RAMAN SPECTROSCOPY AND ITS APPLICATIONS

RAMAN SPECTROSCOPY AND ITS APPLICATIONS by VARINDER KHEPAR (PhD Chemistry), Punjab Agricultural University, Ludhiana, Punjab, INDIA

1. RAMAN SPECTROSCOPY AND ITS APPLICATIONS VARINDER KHEPAR PhD Chemistry 1

2. Contents HISTORY INTRODUCTION RAMAN LINES RAMAN INTENSITY CONCEPT OF POLARISATION CLASSICAL THEORY QUANTUM THEORY SELECTION RULE RULE OF MUTUAL EXCLUSION INSTRUMENTATION ROTATIONAL VIBRATIONAL RAMAN SPECTRA ADVANTAGES OVER IR DISADVANTAGES APPLICATIONS 2

3. SPECTROSCOPY Spectroscopy is that branch of science which deals with the study of interaction of electromagnetic radiations with matter When the different types of electromagnetic radiations are arranged in order of their increasing wavelengths or decreasing frequencies, the complete arrangement is called electromagnetic spectrum. 3

4. What happens when light falls on a material? 4

5. HISTORY 1923 - Inelastic light scattering was predicted by A. Smekel 1928 -Landsberg and Mandelstam see unexpected frequency shifts in scattering from quartz 1928 - C.V Raman discovered the Raman effect on 28 February 1930 -Indian physicist won the Nobel prize in physics 1954 - won Bharat Ratna award NATIONAL SCIENCE DAY is celebrated in India on 28 February 5

6. RAMAN SPECTRA INTRODUCTION It is a type of spectroscopy which not deals with the absorption of electromagnetic radiation but deals with the scattering of light by the molecules. When a substance which may be gaseous, liquid or even solid is irradiated with monochromatic light of a definite frequency v, a small fraction of the light is scattered. Rayleigh found that if the scattered light is found to have the same frequency as that of the incident light. This type of scattering is called Rayleigh scattering . 6

7. When a substance is irradiated with monochromatic light, the light scattered at right angles to the incident light contained lines not only of the incident frequency but also of lower frequency and sometimes of higher frequency as well. The lines with lower frequency are called Stokes' lines whereas lines with higher frequency are called anti-Stokes' lines. 7

8. Raman further observed that the difference between the frequency of the incident light and that of a particular scattered line was constant depending only upon the nature of the substance being irradiated and was completely independent of the frequency of the incident light. If ν i is the frequency of the incident light and ν s that of particular scattered line, the difference Δν = ν i - ν s is called Raman frequency or Raman shift. 8

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10. Raman lines Raman lines formation can be explained on the basis of Planck’s quantum theory of radiation. When photons of frequency ν hit t he nuclei, then three cases will be arises : Rayleigh line . The collision is perfectly elastic, then the scattered photon will have the same frequency as that of incident photon. ν i = ν s , this explains the occurrence of Rayleigh line. 10

11. Stoke’s lines: The collision is inelastic, then by holding the law of conservation, the total energy before and after the collision must be same, however some redistribution of energy take place. When molecule will gains some energy, then photon will be scattered with lesser energy and lower frequency. ν i ≥ ν s , this explains the occurrence of Stoke’s lines. Anti Stoke’s lines. If the collision is inelastic and molecule may lose energy to photon, then scattered photon will have higher energy and higher frequency than incident beam. ν i < ν s , this explains the occurrence of anti Stoke’s lines. 11

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13. Raman Intensity Intensity depends upon population of intial vibrational energy levels : I s = N i σ R( i→f ) I L N i = initial state population σ R( i→f ) : Raman cross section for transition ( E i →E f ) I L : Laser intensity Boltzmann distribution for state N i at T N i = N exp(-h ν vib / kT ) Lower energy state: higher initial state population I(Stokes) > I(Anti-Stokes) 13

14. Concept of Polarisability in Raman Effect How molecule become polarised : When a molecule interacts with electric field of electromagnetic radiation, it undergoes distortion in its electron cloud resulting in the formation of a dipole.. Isotropically polarisable : When spherically symmetrical molecule i.e. CH 4 , CCl 4 etc. interacts with radiation, the electron forming the bonds are displaced by an electric field symmetrically in all directions. Thus molecule become polarised , whatever will be the direction of applied electric field. 14

15. Anisotropically polarisable : Non spherical molecules i.e. diatomic H 2 , HCl , CO and linear polyatomic molecules CO 2 ,HC≡CH, etc. The electron forming the bonds are more easily displaced by an electric field applied in parallel or perpendicular to bond. 15

16. CLASSICAL THEORY OF RAMAN EFFECT The induced dipole moment µ of polarized molecule is given by: µ = αE ……(1) α = polarisability of molecule The electric field vector E is given by: E = E sin 2πνt ……(2) E = amplitude of oscillating electric field ν = frequency of incident radiation From (1) and (2) µ = αE sin 2πνt ……..(3) 16

