UNIT 4 RAMAN SP SPECTROSCOPY
Theo Theory ry of of Ram Raman Spe Spect ctro rosc scop opy y Quantum or Partice Theory !assica or "ave Theory Raman #ctivity of $ibrations Rue of %utua &'cusion (epoarisation Ratio
Instr Instrum umen entat tatio ion n for Ram Raman an Spec Spectr trosc oscop opy y Radiation Sources for Raman Spectroscopy Sampe *andin+ (evices Transducers or (etectors
&nhanc &nhanceme ement nt of Raman Raman Spectr Spectra a Intens Intensitie itiess Resonance Raman Spectroscopy !oherent #nti,Sto-es Raman Spectroscopy Surface &nhanced Raman Scatterin+
4. 4. 4./ 4.0 4.
#pp #ppic icat atio ions ns of of Ram Raman an Spe Spect ctro rosc scop opy y Summary Termin rmina a Ques Questi tio ons #nsers
In the previous unit you earnt about IR spectrometry in terms of its principe3 instrumentation and appications. ou ou oud reca that the IR spectrum is a conse5uence of transitions amon+st the 5uantised vibrationa ener+y eves of the moecues on absorbin+ IR radiation. In this unit you oud earn about Raman Raman spectroscopy hich aso invoves same ener+y eves but it differs on many other counts. The nature of radiation used3 the mechanism of interaction beteen the radiation and matter3 the necessary condition for the interaction3 the seection rues3 the t he sensitivity3 re5uired instrumentation and the resutin+ spectra are different in case of Raman spectroscopy. 6urther3 6urther3 even the information avaiabe from it is different7 in fact it is compimentary to the one avaiabe from IR spectroscopy. #s in the previous unit3 e sha be+in by understandin+ the theory behind Raman spectroscopy in terms of the ori+in of the spectrum and its characteristic features. It i be fooed by an account of the essentia components of Raman spectrometers. Thereafter e sha ta-e up the appications of Raman spectrometry in diverse areas. In the ne't boc- you i earn about spectrometric methods based on moecuar fuorescence and moecuar phosphorescence hich are i mportant spectroscopic method of anaysis.
Obect!"es #fter studyin+ this unit3 you shoud be abe to8 •
define Raman effect3
e'pain the ori+in of Rayei+h scatterin+3
describe Raman effect in terms of cassica ave theory and 5uantum theory of radiation3
define Sto-es and anti,Sto-es ines and e'pain their ori+in3 )
Mo&ecu&ar Spectroscop!c Met(os-I
compare and contrast Raman spectra ith IR spectra3
state the rue of mutua e'cusion and and e'pain its
enist the essentia components of the instrument re5uired for Raman spectroscopy3
describe advanta+es of Raman spectroscopy3 and
enumerate and discuss various various appications of Raman spectroscopy. spectroscopy.
T$EO T$ EOR RY O% RAMA RAMAN N SPEC SPECTR TROS OSCO COPY PY
#n eectroma+netic radiation hen passed throu+h a transparent medium interacts ith the partices 9e.+.3 moecues3 atoms3 or ions: constitutin+ it. If the dimensions of the partices are e5ua to or smaer than that of its aveen+th then it under+oes scatterin+. It has been observed that most of the scattered radiation has e'acty the same aveen+th as that of the incident radiation. Such a scatterin+ is referred to as Ray&e!'( scatter!n'. *oever3 a very sma fraction 9to the tune of about 1 in 1; 0: of the scattered radiation is found to have a aveen+th different from that of the incident radiation. This is caed Raman scatter!n' and the e'istence of Raman scatterin+ is caed Raman e))ect. 6i+ 4.1 +ives the spectra of the scattered radiation obtained hen a sampe of !!4 as interacted ith a aser beam havin+ a aveen+th of !.$. Raman ‒ the discoverer of Raman &ffect in 1e.
