcode=2008ARAC....1..883E | url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:33471118 }}</ref> * Cold vapour atomic fluorescence spectroscopy is a subclass of the atomic emission spectroscopy technique that measures trace amounts of volatile heavy metals in the air, such as mercury.<ref>{{cite journal | last1=Kopysc | first1=Edyta | last2=Pyrzynska | first2=Krystyna | last3=Garbos | first3=Slawomir | last4=Bulska | first4=Ewa | title=Determination of Mercury by Cold-Vapor Atomic AbsorptionSpectrometry with Preconcentration on a Gold-Trap | journal=Analytical Sciences | date=December 17, 2000 | volume=16 | issue=12 | pages=1309–1312 | doi=10.2116/analsci.16.1309|doi-access=free}}</ref> * Correlation spectroscopy encompasses several types of two-dimensional NMR spectroscopy.<ref>{{cite journal | title=Advances in two-dimensional correlation spectroscopy | first=Isao | last=Noda | journal=Vibrational Spectroscopy | volume=36 | issue=2 | year=2004 | pages=143–165 | doi=10.1016/j.vibspec.2003.12.016 }}</ref> * Deep-level transient spectroscopy measures concentration and analyzes parameters of electrically active defects in semiconducting materials.<ref>{{cite journal | title=Defect identification based on first-principles calculations for deep level transient spectroscopy | first1=Darshana | last1=Wickramaratne | first2=Cyrus E. | last2=Dreyer | first3=Bartomeu | last3=Monserrat | first4=Jimmy-Xuan | last4=Shen | first5=John L. | last5=Lyons | first6=Audrius | last6=Alkauskas | first7=Chris G. | last7=Van de Walle | journal=Applied Physics Letters | volume=113 | article-number=192106 | year=2018 | issue=19 | doi=10.1063/1.5047808 | arxiv=1810.05302 }}</ref> * Dielectric spectroscopy measures the dielectric properties of a medium as a function of frequency.<ref>{{cite journal | last1=Volkov | first1=A. A. | last2=Prokhorov | first2=A. S. | title=Broadband Dielectric Spectroscopy of Solids | journal=Radiophysics and Quantum Electronics | volume=46 | pages=657–665 | year=2003 | issue=8–9 | doi=10.1023/B:RAQE.0000024994.15881.c9 }}</ref> * Dual-polarization interferometry measures the real and imaginary components of the complex refractive index.<ref>{{cite journal | title=Dual-Polarization Interferometry: A Novel Technique To Light up the Nanomolecular World | journal=Chemical Reviews | volume=115 | issue=1 | pages=265–294 | date=December 2, 2014 | first1=Jorge | last1=Escorihuela | first2=Miguel Ángel | last2=González-Martínez | first3=José Luis | last3=López-Paz | first4=Rosa | last4=Puchades | first5=Ángel | last5=Maquieira | first6=David | last6=Gimenez-Romero | series=ACS Publications | doi=10.1021/cr5002063 | pmid=25456305 }}</ref> * Electron energy loss spectroscopy in transmission electron microscopy.<ref>{{cite conference | title=Fundamentals of electron energy-loss spectroscopy | first1=F. | last1=Hofer | first2=F. P. | last2=Schmidt | first3=W. | last3=Grogger | first4=G. | last4=Kothleitner | series=IOP Conference Series: Materials Science and Engineering | volume=109 | conference=14th European Workshop on Modern Developments and Applications in Microbeam Analysis (EMAS 2015 Workshop) 3–7 May 2015, Portorož, Slovenia | doi=10.1088/1757-899X/109/1/012007 }}</ref> * Electron phenomenological spectroscopy measures the physicochemical properties and characteristics of the electronic structure of multicomponent and complex molecular systems.<ref name=Dolomatov_et_al_2025>{{cite journal | last1=Dolomatov | first1=M. Y. | last2=Subkhankulov | first2=V. R. | last3=Dolomatova | first3=M. M. | first4=E. A. | last4=Kovaleva | first5=S. S. | last5=Vershinin | first6=O. A. | last6=Belotelov | first7=I. V. | last7=Kazaev | title=Use of Electron Phenomenological Spectroscopy for the Rapid Determination of the Properties of Raw Material for the Preparation of Multifunctional Carbon Materials | journal=Chemistry and Technology of Fuels and Oils | volume=61 | pages=340–345 | year=2025 | issue=2 | doi=10.1007/s10553-025-01872-5 }}</ref> * Electron paramagnetic resonance spectroscopy is similar to nuclear magnetic resonance (NMR), except it measures the spin excitement of unpaired electrons.