Editing Raman spectroscopy (section)

== Instrumentation ==
[[File:1928 Benzene Raman Spectrum.png|thumb|An early Raman spectrum of [[benzene]] published by Raman and Krishnan.<ref>{{Cite journal|last1=K. S. Krishnan|last2=Raman|first2=C. V.|date=1928|title=The Negative Absorption of Radiation|journal=Nature|volume=122|issue=3062|pages=12–13|doi=10.1038/122012b0|issn=1476-4687|bibcode=1928Natur.122...12R|s2cid=4071281}}</ref>]]
[[File:Setup Raman Spectroscopy adapted from Thomas Schmid and Petra Dariz in Heritage 2(2) (2019) 1662-1683.png|thumb|Schematic of one possible dispersive Raman spectroscopy setup.<ref name="Heritage">{{cite journal |author=Thomas Schmid |author2=Petra Dariz |title=Raman Microspectroscopic Imaging of Binder Remnants in Historical Mortars Reveals Processing Conditions |journal=Heritage |volume=2 |issue=2 |pages=1662–1683 |date=2019 |doi=10.3390/heritage2020102 |issn=2571-9408  |doi-access=free }}</ref>]]
Modern Raman spectroscopy nearly always involves the use of [[laser]]s as excitation light sources. Because lasers were not available until more than three decades after the discovery of the effect, Raman and Krishnan used a [[mercury lamp]] and [[photographic plate]]s to record spectra. Early spectra took hours or even days to acquire due to weak light sources, poor sensitivity of the detectors and the weak Raman scattering cross-sections of most materials. Various colored filters and chemical solutions were used to select certain wavelength regions for excitation and detection but the photographic spectra were still dominated by a broad center line corresponding to Rayleigh scattering of the excitation source.<ref name="Long" />

Technological advances have made Raman spectroscopy much more sensitive, particularly since the 1980s. The most common modern detectors are now [[charge-coupled devices]] (CCDs). [[Photodiode array]]s and [[photomultiplier tube]]s were common prior to the adoption of CCDs. The advent of reliable, stable, inexpensive lasers with narrow bandwidths has also had an impact.<ref name="McCreery">{{Cite book|title=Raman spectroscopy for chemical analysis|last=McCreery, Richard L.|date=2000|publisher=John Wiley & Sons|isbn=0471231878|location=New York|oclc=58463983}}</ref>

=== Lasers ===
Raman spectroscopy requires a light source such as a laser. The resolution of the spectrum relies on the bandwidth of the laser source used.<ref name="MathiesFSRS">{{Cite journal|last1=Kukura|first1=Philipp|last2=McCamant|first2=David W.|last3=Mathies|first3=Richard A.|date=2007|title=Femtosecond Stimulated Raman Spectroscopy|journal=Annual Review of Physical Chemistry|volume=58|issue=1|pages=461–488|doi=10.1146/annurev.physchem.58.032806.104456|pmid=17105414|issn=0066-426X|bibcode=2007ARPC...58..461K}}</ref> Generally shorter wavelength lasers give stronger Raman scattering due to the {{mvar|ν}}<sup>4</sup> increase in Raman scattering cross-sections, but issues with sample degradation or fluorescence may result.<ref name="McCreery" />

[[Continuous wave]] lasers are most common for normal Raman spectroscopy, but [[pulsed laser]]s may also be used. These often have wider bandwidths than their CW counterparts but are very useful for other forms of Raman spectroscopy such as transient, time-resolved and resonance Raman.<ref name="MathiesFSRS" /><ref name="KCG">{{Cite journal|last1=Elliott|first1=Anastasia B. S.|last2=Horvath|first2=Raphael|last3=Gordon|first3=Keith C.|date=2012|title=Vibrational spectroscopy as a probe of molecule-based devices|journal=Chem. Soc. Rev.|volume=41|issue=5|pages=1929–1946|doi=10.1039/C1CS15208D|pmid=22008975|issn=0306-0012}}</ref>

=== Detectors ===
Raman scattered light is typically collected and either dispersed by a [[spectrograph]] or used with an [[interferometer]] for detection by Fourier Transform (FT) methods. In many cases commercially available FT-IR spectrometers can be modified to become FT-Raman spectrometers.<ref name="McCreery" />

==== Detectors for dispersive Raman ====
In most cases, modern Raman spectrometers use array detectors such as CCDs. Various types of CCDs exist which are optimized for different wavelength ranges. [[Intensified CCD]]s can be used for very weak signals and/or pulsed lasers.<ref name="McCreery" /><ref>{{Cite journal|last1=Efremov|first1=Evtim V.|last2=Buijs|first2=Joost B.|last3=Gooijer|first3=Cees|last4=Ariese|first4=Freek|date=2007|title=Fluorescence Rejection in Resonance Raman Spectroscopy Using a Picosecond-Gated Intensified Charge-Coupled Device Camera|journal=Applied Spectroscopy|volume=61|issue=6|pages=571–578|doi=10.1366/000370207781269873|pmid=17650366|issn=0003-7028|bibcode=2007ApSpe..61..571E|s2cid=45754275}}</ref>
The spectral range depends on the size of the CCD and the focal length of spectrograph used.<ref>{{Cite web|url=https://www.princetoninstruments.com/calculators/grating-dispersion.cfm|title=Grating Dispersion/Resolution Calculator|website=princetoninstruments.com|access-date=2019-07-22}}</ref>
 
It was once common to use [[monochromator]]s coupled to photomultiplier tubes. In this case the monochromator would need to be moved in order to scan through a spectral range.<ref name="McCreery" />

==== Detectors for FT–Raman ====
FT–Raman is almost always used with NIR lasers and appropriate detectors must be used depending on the exciting wavelength. [[Germanium detector|Germanium]] or [[Indium gallium arsenide]] (InGaAs) detectors are commonly used.<ref name="McCreery" />

=== Filters ===
It is usually necessary to separate the Raman scattered light from the Rayleigh signal and reflected laser signal in order to collect high quality Raman spectra using a laser rejection filter. [[Notch filter|Notch]] or [[long-pass filter|long-pass]] optical filters are typically used for this purpose. Before the advent of holographic filters it was common to use a triple-grating monochromator in subtractive mode to isolate the desired signal.<ref name="McCreery" /> This may still be used to record very small Raman shifts as holographic filters typically reflect some of the low frequency bands in addition to the unshifted laser light. However, [[Volume hologram]] filters are becoming more common which allow shifts as low as 5&nbsp;cm<sup>−1</sup> to be observed.<ref>{{Cite journal|
url=http://www.spectroscopyonline.com/investigating-crystallinity-using-low-frequency-raman-spectroscopy-applications-pharmaceutical-analy|
title=Investigating Crystallinity Using Low Frequency Raman Spectroscopy: Applications in Pharmaceutical Analysis|last=Gordon|first=Geoffrey P. S. Smith Gregory S. Huff Keith C.|journal=Spectroscopy|
series=Spectroscopy-02-01-2016|
date=February 2016|
volume=31|
issue=2|
pages=42–50|access-date=2019-07-21}}</ref><ref>{{Cite web|url=https://optigrate.com/BragGrate_Bandpass.html|title=BragGrate- Bandpass ASE Suppression Filters|website=optigrate.com|access-date=2019-07-21}}</ref><ref>{{Cite web|url=https://www.coherent.com/lasers/laser/sureblock-notch-filter|title=SureBlock- Ultra Narrow-band Notch Filters|website=coherent.com|access-date=2021-03-25}}</ref>