%!'. 4.1* Raman spectra )or CC&4 us!n' 4++ nm &aser source
ou ou may note that the intense pea- in the centre of the spectrum has the same aveen+th 9or avenumber: as that of the incident radiation. This ?i-e IR spectrum3 the Raman is the Ray&e!'( pea, and and the other si+nas on either side of the spectra are aso +iven in Rayei+h pea- are the Raman &!nes. The ines to the eft of avenumber units. This is Rayei+h pea- and havin+ oer vaue of avenumber are caed convenient because the Sto,es &!nes hie the ones to the ri+ht and havin+ hi+her vaue of position of other scattered avenumber are caed ant!-Sto,es &!nes. Sto-es ines are at oer radiations 9Raman scatterin+: ener+y hie the anti,Sto-es ines are at ener+y +reater than the can be convenienty e'pressed in terms of Raman Rayei+h pea-. shifts.
The positions of Raman ines are e'pressed in terms of Raman s(!)t 3 ∆ν hich is defined as per the fooin+ e5uation.
∆ ν = 9 ν s − ν ; : cm −1
"here3 ν s and ν ; are the avenumbers of the source 9or incident: radiation and the observed scattered ines respectivey. It is obvious that the Raman shifts of the Sto-es ines oud be positive hie for anti,Sto-es ines3 these oud be ne+ative. There are to more features in the spectrum +iven above. ?ocate and rite them in the space provided beo. @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... "e are are sure that you coud notice that3 firsty3 the Sto-es and anti,Sto-es ines are e5uidistant from the Rayei+h ine and secondy3 the Sto-es ines are much more intense as compared to the anti,Sto-es ines. ?et us understand the mechanism of ori+in of the Raman ines3 the reasons for the Sto-es and anti,Sto-es ines bein+ e5uidistant from Rayei+h ine and their reative intensities. There are to theories of Raman effect. These are as foos8 •
Quantum or partice theory
!assica or ave theory
?et us discuss these one by one.
4.#. 4.#.1 1
/uan /u antu tum m or Part Part!c !c&e &e T(e T(eor ory y
ou ou -no that accordin+ to the 5uantum theory of radiation3 an eectroma+netic radiation is considered to be consistin+ of a stream of partices caed photons. The photons constitutin+ a radiation of fre5uency fre5uency ν oud have ener+y e5ua to hν here h is the Panc-As constant. The interaction of the radiation ith the interactin+ species can be visuaised in terms of coisions beteen them and the photons. If the coisions are e&ast!c 9i.e.3 they do not invove any e'chan+e of ener+y: the photons oud be scattered 9defected: ith their incident fre5uency remainin+ unchan+ed. This e'pains the observance of Rayei+h ine in the spectrum. Since most of the coisions are eastic in nature3 most of the scatt ered photons oud have same fre5uency as the incident fre5uency. Therefore the Rayei+h ine is observed to be 5uite intense. # sma fraction of the coisions beteen the photons and partices of matter is found to be !ne&ast!c in nature i.e.3 these invove e'chan+e of ener+y. "hen "hen the photons constitutin+ the radiation under+o ineastic coisions ith the absorbin+ species3 they either +ain or ose ener+y. ener+y. These ener+y e'chan+es brin+ about transitions in the 5uantised ener+y eves of the moecues. In such an event of ineastic coisions3 the moecues are either vibrationay 9andBor rotationay: e'cited or they may under+o vibrationa 9andBor rotationa: rea'ation. In both the cases the photons +et scattered
E&ast!c co&&!s!ons* The coisions that do not invove any e'chan+e of ener+y.
Ine&ast!c co&&!s!ons* The coisions that invove e'chan+e of ener+y.
Mo&ecu&ar Spectroscop!c Met(os-I
ith a fre5uency different from their initia fre5uency. fre5uency. In former case i.e.3 hen the moecues under+o e'citation3 they are of oer fre5uency. fre5uency. In the ater case3 here the moecues under+o rea'ation3 the scattered photons are of a hi+her fre5uency. These scattered photons +ive rise3 respectivey to the Sto,es and ant!-Sto,es Raman &!nes in the spectrum.
