UNIT 4 Raman Spectroscopy 12309

UNIT 4 RAMAN SP SPECTROSCOPY

Raman Spectroscopy

Structure 4.1

Introduction Objectives

4.2 4.2

Theo Theory ry of of Ram Raman Spe Spect ctro rosc scop opy y Quantum or Partice Theory !assica or "ave Theory Raman #ctivity of $ibrations Rue of %utua &'cusion (epoarisation Ratio

4.) 4.)

Instr Instrum umen entat tatio ion n for Ram Raman an Spec Spectr trosc oscop opy y Radiation Sources for Raman Spectroscopy Sampe *andin+ (evices Transducers or (etectors

4.4

&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.

4.1

#pp #ppic icat atio ions ns of of Ram Raman an Spe Spect ctro rosc scop opy y Summary Termin rmina a Ques Questi tio ons #nsers

INTRODUCTION

In the previous unit you earnt about IR spectrometry in terms of its principe3 instrumentation and appications. ou ou oud reca that the IR spectrum is a conse5uence of transitions amon+st the 5uantised vibrationa ener+y eves of the moecues on absorbin+ IR radiation. In this unit you oud earn about Raman Raman spectroscopy hich aso invoves same ener+y eves but it differs on many other counts. The nature of radiation used3 the mechanism of interaction beteen the radiation and matter3 the necessary condition for the interaction3 the seection rues3 the t he sensitivity3 re5uired instrumentation and the resutin+ spectra are different in case of Raman spectroscopy. 6urther3 6urther3 even the information avaiabe from it is different7 in fact it is compimentary to the one avaiabe 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 fooed by an account of the essentia components of Raman spectrometers. Thereafter e sha ta-e up the appications of Raman spectrometry in diverse areas. In the ne't boc- you i earn about spectrometric methods based on moecuar fuorescence and moecuar phosphorescence hich are i mportant spectroscopic method of anaysis.

Obect!"es #fter studyin+ this unit3 you shoud be abe to8 •

define Raman effect3

•

e'pain the ori+in of Rayei+h scatterin+3

•

describe Raman effect in terms of cassica ave theory and 5uantum theory of radiation3

•

define Sto-es and anti,Sto-es ines and e'pain their ori+in3 )

Mo&ecu&ar Spectroscop!c Met(os-I

•

compare and contrast Raman spectra ith IR spectra3

•

state the rue of mutua e'cusion and and e'pain its

si+nificance3 •

enist the essentia components of the instrument re5uired for Raman spectroscopy3

•

describe advanta+es of Raman spectroscopy3 and

•

enumerate and discuss various various appications of Raman spectroscopy. spectroscopy.

4.# 4.#

T$EO T$ EOR RY O% RAMA RAMAN N SPEC SPECTR TROS OSCO COPY PY

#n eectroma+netic radiation hen passed throu+h a transparent medium interacts ith the partices 9e.+.3 moecues3 atoms3 or ions: constitutin+ it. If the dimensions of the partices are e5ua to or smaer than that of its aveen+th then it under+oes scatterin+. It has been observed that most of the scattered radiation has e'acty the same aveen+th as that of the incident radiation. Such a scatterin+ is referred to as Ray&e!'( scatter!n'. *oever3 a very sma fraction 9to the tune of about 1 in 1; 0: of the scattered radiation is found to have a aveen+th different from that of the incident radiation. This is caed Raman scatter!n'  and the e'istence of Raman scatterin+ is caed Raman e))ect. 6i+ 4.1 +ives the spectra of the scattered radiation obtained hen a sampe of !!4 as interacted ith a aser beam havin+ a aveen+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 aveen+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+nas on either side of the spectra are aso +iven in Rayei+h pea- are the Raman &!nes. The ines to the eft of avenumber units. This is Rayei+h pea- and havin+ oer vaue of avenumber are caed convenient because the Sto,es &!nes hie the ones to the ri+ht and havin+ hi+her vaue of position of other scattered avenumber are caed ant!-Sto,es &!nes. Sto-es ines are at oer radiations 9Raman scatterin+: ener+y hie the anti,Sto-es ines are at ener+y +reater than the can be convenienty e'pressed in terms of Raman Rayei+h pea-. shifts.

4

The positions of Raman ines are e'pressed in terms of Raman s(!)t 3 ∆ν   hich is defined as per the fooin+ e5uation.