<ref>{{cite journal | last=Lancaster | first=G. | title=Electron paramagnetic resonance (a review) | journal=Journal of Materials Science | volume=2 | pages=489–495 | year=1967 | issue=5 | doi=10.1007/BF00562955 }}</ref> * Force spectroscopy is a set of techniques for the study of the interactions and the binding forces between individual molecules, although the name is somewhat misleading because there is no true matter-radiation interaction.<ref>{{cite journal | title=Force Spectroscopy and Beyond: Innovations and Opportunities | first1=Bhavik | last1=Nathwani | first2=William M. | last2=Shih | first3=Wesley P. | last3=Wong | journal=Biophysical Perspective | volume=115 | issue=12 | pages=2279–2285 | date=December 18, 2018 | pmid=30447991 | pmc=6302248 | doi=10.1016/j.bpj.2018.10.021 }}</ref> * Fourier-transform spectroscopy is an efficient method for processing spectra data obtained using interferometers. Fourier-transform infrared spectroscopy is an implementation of infrared spectroscopy.<ref>{{cite book | last=Faix | first=O. | year=1992 | chapter=Fourier Transform Infrared Spectroscopy | editor1-last=Lin | editor1-first=S. Y. | editor2-last=Dence | editor2-first=C.W. | title=Methods in Lignin Chemistry | series=Springer Series in Wood Science | pages=83–109 | publisher=Springer | location=Berlin, Heidelberg | doi=10.1007/978-3-642-74065-7_7 | isbn=978-3-642-74067-1 }}</ref> NMR also employs Fourier transforms. * Gamma spectroscopy measures the gamma ray emissions from high energy processes including radioactive and astrophysical sources.<ref>{{cite web | title=Gamma-ray spectrometers | publisher=Space Science Institute | url=https://space-science.llnl.gov/research/gamma-ray-spectrometers | access-date=2025-12-29 }}</ref> * Hadron spectroscopy studies the energy/mass spectrum of hadrons according to spin, parity, and other particle properties. Baryon spectroscopy and meson spectroscopy are types of hadron spectroscopy.{{cn|date=January 2026}} * Multispectral imaging and hyperspectral imaging is a method to create a complete picture of the environment or various objects, each pixel containing a full visible, visible near infrared, near infrared, or infrared spectrum.<ref>{{cite journal | title=Advances in Spectral Imaging: A Review of Techniques and Technologies | first1=Sani | last1=Mukhtar | first2=Amir | last2=Arbabi | first3=Jaime | last3=Viegas | journal=IEEE Access | volume=13 | pages=35848–35902 | year=2025 | doi=10.1109/ACCESS.2025.3544476 | doi-access=free}}</ref> * Inelastic electron tunneling spectroscopy uses the changes in current due to inelastic electron-vibration interaction at specific energies that can also measure optically forbidden transitions.<ref>{{cite journal | title=Inelastic Electron Tunneling Spectroscopy | first1=S. K. | last1=Khanna | first2=John | last2=Lambe | journal=Science | date=June 24, 1983 | volume=220 | issue=4604 | pages=1345–1351 | doi=10.1126/science.220.4604.1345 | pmid=17730635 }}</ref> * Inelastic neutron scattering is similar to Raman spectroscopy, but uses neutrons instead of photons.<ref>{{cite journal | title=Inelastic Neutron Scattering: A Tool in Molecular Vibrational Spectroscopy and a Test of ab Initio Methods | first=Bruce S. | last=Hudson | date=March 24, 2001 | journal=The Journal of Physical Chemistry A | volume=105 | issue=16 | pages=3949–3960 | doi=10.1021/jp004429o }}</ref> * Laser-induced breakdown spectroscopy, also called laser-induced plasma spectrometry, is a type of atomic emission spectroscopy which uses a highly energetic laser pulse as the excitation source<ref>{{cite journal | title=Laser-Induced Breakdown Spectroscopy: Fundamentals, Applications, and Challenges | first1=F. | last1=Anabitarte | first2=A. | last2=Cobo | first3=J. M. | last3=Lopez-Higuera | journal=International Scholarly Research Notices | date=October 30, 2012 | editor1-first=H. J. | editor1-last=Byrne | editor2-first=G. | editor2-last=Louarn | series=Wiley Online Library | pages=1–12 | doi=10.5402/2012/285240 | doi-access=free }}</ref> * Laser spectroscopy uses tunable lasers<ref>{{cite book | first=W. | last=Demtröder | author-link=W. Demtröder | title=Laser Spectroscopy | edition=4th | publisher=Springer Science & Business Media | year=2008 | page=314 | isbn=978-3-540-73418-5 | url=https://books.google.com/books?id=5vuqvvb9YxkC&pg=PA314 }}</ref> and other types of coherent emission sources, such as optical parametric oscillators,<ref>{{cite book | editor-first=F. J. | editor-last=Duarte | editor-link=F. J. Duarte | title=Tunable Laser Applications | edition=3rd | publisher=CRC Press | location=Boca Raton | year=2016 | isbn=978-1-4822-6106-6 | first1=Brian | last1=Orr | first2=J. G. | last2=Haub | first3=Y. | last3=He | first4=R. T. | last4=White | chapter=Spectroscopic Applications of Pulsed Tunable Optical Parametric Oscillators | pages=17–142| author1-link=Brian Orr }}</ref> for selective excitation of atomic or molecular species. * Light scattering spectroscopy (LSS) is a spectroscopic technique typically used to evaluate morphological changes in epithelial cells in order to study mucosal tissue and detect early cancer and precancer.<ref name="link.aps.org"/><ref>{{Cite journal |last1=Backman |first1=V. |last2=Wallace |first2=M. B. |last3=Perelman |first3=L. T. |last4=Arendt |first4=J. T. |last5=Gurjar |first5=R. |last6=Müller |first6=M. G. |last7=Zhang |first7=Q. |last8=Zonios |first8=G. |last9=Kline |first9=E. |last10=McGillican |first10=T. |last11=Shapshay |first11=S. |last12=Valdez |first12=T. |last13=Badizadegan |first13=K. |last14=Crawford |first14=J. M. |last15=Fitzmaurice |first15=M. |date=July 2000 |title=Detection of preinvasive cancer cells |url=https://www.nature.com/articles/35017638 |journal=Nature |language=en |volume=406 |issue=6791 |pages=35–36 |doi=10.1038/35017638 |pmid=10894529 |bibcode=2000Natur.406...35B |s2cid=4383575 |issn=1476-4687|url-access=subscription }}</ref> * Mass spectroscopy is a historical term used to refer to mass spectrometry. The current recommendation is to use the latter term.<ref>{{cite journal | last1=Murray | first1=Kermit K. | last2=Boyd | first2=Robert K. | last3=Eberlin | first3=Marcos N. | last4=Langley | first4=G. John | last5=Li | first5=Liang | last6=Naito | first6=Yasuhide | title=Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013) | journal=Pure and Applied Chemistry | year=2013 | page=1 | issn=0033-4545 | doi=10.1351/PAC-REC-06-04-06 | volume=85 | issue=7 | url=http://www.degruyter.com/downloadpdf/j/pac.2013.85.issue-7/pac-rec-06-04-06/pac-rec-06-04-06.xml| doi-access=free }}</ref> The term "mass spectroscopy" originated in the use of phosphor screens to detect ions. * Mössbauer spectroscopy probes the properties of specific isotopic nuclei in different atomic environments by analyzing the resonant absorption of gamma rays.<ref>{{cite book | last=Nasu | first=S. | year=2013 | chapter=General Introduction to Mössbauer Spectroscopy | editor1-last=Yoshida | editor1-first=Y. | editor2-last=Langouche | editor2-first=G. | title=Mössbauer Spectroscopy | pages=1–22 | publisher=Springer | location=Berlin, Heidelberg | doi=10.1007/978-3-642-32220-4_1 | isbn=978-3-642-32219-8 }}</ref> See Mössbauer effect. * Neutron spin echo spectroscopy measures internal dynamics in proteins and other soft matter systems.