17. As the molecule undergoes vibrational or rotational motion with frequency ν n ,the polarisabilty of the molecule changes : α =α + βsin2πν n t α = equilibrium polarisabilty β = rate of change of polarisabilty with vibration , then eq. (3) can be written as: µ = (α + βsin2πν n t) (E sin 2πνt) = α E sin 2πνt + βE sin 2πνtsin2πν n t µ = α E sin 2πνt +(1/2) βE {cos2π( ν- ν n )t - cos2π( ν+ν n )t} Here, α E sin 2πνt has frequency of incident radiation, represents Rayleigh scattering . The (ν- ν n ) represents Stoke’s lines and ( ν+ν n ) represents anti Stoke’s lines 17

18. LIMITATIONS OF CLASSICAL THEORY Stokes and antistokes , arise from same molecule, having same frequency, which is not true. Intensity of individual rotational raman line cannot be calculated separately. At ordinary temperature, there are very few vibrational states and thus Raman lines of Large frequency shifts are not observed. 18

19. QUANTUM THEORY OF RAMAN EFFECT hv i Molecule may deviate the photon without absorbing its energy unmodified line. May absorb part of energy of incident photon modified Stokes line. Molecule, itself being in an excited state, imparts some of its intrinsic energy to incident photon modified Antistokes line. Let a molecule of energy Ep be exposed to radiation ( ν ) and after collision the energy of molecule be Eq and the frequency of scattered photon be ν ’. 19

20. From law of conservation of energy hν + Ep + 1/2mv 2 = hν ’ + Eq + 1/2mv’ 2 …..(1) Since, there is no change in temperature due to collision so, 1/2mv 2 =1/2mv’ 2 So equation (1) becomes hν + Ep = hν ’ + Eq ν ’ = ν + Ep - Eq /h Ep = Eq , ν ’ =ν (unmodified line) Ep < Eq , ν ’ < ν (Stokes line) Ep > Eq , ν ’ > ν ( Antistokes line) 20

21. Selection Rule The polarisability of molecule must be change. For vibrational  Raman spectra, the vibration must change the polarisability of the molecule ∆v = ±1 where, ∆v = 1 corresponds to Stokes lines and ∆v = −1 corresponds to Anti-Stokes lines. For rotational Raman spectra, the polarisibilty of molecule must be anisotropic. ∆J = 0, ±2 All diatomic, linear and non spherical molecules are Raman active. All spherical molecules are rotational Raman inactive. E.g CH 4 and CCl 4 . 21

22. Rule of Mutual Exclusion “ If a molecule has a centre of symmetry then the raman active vibrations are inactive in infra red and vice verca ῎ e.g. CO 2 Modes of vibration of CO 2 Raman Infra-red Symmetric stretch Active Inactive Bend Inactive Active Asymmetric stretch Inactive Active 22

23. Instrumentation There are following component involves: 1. Laser or source of light 2. Filter 3. Sample holder 4. Detector 23

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25. 1. Laser or source of light • Lasers are strong source strong to detect Raman scattering. • Lasers operate using the principle of stimulated emission. • Electronic population inversion is required. • Population inversion is achieved by “pumping” using lots of photons in a variety of laser gain media  25

26. List of Various laser source S.No . Laser wavelength 1 Nd:YAG 1064nm 2 He:Ne 633nm 3 Argon ion 488nm 4 GaAlAs diode 785nm 5 CO 2 10600nm 26

27. 2. Filter For getting monochromatic radiations filters are used. They may be made of nickel oxide glass or quartz glass. Sometimes a suitable colored solution such as an aqueous solution of iodine in CCl 4 may be used as a monochromator . 27

28. 3. Sample holder For the study of raman effect the type of sample holder to be used depends upon the intensity of sources ,the nature and availability of the sample. The study of raman spectra of gases requires samples holders which are bigger in size than those for liquids. Solids are powdered or dissolved before subjecting to Raman spectrograph. Any solvents which is suitable for the ultraviolet spectra can be used for the study of Raman spectra. Water is regarded as good solvents for the study of compounds in Raman spectroscopy. 28

29. 4. Detector Because of the weakness of a typical Raman signal. Now days multichannel detectors like photodiode arrays(PDA), charged couple devices(CCD) are used. Sensitivity & performance of modern CCD detectors are high. 29

30. Rotational and Vibrational Raman Spectra Pure rotational Raman spectrum Pure vibrational Raman spectrum Rotational vibrational Raman spectrum 30

31. Pure rotational Raman spectrum The rotational energy levels of a non-rigid diatomic rotator are given by ν J = BJ(J+1)cm - 1 (J = 0,1,2…) For transition taking place from lower rotational level J to higher rotational level with quantum number J’, the energy absorbed is Δ ν = ν J' - ν J =BJ’(J’+1)- BJ(J+1) cm - 1 Applying selection rule, Δ J= +2 J’-J =2, J’= J+2 We get, Δ ν = B(J+2)(J+3) – BJ(J+1) = B(4J+6) cm - 1 31

32. Wave number of these spectral lines is ν s = ν i ±B (4J+6) cm -1 Where, ν i = wave number of Rayleigh line - ve = for stokes lines (appearing at low wave number side) + ve = for antistokes lines (appearing at high wave number side) Now, if J=0 in above eq. gives the separation of first line from the rayleigh line is 6B cm -1 , while the separation between successive lines is 4B cm -1 . 32