?et us understand the mechanism of Raman and Rayei+h scatterin+ ith the hep of an ener+y eve dia+ram. 6i+. 4.2 +ives a schematic ener+y eve dia+ram of a moecue. The set of ines at the bottom end represent the eectronic +round state and the correspondin+ vibrationa ener+y eves. Simiary the set of ines at the top end represent the first eectronic e'cited state and the correspondin+ vibrationa ener+y eves. The re+ion of continuum in the midde represents virtua states hose ener+ies are not 5uantised.
%!'. 4.#* Sc(emat!c representat!on o) t(e ener'y c(an'es assoc!ate 0!t( t(e ec!tat!on2 Ray&e!'( scatter!n' an Raman scatter!n'
"hen a radiation3 havin+ a aveen+th that is different from any of the absorption ma'ima of the moecue3 interacts ith the moecue7 the ener+y of the moecue increases by an amount e5ua to hν . The ener+y chan+e can be represented by the upard arro 9bac-: to the far eft in the fi+ure. This su++ests that the moecue in the oest vibrationa state of the eectronic +round state +ets e'cited to a virtua state. The second arro from the ‒ eft represents a situation in hich the incident photon photon 9of same ener+y: encounters the moecue in its first vibrationay e'cited state and accordin+y the virtua state has hi+her ener+y. ener+y. #s the virtua state is e'tremey short , ‒ 14 ived 9of the order of 1; seconds:3 the moecue soon drops bac- don to its +round state3 reeasin+ a photon. The ne't to don arros 9purpe: indicate these processes in hich there is no transfer of ener+y and the scattered photon has the same fre5uency as that of the incident photon. This e'pains the Rayei+h scatterin+. The ne't to don arros 9+reen and bue: represent the situations hich arise in case of ineastic coisions. The first 9+reen: of these corresponds to the rea'ation from the virtua state that as ac5uired by the moecue in the eectronicay as e as vibrationay +round state by absorbin+ the incident photon. In this case3 the moecue does not rea' bac- to the vibrationa +round state7 instead it comes don to a vibrationay e'cited state. In other ords the ineastic coision has brou+ht in a vibrationa e'citation in the moecue. The scattered photon oud be of oer fre5uency i.e.3 it i +ive a Sto-es ine. /
The arro on the e'treme ri+ht 9bue: represents the situation herein the moecue in the virtua state obtained by absorption of incident photon by the moecue in the vibrationa e'cited state of the +round eectronic state reeases a photon. On reeasin+ the photon the moecue comes don to the vibrationa +round state of the +round eectronic state. The overa process of e'citation and rea'ation is e5uivaent to the vibrationa rea'ation of a moecue as a conse5uence of the ineastic coision. The ener+y of the scattered photon is +reater than that of the incident photon7 this +ives rise to anti,Sto-es ine.
The most i-ey event in these interactions is t he ener+y absorption and re,emission by the moecues in the +round state. Therefore3 the Rayei+h ines are the most intense. 6urther3 since the virtua state from hich the Sto-es ine ori+inates is more popuated than the one for anti,Sto-es ines3 the Sto-es ines are more intense than the anti,Sto-es ines 96i+. 4.1:. Cefore proceedin+ to the cassica theory3 theory3 try to anser the t he fooin+ S#Qs.
SA/ 1 !acuate the avenumber 9in cm ‒ 1: of the radiation radiation havin+ a aveen+th aveen+th of 4 nm. @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@...
SA/ # "hat is Raman shiftD The Raman shift positions of Sto-es and anti,Sto-es ines are e5ua but opposite. !omment. @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@...
4.#. 4.#.# #
C&as C&ass! s!ca ca&& or 3a"e T(e T(eor ory y
The concept of the poarisabiity of a moecue is fundamenta to the understandin+ of the cassica theory of Raman effect. ?et us first understand it before e move to the e'panation of the Raman effect. Po&ar!sab!&!ty o) mo&ecu&es
"hen a moecue is paced in a static eectric fied3 the positive and ne+ativey char+ed partices constitutin+ the moecue +et attracted to the opposite poes poes of the appied fied. This eads to a char+e separation and conse5uenty to the deveopment of an induced dipoe moment. In simpe ords3 e say that the moecue has become poarised. This tendency of the moecue moecue to +et poarised is caed po&ar!sab!&!ty i.e.3 the abiity to +et poarised. The ma+nitude of the induced dipoe moment is proportiona to the stren+th of the appied appied fied and is +iven by the fooin+ e5uation. e5uation. µ = α E
The proportionaity constant α is caed polarisability. caed polarisability. It is a measure of the ease ith hich the moecue 9or a bond in it: can +et poarised. # ea-er bond ith
The poarisabiity 9E: of the moecue depends on the bond en+th7 shorter bonds0 bein+ difficut to poarise poarise than on+er bonds.