<ref>{{cite book | last=Mezei | first=F. | year=2002 | chapter=Fundamentals of Neutron Spin Echo Spectroscopy | editor1-last=Mezei | editor1-first=F. | editor2-last=Pappas | editor2-first=C. | editor3-last=Gutberlet | editor3-first=T. | title=Neutron Spin Echo Spectroscopy | series=Lecture Notes in Physics | volume=601 | pages=5–14 | publisher=Springer | location=Berlin, Heidelberg | doi=10.1007/3-540-45823-9_2 | isbn=978-3-540-44293-6 }}</ref> * Nuclear quadrupole resonance is a chemical spectroscopy method mediated by NMR of the electric field gradient (EFG) in the absence of magnetic field * Perturbed angular correlation (PAC) uses radioactive nuclei as probe to study electric and magnetic fields (hyperfine interactions) in crystals (condensed matter) and bio-molecules.{{cn|date=January 2026}} * Photoacoustic spectroscopy is the measurement of the effect of absorbed electromagnetic energy on matter by means of acoustic detection.<ref>{{cite web | title=Photoacoustic Spectroscopy | website=Spectroscopy Online | first=David W. | last=Ball | date=September 2006 | volume=21 | issue=9 | url=https://www.spectroscopyonline.com/view/photoacoustic-spectroscopy | access-date=2025-11-25 }}</ref> * Acoustic emission spectroscopy is the measurement of acoustic waves as a material is deformed.<ref>{{cite journal | title=Acoustic Emission Spectroscopy: Applications in Geomaterials and Related Materials | first1=Ekhard K. H. | last1=Salje | first2=Xiang | last2=Jiang | first3=Jack | last3=Eckstein | first4=Lei | last4=Wang | journal=Applied Sciences | year=2021 | volume=11 | issue=19 | page=8801 | doi=10.3390/app11198801 | doi-access=free }}</ref> * Photoemission spectroscopy measures the energy or spin of electrons emitted from materials by the photoelectric effect.<ref>{{cite journal | title=Photoelectron spectroscopy—An overview | first1=Stefan | last1=Hüfner | first2=Stefan | last2=Schmidt | first3=Friedrich | last3=Reinert | journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | volume=547 | issue=1 | date=July 21, 2005 | pages=8–23 | publisher=Elsevier | doi=10.1016/j.nima.2005.05.008 }}</ref> ** X-ray photoelectron spectroscopy is a surface analysis technique that uses a beam of X-rays to perform photoemission spectroscopy<ref>{{cite journal | title=Introduction to x-ray photoelectron spectroscopy | first1=Fred A. | last1=Stevie | first2=Carrie L. | last2=Donley | journal=Journal of Vacuum Science & Technology A | volume=38 | article-number=063204 | year=2020 | issue=6 | doi=10.1116/6.0000412 }}</ref> * Photothermal spectroscopy measures heat evolved upon absorption of radiation.<ref>{{cite journal | title=Thermal effects in photothermal spectroscopy and photothermal imaging | first1=L. C. | last1=Aamodt | first2=J. C. | last2=Murphy | journal=Journal of Applied Physics | volume=54 | pages=581–591 | year=1983 | doi=10.1063/1.332062 }}</ref> * Pump-probe spectroscopy can use ultrafast laser pulses to measure reaction intermediates in the femtosecond timescale.<ref>{{cite book | last=Beeby | first=A. | year=2002 | chapter=Pump-Probe Laser Spectroscopy | editor1-last=Andrews | editor1-first=D. L. | editor2-last=Demidov | editor2-first=A. A. | title=An Introduction to Laser Spectroscopy | publisher=Springer | location=Boston, MA. | doi=10.1007/978-1-4615-0727-7_4 }}</ref> * Raman optical activity spectroscopy exploits Raman scattering and optical activity effects to reveal detailed information on chiral centers in molecules. * Raman spectroscopy is used to determine vibrational modes of molecules, providing a structural fingerprint by which molecules can be identified.<ref>{{cite journal | title=Chemical Applications of Raman Spectroscopy | first=Bernhard | last=Schrader | journal=Angewandte Chemie | volume=12 | issue=11 | date=November 1973 | pages=884–908 | doi=10.1002/anie.197308841 }}</ref> * Saturated spectroscopy uses a dual laser technique to remove the Doppler profiles from spectroscopic signals of hot atoms. It does so by measuring the spectral signal of atoms moving perpendicular to the laser.