∆ ν = 9 ν s −  ν ; : cm −1

Raman Spectroscopy

@ 94.1:

"here3 ν   s  and ν  ;  are the avenumbers of the source 9or incident: radiation and the observed scattered ines respectivey. It is obvious that the Raman shifts of the Sto-es ines oud be positive hie for anti,Sto-es ines3 these oud be ne+ative. There are to more features in the spectrum +iven above. ?ocate and rite them in the space provided beo. @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@... "e are are sure that you coud notice that3 firsty3 the Sto-es and anti,Sto-es ines are e5uidistant from the Rayei+h ine and secondy3 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 Rayei+h ine and their reative intensities. There are to theories of Raman effect. These are as foos8 •

Quantum or partice 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 eectroma+netic radiation is considered to be consistin+ of a stream of partices caed photons. The  photons constitutin+ a radiation of fre5uency fre5uency ν  oud have ener+y e5ua to hν  here h is the Panc-As constant. The interaction of the radiation ith the interactin+ species can be visuaised in terms of coisions beteen them and the photons. If the coisions are e&ast!c 9i.e.3 they do not invove any e'chan+e of ener+y: the photons oud be scattered 9defected: ith their incident fre5uency remainin+ unchan+ed. This e'pains the observance of Rayei+h ine in the spectrum. Since most of the coisions are eastic in nature3 most of the scatt ered photons oud have same fre5uency as the incident fre5uency. Therefore the Rayei+h ine is observed to be 5uite intense. # sma fraction of the coisions beteen the photons and partices of matter is found to be !ne&ast!c in nature i.e.3 these invove e'chan+e of ener+y. "hen "hen the photons constitutin+ the radiation under+o ineastic coisions 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 eves of the moecues. In such an event of ineastic coisions3 the moecues are either vibrationay 9andBor rotationay: e'cited or they may under+o vibrationa 9andBor rotationa: rea'ation. In both the cases the photons +et scattered

E&ast!c co&&!s!ons* The coisions that do not invove any e'chan+e of ener+y.

Ine&ast!c co&&!s!ons* The coisions that invove e'chan+e of ener+y.



Mo&ecu&ar Spectroscop!c Met(os-I

ith a fre5uency different from their initia fre5uency. fre5uency. In former case i.e.3 hen the moecues under+o e'citation3 they are of oer fre5uency. fre5uency. In the ater case3 here the moecues under+o rea'ation3 the scattered photons are of a hi+her fre5uency. These scattered photons +ive rise3 respectivey to the Sto,es and ant!-Sto,es Raman &!nes  in the spectrum.

?et us understand the mechanism of Raman and Rayei+h scatterin+ ith the hep of an ener+y eve dia+ram. 6i+. 4.2 +ives a schematic ener+y eve dia+ram of a moecue. The set of ines at the bottom end represent the eectronic +round state and the correspondin+ vibrationa ener+y eves. Simiary the set of ines at the top end represent the first eectronic e'cited state and the correspondin+ vibrationa ener+y eves. The re+ion of continuum in the midde 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 ec!tat!on2 Ray&e!'( scatter!n' an Raman scatter!n'

"hen a radiation3 havin+ a aveen+th that is different from any of the absorption ma'ima of the moecue3 interacts ith the moecue7 the ener+y of the moecue increases by an amount e5ua to hν  . The ener+y chan+e can be represented by the upard arro 9bac-: to the far eft in the fi+ure. This su++ests that the moecue in the oest vibrationa state of the eectronic +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 moecue in its first vibrationay e'cited state and accordin+y the virtua state has hi+her ener+y. ener+y. #s the virtua state is e'tremey short , ‒ 14 ived 9of the order of 1;  seconds:3 the moecue soon drops bac- don to its +round state3 reeasin+ a photon. The ne't to don arros 9purpe: 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'pains the Rayei+h scatterin+. The ne't to don arros 9+reen and bue: represent the situations hich arise in case of ineastic coisions. The first 9+reen: of these corresponds to the rea'ation from the virtua state that as ac5uired by the moecue in the eectronicay as e as vibrationay +round state by absorbin+ the incident photon. In this case3 the moecue does not rea' bac- to the vibrationa +round state7 instead it comes don to a vibrationay e'cited state. In other ords the ineastic coision has brou+ht in a vibrationa e'citation in the moecue. The scattered photon oud be of oer fre5uency i.e.3 it i +ive a Sto-es ine. /