Mo&ecu&ar Spectroscop!c Met(os-I
oosey hed eectrons is more poarisabe7 a sin+e bond is more poarisabe than a doube bond beteen beteen the same set of atoms. 6urther3 the poarisabiity can be anisotropic3 that is the eectrons of a bond may be poarised to different e'tents hen the fied is appied in different directions. 6or e'ampe3 the hydro+en h ydro+en moecue +ets poarised to different e'tents hen the fied is appied in the direction of the bond or in the direction perpendicuar to it3 as shon in 6i+. 4.).
%!'.4.* Sc(emat!c representat!on o) t(e an!sotrop!c po&ar!sab!&!ty o) a mo&ecu&e
"hat oud happen if e pace a moecue in an osciatin+ eectric fied i-e that of an eectroma+netic radiationD ?et us see. The eectric fied associated ith a radiation havin+ a fre5uency fooin+ e5uation. E = E ;
cos92πν ext :
varies as per the
here E here E is the eectrica eectrica fied stren+th stren+th at any time t and E and E ; is the ampitude of the ave. "hen such a radiation is made to interact ith a moecue it oud induce a dipoe moment in it. 6urther3 as the eectrica fied of the radiation is osciatin+ the resutin+ induced dipoe moment oud aso osciate at the same fre5uency as that of the radiation. That is3 µ = α E = α E ; cos9 2πν ext :
This osciatin+ dipoe oud immediatey +enerate a radiation of the fre5uency3 and e have a ready e'panation for Rayei+h scatterin+.
#s you are aare3 the moecues are never stationary and aays under+o vibration 9and may be rotation: motions. These motions may cause a chan+e in the poarisabiity of the moecue. 6or e'ampe3 in the process of a vibration the bond en+th of a bond in the moecue -eeps varyin+ and as the poarisabiity depends on the stren+th of the bond3 it may chan+e the poarisabiity of the the moecue. It can be shon 9see the bo' +iven beo: that hen such a system vibratin+ ith a fre5uency of ν vib interacts ith the osciatory eectric fied of the eectroma+netic radiation then the scattered radiation may have three different components. One of these has fre5uency e5ua to to the incident fre5uency 9 ν ex : hie the fre5uencies of the other to are e5ua to 9ν ex + ν vib : and 9ν ex − ν vib : respectivey. The The first component ith a fre5uency of is the reason for the Rayei+h scatterin+. The other to components are the sources of the anti,Sto-es and Sto-es ines respectivey.
Interact!on Interact!on o) osc!&&atory e&ectr!c )!e& 0!t( "!brat!n' mo&ecu&e m o&ecu&e
Suppose a bond ith an e5uiibrium bond en+th of r 0 and e5uiibrium bond poarisabiity of α ; is vibratin+ ith a vibrationa fre5uency e5ua to ν vib . If the rate of chan+e in the poarisabiity ith the chan+e in bond en+th r 3 durin+ vibration is 9∂α B ∂r : 3 then the poarisabiity of the bond as a function of the chan+e in bond en+th is +iven as
µ = α ; + 9∂α B ∂r :r ; cos92πν vibt :
Substitutin+ &5. 94#: in &5. 94.): e +et
µ = Gα ; + 9∂α B ∂r :r ; cos9 2πν vibt :F E ; cos 2πν ex t
&5. 94C: on e'pansion +ives3
µ = α ; E ; cos 2πν ext + 9∂α B ∂r :r ; E ; cos92πν vibt : cos9 2πν ex t : @ 94!:
In order to simpify &5.94!:3 e can ma-e use of an identity from tri+onometry3 i.e.3
cos A cos B =
1 Gcos9 A + B : + cos9 A − B :F 2
Substitutin+ this identity in &+. 94!: e +et3
µ = α ;& ; cos 2π ν e' t + 9∂α B ∂r :r ;
cosG 2π9 ν e' + ν vib :t F + 9∂α B ∂r : r ;
cosG 2π9 ν e' − ν vib :t F
@ 94&: The three terms in the e5uation su++est for the presence of three different components of the osciatin+ eectrica dipoe moment. #ccordin+y3 #ccordin+y3 there are three components in in the scattered radiation. These osciate ith the fre5uency e5ua to ν ex 3 9ν ex + ν vib : and 9ν ex − ν vib : respectivey. In other ords e may say that the incident fre5uency of the radiation has been moduated by the vibration fre5uency as
± ν vib .