<ref>{{cite book | last=Letokhov | first=V. S. | year=1976 | chapter=Saturation spectroscopy | editor-last=Shimoda | editor-first=K. | title=High-Resolution Laser Spectroscopy | series=Topics in Applied Physics | volume=13 | publisher=Springer | location=Berlin, Heidelberg | doi=10.1007/3540077197_20 }}</ref> * Scanning tunneling spectroscopy uses a scanning tunneling microscope to measure the current–voltage characteristics of a surface and build a map of the electronic structure.<ref name=Zandvliet_van_Houselt_2009>{{cite journal | title=Scanning Tunneling Spectroscopy | first1=Harold J. W. | last1=Zandvliet | first2=Arie | last2=van Houselt | journal=Annual Review of Analytical Chemistry | volume=2 | date=July 2009 | pages=37–55 | doi=10.1146/annurev-anchem-060908-155213 }}</ref> * Spectrophotometry combines a spectrometer with a photometer to measure the energy intensity at different wavelengths. This can be used to determine the reflection or transmission properties of a substance.<ref>{{cite journal | title=Spectrophotometry | first=Rob | last=Morris | date=November 2, 2015 | journal=Essential Laboratory Techniques | publisher=Wiley | doi=10.1002/9780470089941.et0201s11 }}</ref> * Spin noise spectroscopy traces spontaneous fluctuations of electronic and nuclear spins.<ref>{{cite journal | doi=10.1088/0034-4885/79/10/106501 | pmid=27615689 | title=The theory of spin noise spectroscopy: a review | journal=Reports on Progress in Physics | volume=79 | issue=10 | article-number=106501 | year=2016 | first1=N. A. | last1=Sinitsyn | first2=Y. V. | last2=Pershin | bibcode=2016RPPh...79j6501S | arxiv=1603.06858| s2cid=4393400 }}</ref> * Time-resolved spectroscopy measures the decay rates of excited states using various spectroscopic methods.<ref>{{cite web | title=Time-Resolved Spectroscopy Comes of Age | first=Jennifer | last=Ouellette | work=The Industrial Physicist | pages=16–19 | publisher=Center for Chemical Dynamics, Case Western Reserve University | url=https://case.edu/artsci/chem/facilities/ccd/methods/Time_Resolved_Spectroscopy.pdf | access-date=2026-01-05 }}</ref> * Time-stretch spectroscopy<ref>{{cite journal | pages=48–51 | doi=10.1038/nphoton.2007.253 | title=Amplified wavelength–time transformation for real-time spectroscopy | journal=Nature Photonics | volume=2 | issue=1 | year=2008 | last1=Solli | first1=D. R. | last2=Chou | first2=J. | last3=Jalali | first3=B. | bibcode=2008NaPho...2...48S}}</ref><ref>{{cite journal | doi=10.1063/1.2896652 | title=Real-time spectroscopy with subgigahertz resolution using amplified dispersive Fourier transformation | journal=Applied Physics Letters | volume=92 | issue=11 | page=111102 | year=2008 | last1=Chou | first1=Jason | last2=Solli | first2=Daniel R.| last3=Jalali | first3=Bahram | bibcode=2008ApPhL..92k1102C | arxiv=0803.1654| s2cid=53056467 }}</ref> * Thermal infrared spectroscopy measures thermal radiation emitted from materials and surfaces and is used to determine the type of bonds present in a sample as well as their lattice environment. The techniques are widely used by organic chemists, mineralogists, and planetary scientists. * Transient grating spectroscopy measures quasiparticle propagation. It can track changes in metallic materials as they are irradiated. * Ultraviolet photoelectron spectroscopy * Ultraviolet–visible spectroscopy * Vibrational circular dichroism spectroscopy * Video spectroscopy

== Applications == [[File:UVES on UT2-KUEYEN.jpg|thumb|UVES is a high-resolution spectrograph on the Very Large Telescope.<ref>{{cite news | title=Media advisory: Press Conference to Announce Major Result from Brazilian Astronomers | date=20 August 2013 | url=http://www.eso.org/public/announcements/ann13066/ | access-date=21 August 2013 | newspaper=ESO Announcement | publisher=The European Southern Observatory }}</ref> ]]

