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FundamentalsofRamanscattering
spectroscopy
KamillaMalek,MałgorzataBarańska,KamilaKochan
Ramanscatteringspectroscopyisbasedontransitionsbetweenvibrationaland
rotationallevelsofmolecules,whichoccurasaresultofinelasticlightscattering.
ThisefectwasdescribedtheoreticallybyA.Smékalin1923andconfirmedexperi-
mentallybyanIndianphysicistChandrashekharV.Ramanin1928.Forthisdis-
coveryhewasawardedwiththeNobelPrizeinphysicsin1930.Thesameefect
oflightscatteringisobservedinthemacro-andmicroscopicworld.
Onecanimagineitlikescatteringoflightbycolloidalsolutioniftheparti-
clesizeissimilartothewavelengthoftheincidentlight.Lightscatteringisalso
observedonenergylevelsofmolecules,andwillbeexplainedforthevibration-
allevels.Fundamentalphenomenonofthisspectroscopyistheexcitationofmo-
lecularvibrationsthroughilluminationofasamplebylightwithfrequencyofν
0
fromUV
,VisorNIRregion.Quantumdescriptionofmolecularvibrationsisgiven
inChapter1.Aresultofthe“molecule-electromagneticwave”interactionsis
scatteredlightwiththesameenergy(Rayleighscattering)anddiscretecompo-
nentswithfrequencydiferentthanν
0(Ramanscattering).Thus,Ramanspec-
trumconsistsoflinesevenlylocatedaroundaRayleighbandwithfrequencies
ν
0
±ν
osc
whereν
osc
correspondstothetransitionfrequencybetweenvibrational
levels.AmagnitudeoftheRamanshiftfromtheRayleighbanddoesnotdependon
frequencyoftheincidentradiation,butresultssolelyfrompropertiesofscattering
molecules.Ramanbandsappearingatthefrequencylowerthanν
0
arecalledStokes
bands,whereasthoseatthehigherfrequencyareanti-Stokesbands.Lightscatter-
ingisaveryweakefectandthisisreflectedbylowintensityofbands.Intensities
ofRayleighandRamanscatteringisapproximatelythousandandmilliontimeslow-
er,respectively,thanintensityoftheexcitinglight.MostofRamanspectroscopy
techniquesemploymeasurementsoftheStokespartofRamanspectra,presented
inascaleofwavenumbers(expressedinΔcm
-1
,so-calledRamanshift)inthespec-
tralregionfrom0to4000cm
-1
.Ramanspectrumexpressedinthatscalecorres-
pondstoIRspectrum,whatenablestheircomparisonsincebothtechniquesare
complementarytoeachother.Ramanspectra,similarlytoIRspectra,primarily
provideencryptedinformationaboutmolecularstructure.Theyareusedforiden-
tificationandquantificationofunknownsubstancesandspecificgroupsofatoms,