If 9∂α B ∂r : is e5ua to >ero then &5. 94&: reduces to µ = α ; E ; cos 2πν ext hich means that ony Rayei+h scatterin+ is observed.
The three components in the scattered radiation are observed ony hen the vibration causes a chan+e in the poarisabiity. If the poarisabiity does not chan+e in the course of vibration ony Rayei+h scatterin+ is observed. Therefore3 e can concude that in order to observe Sto-es and anti,Sto-es ines the poarisabiity of the bond must chan+e in the course of vibration.
The necessary condition to observe vibration Raman spectrum is that the poarisabiity of the bond must chan+e in the course of vibration
?et us raise a 5uestion. Since both the IR and Raman spectra invove transitions amon+st the same vibrationa eves3 oud the to spectra be the sameD *oever3 before proceedin+ further hy donAt you assess your understandin+ of the meanin+ and the e'panation of the Raman effect by anserin+ the fooin+ S#Q.
Mo&ecu&ar Spectroscop!c Met(os-I
SA/ 6i up the ban-s in the fooin+.
"hen "hen the the cois coision ionss bete beteen en the photon photonss and and moe moecu cues es are are @@@. @@@.33 the the photons oud [email protected]
@@@..ith the same fre5uency. fre5uency.
If in the event of an ineastic coision ith photons photons the moecues +et vibrationay e'cited3 the fre5uency of the scattered photon i be @@@@.than the incident photon.
In order order to to obse observe rve Raman Raman scat scatter terin+ in+ the @@@@@ @@@@@ of of the the bond bond must must chan+ chan+ee in the course of vibration.
Raman Raman Act!" Act!"!ty !ty o) 5!brat! !brat!ons ons
#s a first response3 the anser to the 5uestion about the simiarity of IR and Raman spectra raised above3 coud be in a firm affirmative or one may respond3 Hmay beA. *oever in actua practice the to spectra are +eneray not identica and in case of a number of moecues3 the spectra in fact do not have anythin+ in common 9Sec. 4.2.4:. This is so because the very mechanisms by hich t he to spectra arise are different. This feature actuay is an important indicator about the symmetry of the moecue. The Raman and IR spectra for a simpe moecue !! 4 is +iven in 6i+. 4.4. ou may note that ony some of the si+nas are common in the to s pectra and aso the common si+nas are of different reative intensities.
%!'. 4.4* IR an Raman spectra o) CC& 46 note t(e s!m!&ar!t!es an t(e !))erences
ou ou oud reca from t he previous unit on IR spectrometry s pectrometry that a the possibe possi be vibrationa modes of a moecue may not be IR active. Ony those modes hich have an associated chan+e in dipoe moment are IR active and sho up in the spectrum hie the other modes are absent. #s mentioned above3 in Raman spectrum ony those vibrationa modes are active hich are associated ith a chan+e in poarisabiity. (ue to this difference in the re5uirements for shoin+ the to types of spectra3 the IR and Raman activities of the vibration modes may differ from each other. The Raman activity of different vibrationa modes can be predicted on the basis of hether or not the poarisabiity chan+es durin+ the vibration. *oever3 it is not very