The arro on the e'treme ri+ht 9bue: represents the situation herein the moecue in the virtua state obtained by absorption of incident photon by the moecue in the vibrationa e'cited state of the +round eectronic state reeases a photon. On reeasin+ the photon the moecue comes don to the vibrationa +round state of the +round eectronic state. The overa process of e'citation and rea'ation is e5uivaent to the vibrationa rea'ation of a moecue as a conse5uence of the ineastic coision. The ener+y of the scattered photon is +reater than that of the incident photon7 this +ives rise to anti,Sto-es ine.

Raman Spectroscopy

The most i-ey event in these interactions is t he ener+y absorption and re,emission by the moecues in the +round state. Therefore3 the Rayei+h ines are the most intense. 6urther3 since the virtua state from hich the Sto-es ine ori+inates is more popuated 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 cassica theory3 theory3 try to anser the t he fooin+ S#Qs.

SA/ 1 !acuate the avenumber 9in cm ‒ 1: of the radiation radiation havin+ a aveen+th aveen+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 poarisabiity of a moecue is fundamenta to the understandin+ of the cassica theory of Raman effect. ?et us first understand it before e move to the e'panation of the Raman effect. Po&ar!sab!&!ty o) mo&ecu&es

"hen a moecue is paced in a static eectric fied3 the positive and ne+ativey char+ed  partices constitutin+ the moecue +et attracted to the opposite poes poes of the appied fied. This eads to a char+e separation and conse5uenty to the deveopment of an induced dipoe moment. In simpe ords3 e say that the moecue has become  poarised. This tendency of the moecue moecue to +et poarised is caed po&ar!sab!&!ty i.e.3 the abiity to +et poarised. The ma+nitude of the induced dipoe moment is  proportiona to the stren+th of the appied appied fied and is +iven by the fooin+ e5uation. e5uation.  µ  = α  E 

@ 94.2:

The proportionaity constant α   is caed polarisability. caed polarisability. It is a measure of the ease ith hich the moecue 9or a bond in it: can +et poarised. # ea-er bond ith

The poarisabiity 9E: of the moecue depends on the  bond en+th7 shorter bonds0  bein+ difficut to poarise poarise than on+er bonds.

Mo&ecu&ar Spectroscop!c Met(os-I

oosey hed eectrons is more poarisabe7 a sin+e bond is more  poarisabe than a doube bond beteen beteen the same set of atoms. 6urther3 the poarisabiity can be anisotropic3 that is the eectrons of  a bond may be poarised to different e'tents hen the fied is appied in different directions. 6or e'ampe3 the hydro+en h ydro+en moecue +ets poarised to different e'tents hen the fied is appied in the direction of the bond or in the direction perpendicuar to it3 as shon 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 oud happen if e pace a moecue in an osciatin+ eectric fied  i-e that of an eectroma+netic radiationD ?et us see. The eectric fied associated ith a radiation havin+ a fre5uency fooin+ e5uation.  E  =  E ;

cos92πν ext :

ν  ex

 varies as per the

@ 94.):

here E  here  E   is the eectrica eectrica fied stren+th stren+th at any time t   and E  and  E ; is the ampitude of the ave. "hen such a radiation is made to interact ith a moecue it oud induce a dipoe moment in it. 6urther3 as the eectrica fied of the radiation is osciatin+ the resutin+ induced dipoe moment oud aso osciate at the same fre5uency as that of  the radiation. That is3  µ  = α  E  = α  E ; cos9 2πν ext :

@ 94.4:

This osciatin+ dipoe oud immediatey +enerate a radiation of the fre5uency3 and e have a ready e'panation for Rayei+h scatterin+.

ν  ex

#s you are aare3 the moecues are never stationary and aays under+o vibration 9and may be rotation: motions. These motions may cause a chan+e in the poarisabiity of the moecue. 6or e'ampe3 in the process of a vibration the bond en+th of a bond in the moecue -eeps varyin+ and as the poarisabiity depends on the stren+th of the  bond3 it may chan+e the poarisabiity of the the moecue. It can be shon 9see the bo' +iven beo: that hen such a system vibratin+ ith a fre5uency of ν  vib interacts ith the osciatory eectric fied of the eectroma+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 : hie the fre5uencies of the other to are e5ua to 9ν ex + ν vib : and 9ν ex − ν vib : respectivey. The The first component ith a fre5uency of   is the reason for the Rayei+h scatterin+. The other to components are the sources of the anti,Sto-es and Sto-es ines respectivey.