There are several applications of spectroscopy in the fields of medicine, physics, chemistry, and astronomy. Taking advantage of the properties of absorbance and, with astronomy, emission, spectroscopy can be used to identify certain states of nature. The uses of spectroscopy in so many different scientific fields and for so many different applications has led to the creation of specialized subfields. Such examples include: * Determining the atomic structure of a sample<ref name="Bowers"> {{cite book | title = Sir Charles Wheatstone FRS: 1802–1875 | edition = 2nd | first = Brian | last=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> * Studying spectral emission lines of the sun and distant galaxies<ref name="Brand 57"> {{cite book | chapter=Emission and Absorption, ca. 1800–1870 | title = Lines of Light: The Sources of Dispersive Spectroscopy, 1800 – 1930 | last = Brand | first = John C. D. | publisher = Gordon and Breach Publishers | date = 1995 | isbn = 978-2-88449-162-4 | page = 57 | chapter-url=https://books.google.com/books?id=spYSK-g8DrkC&pg=PA57 }}</ref> * Space exploration<ref>{{cite journal | title=Spectroscopy from Space | first1=Roger N. | last1=Clark | first2=Gregg A. | last2=Swayze | first3=Robert | last3=Carlson | first4=Will | last4=Grundy | first5=Keith | last5=Noll | journal=Reviews in Mineralogy and Geochemistry | year=2014 | volume=78 | issue=1 | pages=399–446 | doi=10.2138/rmg.2014.78.10 }}</ref> * Cure monitoring of composites using optical fibers.<ref>{{cite conference | title=Composite Cure Monitoring With Infrared Transmitting Optical Fibers | first1=Mark A. | last1=Druy | first2=Lucy | last2=Elandjian | first3=W. A. | last3=Stevenson | series=SPIE Conference Proceedings | volume=0986 | conference=Fiber Optic Smart Structures and Skins, O-E/Fiber LASE '88, Boston, MA, United States | publisher=Society of Photo-Optical Instrumentation Engineers | date=September 6, 1988 | editor-first=Eric | editor-last=Udd | doi=10.1117/12.948895 }}</ref> * Estimating weathered wood exposure times using near infrared spectroscopy<ref>{{cite journal | journal=WTCE 2006 – 9th World Conference on Timber Engineering | url=http://www.fpl.fs.fed.us/documnts/pdf2006/fpl_2006_wang002.pdf | title=Using NIR Spectroscopy to Predict Weathered Wood Exposure Times | year=2006 | first1=Xiping | last1=Wang | first2=James P. | last2=Wacker | access-date=2009-06-22 | archive-date=2021-03-01 | archive-url=https://web.archive.org/web/20210301171119/https://www.fpl.fs.fed.us/documnts/pdf2006/fpl_2006_wang002.pdf }}</ref> * Measurement of different compounds in food samples by absorption spectroscopy both in visible and infrared spectrum<ref>{{cite journal | title=Near Infrared Spectroscopy in Natural Products Analysis | first=Daniel | last=Cozzolino | journal=Planta Medica | date=June 2009 | volume=75 | issue=7 | pages=746–756 | doi=10.1055/s-0028-1112220 | pmid=19165716 }}</ref> * Measurement of toxic compounds in blood samples<ref>{{cite journal | last1=Bakr | first1=C. A. | last2=Hussein | first2=Z. A. | title=Determination of toxic element concentrations in human blood samples using X-ray fluorescence spectroscopy | journal=International Journal of Environmental Analytical Chemistry | year=2025 | pages=1–17 | doi=10.1080/03067319.2025.2565656 }}</ref> * Non-destructive elemental analysis by X-ray fluorescence<ref>{{cite journal | title=Recent trends in quantitative aspects of microscopic X-ray fluorescence analysis | first1=Koen | last1=Janssens | first2=Wout | last2=De Nolf | first3=Geert | last3=Van Der Snickt | first4=Laszlo | last4=Vincze | first5=Bart | last5=Vekemans | first6=Roberto | last6=Terzano | first7=Frank E. | last7=Brenker | journal=Trends in Analytical Chemistry | volume=29 | issue=6 | date=June 2010 | pages=464–478 | doi=10.1016/j.trac.2010.03.003 }}</ref> * Electronic structure research with various spectroscopes<ref name=Zandvliet_van_Houselt_2009/><ref name=Dolomatov_et_al_2025/><ref>{{cite journal | title=Real-time mapping of electronic structure with single-shot two-dimensional electronic spectroscopy | first1=Elad | last1=Harel | first2=Andrew F. | last2=Fidler | first3=Gregory S. | last3=Engel | journal=Proceedings of the National Academy of Sciences | volume=107 | issue=38 | date=September 1, 2010 | pages=16444–16447 | doi=10.1073/pnas.1007579107 | jstor=20779686 }}</ref> * Redshift to determine the speed and velocity of a distant object<ref>{{cite journal | title=Optical Spectroscopy of Distant Red Galaxies | first1=Stijn | last1=Wuyts | first2=Pieter G. | last2=van Dokkum | first3=Marijn | last3=Franx | first4=Natascha M. Förster | last4=Schreiber | first5=Garth D. | last5=Illingworth | first6=Ivo | last6=Labbé | first7=Gregory | last7=Rudnick | journal=The Astrophysical Journal | volume=706 | issue=1 | page=885 | year=2009 | doi=10.1088/0004-637X/706/1/885 | arxiv=0910.1836 }}</ref> * Determining the metabolic structure of a muscle * Monitoring dissolved oxygen content in freshwater and marine ecosystems<ref>{{cite journal | title=Review of Dissolved Oxygen Detection Technology: From Laboratory Analysis to Online Intelligent Detection | first1=Yaoguang | last1=Wei | first2=Yisha | last2=Jiao | first3=Dong | last3=An | first4=Daoliang | last4=Li | first5=Wenshu | last5=Li | first6=Qiong | last6=Wei | journal=Sensors | date=September 16, 2019 | volume=19 | issue=18 | article-number=3995 | doi=10.3390/s19183995 | doi-access=free | pmc=6767127 | pmid=31527482 }}</ref> * Altering the structure of drugs to improve effectiveness * Characterization of proteins * Respiratory gas analysis in hospitals<ref name="auto"/> * Finding the physical properties of a distant star or nearby exoplanet using the Relativistic Doppler effect.<ref>{{Cite journal | bibcode=1968JRASC..62..105S | title=The Relativistic Doppler Effect | last1=Sher | first1=D. | journal=Journal of the Royal Astronomical Society of Canada | year=1968 | volume=62 | page=105 }}</ref> * In-ovo sexing: spectroscopy allows to determine the sex of the egg while it is hatching. Developed by French and German companies, both countries decided to ban chick culling, mostly done through a macerator, in 2022.<ref>{{Cite web | first=Clément | last=Vérité | title=Germany and France Will Stop Chick Culling | date=22 July 2021 | website=Newsendip| url=https://www.newsendip.com/germany-and-france-will-stop-chick-culling/ | access-date=2025-11-24 }}</ref> * Process monitoring in Industrial process control<ref name="grauluque2021">{{cite journal |last1=Grau-Luque |first1=Enric |last2=Guc |first2=Maxim |last3=Becerril-Romero |first3=Ignacio |last4=Izquierdo-Roca |first4=Víctor |last5=Pérez-Rodríguez |first5=Alejandro |last6=Bolt |first6=Pieter |last7=Van den Bruele |first7=Fieke |last8=Ruhle |first8=Ulfert |title=Thickness evaluation of AlO x barrier layers for encapsulation of flexible PV modules in industrial environments by normal reflectance and machine learning |journal=Progress in Photovoltaics: Research and Applications |date=March 2022 |volume=30 |issue=3 |pages=229–239 |doi=10.1002/pip.3478 |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/pip.3478 |language=en |issn=1062-7995|url-access=subscription }}</ref>

== History == {{Main|History of spectroscopy}}

The history of spectroscopy began with Isaac Newton's optics experiments (1666–1672). According to Andrew Fraknoi and David Morrison, "In 1672, in the first paper that he submitted to the Royal Society, Isaac Newton described an experiment in which he permitted sunlight to pass through a small hole and then through a prism. Newton found that sunlight, which looks white to us, is actually made up of a mixture of all the colors of the rainbow."<ref name="open-astro">{{cite web | first1=Andrew | last1=Fraknoi | author1-link=Andrew Fraknoi | first2=David | last2=Morrison | author2-link=David Morrison (astrophysicist) | date=October 13, 2016 | title=Astronomy | website=OpenStax | url=http://cnx.org/content/col11992/latest/ | access-date=2025-11-24 }}</ref> Newton applied the word "spectrum" to describe the rainbow of colors that combine to form white light and that are revealed when the white light is passed through a prism.