ν  ex

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 e5uiibrium bond en+th of r 0 and e5uiibrium bond  poarisabiity of α  ;  is vibratin+ ith a vibrationa fre5uency e5ua to ν  vib . If the rate of chan+e in the poarisabiity ith the chan+e in bond en+th r 3 durin+ vibration is 9∂α   B ∂r : 3 then the poarisabiity 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 :

Raman Spectroscopy

@ 94#:

Substitutin+ &5. 94#: in &5. 94.): e +et

 µ  = Gα ; + 9∂α  B ∂r :r ; cos9 2πν vibt :F E ; cos 2πν ex t 

@ 94C:

&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 simpify &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

@ 94(:

Substitutin+ this identity in &+. 94!: e +et3

µ = α ;& ; cos 2π ν e' t + 9∂α B ∂r :r ;

&; 2

cosG 2π9 ν e' + ν vib :t F + 9∂α B ∂r : r ;

&; 2

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 osciatin+ eectrica dipoe moment. #ccordin+y3 #ccordin+y3 there are three components in in the scattered radiation. These osciate ith the fre5uency e5ua to ν  ex 3 9ν ex + ν vib : and 9ν ex − ν vib :  respectivey. In other ords e may say that the incident fre5uency of the radiation has been moduated 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 ony Rayei+h scatterin+ is observed.

The three components in the scattered radiation are observed ony hen the vibration causes a chan+e in the poarisabiity. If the poarisabiity does not chan+e in the course of vibration ony Rayei+h scatterin+ is observed. Therefore3 e can concude that in order to observe Sto-es and anti,Sto-es ines the poarisabiity of the bond must chan+e in the course of vibration.

The necessary condition to observe vibration Raman spectrum is that the  poarisabiity of the bond must chan+e in the course of vibration

?et us raise a 5uestion. Since both the IR and Raman spectra invove transitions amon+st the same vibrationa eves3 oud the to spectra be the sameD *oever3 before proceedin+ further hy donAt you assess your understandin+ of the meanin+ and the e'panation of the Raman effect by anserin+ the fooin+ S#Q.

<

Mo&ecu&ar Spectroscop!c Met(os-I

SA/  6i up the ban-s in the fooin+.

a:

"hen "hen the the cois coision ionss bete beteen en the photon photonss and and moe moecu cues es are are @@@. @@@.33 the the  photons oud be@@@@..ith the same fre5uency. fre5uency.

 b:

If in the event of an ineastic coision ith photons photons the moecues +et vibrationay e'cited3 the fre5uency of the scattered photon i be @@@@.than the incident photon.

c:

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.

4.#. 4.#.

Raman Raman Act!" Act!"!ty !ty o) 5!brat! !brat!ons ons

#s a first response3 the anser to the 5uestion about the simiarity of IR and Raman spectra raised above3 coud be in a firm affirmative or one may respond3 Hmay beA. *oever in actua practice the to spectra are +eneray not identica and in case of a number of moecues3 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 to spectra arise are different. This feature actuay is an important indicator about the symmetry of the moecue. The Raman and IR spectra for a simpe moecue !! 4 is +iven in 6i+. 4.4. ou may note that ony some of the si+nas are common in the to s pectra and aso the common si+nas are of different reative intensities.

%!'. 4.4* IR an Raman spectra o) CC& 46 note t(e s!m!&ar!t!es an t(e !))erences

ou ou oud reca from t he previous unit on IR spectrometry s pectrometry that a the possibe possi be vibrationa modes of a moecue may not be IR active. Ony those modes hich have an associated chan+e in dipoe moment are IR active and sho up in the spectrum hie the other modes are absent. #s mentioned above3 in Raman spectrum ony those vibrationa modes are active hich are associated ith a chan+e in poarisabiity. (ue to this difference in the re5uirements for shoin+ the to 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 poarisabiity chan+es durin+ the vibration. *oever3 it is not very