Fraknoi and Morrison state that "In 1802, William Hyde Wollaston built an improved spectrometer that included a lens to focus the Sun's spectrum on a screen. Upon use, Wollaston realized that the colors were not spread uniformly, but instead had missing patches of colors, which appeared as dark bands in the spectrum."<ref name="open-astro" /> During the early 1800s, Joseph von Fraunhofer made experimental advances with dispersive spectrometers that enabled spectroscopy to become a more precise and quantitative scientific technique. Since then, spectroscopy has played and continues to play a significant role in chemistry, physics, and astronomy. Per Fraknoi and Morrison, "Later, in 1815, German physicist Joseph Fraunhofer examined the solar spectrum, and found about 600 such dark lines (missing colors), are now known as Fraunhofer lines, or Absorption lines."<ref name="open-astro" />{{Better source needed|date=November 2020}}

Spectra of atoms and molecules often consist of a series of spectral lines, each one representing a resonance between two different quantum states. The explanation of these series, and the spectral patterns associated with them, were one of the experimental enigmas that drove the development and acceptance of quantum mechanics. The hydrogen spectral series in particular was first successfully explained by the Rutherford–Bohr quantum model of the hydrogen atom. In some cases spectral lines are well separated and distinguishable, but spectral lines can overlap and appear to be a single transition if the density of energy states is high enough. Named series of lines include the principal, sharp, diffuse and fundamental series.

== Hobbyist == Spectroscopy has emerged as a growing practice within the maker movement, enabling hobbyists and educators to construct functional spectrometers using readily available materials.<ref>{{Cite web |title=DIY Webcam Diffraction Grating Spectrometer |url=https://physicsopenlab.org/2015/11/26/webcam-diffraction-grating-spectrometer/ |access-date=2025-03-04 |website=PhysicsOpenLab |language=en-US}}</ref> Utilizing components like CD/DVD diffraction gratings, smartphones, and 3D-printed parts, these instruments offer a hands-on approach to understanding light and matter interactions. Smartphone applications<ref>{{Cite web |date=27 February 2025 |title=Spectroscope |url=https://apps.apple.com/us/app/spectroscope/id6741684078 |access-date=2025-03-04 |website=App Store |language=en-US}}</ref><ref>{{Cite web |title=Spectroscope - Apps on Google Play |url=https://play.google.com/store/apps/details?id=spectroscope.spectroscope |access-date=2025-03-04 |website=play.google.com |language=en-US}}</ref> along with open-source tools<ref>{{Cite web |last=Wright |first=Les |title=leswright1977/PySpectrometer2 |website=GitHub |date=16 February 2025 |url=https://github.com/leswright1977/PySpectrometer2 |access-date=2025-03-04}}</ref> facilitate integration, greatly simplify the capturing and analysis of spectral data. While limitations in resolution, calibration accuracy, and stray light management exist compared to professional equipment, DIY spectroscopy provides valuable educational experiences<ref>{{Cite web |title=Project Spectra! |url=https://lasp.colorado.edu/information/k-12-educators/project-spectra/ |access-date=2025-03-04 |website=Laboratory for Atmospheric and Space Physics |language=en-US}}</ref> and contributes to citizen science initiatives, fostering accessibility to spectroscopic techniques.

== References == {{reflist|30em}}

== Further reading == * {{cite book|doi=10.1002/0470027320|title=Handbook of Vibrational Spectroscopy|year=2006|isbn=978-0-471-98847-2|editor1=John M. Chalmers|editor2= Peter Griffiths|publisher=Wiley|location= New York}} * {{cite book|url=https://books.google.com/books?id=OzAnX25h4soC&pg=PR4 |title=Applied Spectroscopy|isbn=978-0-08-052749-9|editor1=Jerry Workman|date=1998|editor2=Art Springsteen|publisher=Academic Press|location =Boston}} * {{cite book|author=Peter M. Skrabal|url=https://vdf.ch/index.php?route=product/search&search=skrabal |title=Spectroscopy - An interdisciplinary integral description of spectroscopy from UV to NMR|format= e-book|date= 2012|publisher= vdf Hochschulverlag AG |location= ETH Zurich|isbn= 978-3-7281-3385-4|doi=10.3218/3385-4|s2cid = 244026324}}

== External links == {{Wikiquote}} {{Wiktionary}} {{Commons category}}  * [http://nist.gov/pml/data/atomspec.cfm NIST Atomic Spectroscopy Databases] * [http://web.mit.edu/spectroscopy/history/spec-history.html MIT Spectroscopy Lab's History of Spectroscopy] * [https://archive.today/20130418233543/http://spectroscopyonline.findanalytichem.com/spectroscopy/article/articleDetail.jsp?id=381944&sk=&date=&pageID=1 Timeline of Spectroscopy] * [https://hubblesite.org/contents/articles/spectroscopy-reading-the-rainbow Spectroscopy: Reading the Rainbow]

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