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34. Pure vibrational raman spectrum The vibrational energy is E v = (v+1∕ 2) ω e -(v+1∕ 2) 2 ω e x e cm -1 (v =0,1,2…) ω e =equilibrium vibrational frequency x e = anharmonicity constant The selection rule for pure vibrational raman spectrum of diatomic molecules is Δv = ±1 Thus for transition, from v=0 to v=1 ΔE fundamental =(1-2x e ) ω e cm -1 The Rayleigh lines appears at fundamental frequency. The Raman lines appears at distance from fundamental vibration as ν raman = ν incident ± ΔE fundamental - ve for stokes, appear on lower frequency side of fundamental line + ve for antistokes lines appear on higher frequency side of fundamental line. 34

35. Are you getting the concept ? If HCl molecule is irradiated with 434.8nm mercury line, calculate the frequency of stokes Raman line if the fundamental vibrational frequency of HCl is 8.667 X 10 13 sec -1 . 35

36. Rotational and vibrational raman spectra The vibrational -rotational energy levels of a diatomic molecule are ΔE J,v =BJ(J+1)+ (v+1∕ 2) ω e -(v+1∕ 2) 2 ω e x e cm -1 The selection rules are Δv = ±1 ΔJ= 0,±2 For, ΔJ= 0, ΔE J,v = Δ ν Q = (1- 2 x e ) ω e (Q-BRANCH) ΔJ= +2, ΔE J,v = Δ ν S =B(4J+6)+ (1- 2 x e ) ω e (S-BRANCH) ΔJ= -2, ΔE J,v = Δ ν O =B(-4J+2)+ (1- 2 x e ) ω e (O-BRANCH)   36

37. ADVANTAGES OF RAMAN OVER IR Raman lines are polarized. Raman spectra can be obtained even for molecules such as O 2 , N 2 , CO 2 etc. which have no permanent dipole moment. Such a study has not been possible by infra-red spectroscopy. Raman spectra can be obtained not only for gases but even for liquids and solids whereas infra-red spectra for liquids and solids are quite diffuse. 37

38. Water can be used as solvent. Very suitable for biological samples in native state (because water can be used as solvent) Frequencies, spectrum is obtained using visible light or NIR •Glass and quartz lenses, cells, and optical fibers can be used. • Standard detectors can be used. 38

39. Disadvantages of Raman Spectroscopy Instrumentation is vey expensive. Can not be used for metals or alloys. The Raman effect is very weak, so the detection needs sensitive detectors. Fluorescense compounds when irradiated by the laser beam , can hide the Raman spectrum. Sample heating through the intense laser radiation can destroy the sample. 39

40. APPLICATIONS Application in inorganic chemistry Vibrational spectroscopy still plays an important role in inorganic systems. For example, some small reactive molecules only exist in gas phase and XRD can only be applied for solid state. Raman Spectroscopy is utilized to study the structure of homonuclear diatomic molecules (i.e. bond length) are all IR inactive 40

41. Organic chemistry - usеd to obtain information rеgarding thе prеsеncе or absеncе of spеcific linkagеs in a molеculе , thе structurе of simplе compounds, study of isomеrs (i.e. cis and trans). Physical Chemistry- helps to study physical chemistry concerning electrolytic dissociation, hydrolysis and transition from crystalline to amorphous state. 41

42. Polymer Chemistry - usеd for charactеrization of polymеr compounds, by rеvеaling thе physical propеrtiеs and tacticity . Forensic science including the identification of illicit drugs, gunshot residue, inks used in explosives. 42

43. Pharmaceutical Sector- Raman Spectroscopy is widely adopted analytical technique in the pharmaceutical market. . It can analyze excipients and active pharmaceutical ingredients (APIs) of whole tablet within seconds, to obtain an overview of the tablet through to a few tens of micrometers to provide an in-depth analysis of individual grains and phase boundaries. 43

44. Biology- It helped to confirm the existence of low-frequency phonons in proteins and DNA, and their biological functions. used as a technique for, biochemical characterization of wounds. used to detect cancer cells using bodily fluids such as urine and blood samples  44

45. SUMMARY Raman spectroscopy deals not with the absorption of electromagnetic radiation but deals with the scattering of light by the molecules. If the scattered light have same frequency as that of the incident light,known as Rayleigh scattering while the lines with lower frequency are called Stokes' lines whereas lines with higher frequency are called anti-Stokes' lines. I(Stokes) > I(Anti-Stokes). Classical theory of Raman effect gives µ = α E sin 2πνt +(1/2) βE {cos2π( ν- ν n )t - cos2π( ν+ν n )t} Quantum theory of Raman effect : ν ’ = ν + Ep - Eq /h Selection Rule - The polarisability of molecule must be change. For vibrational  Raman spectra (∆v = ±1) and for rotational Raman spectra is (∆J = 0, ±2) Raman spectra can be obtained not only for gases but even for liquids and solids whereas infra-red spectra for liquids and solids. Water can be regarded as good solvent. Gaseous samples have sharper peaks than liquids. Raman spectroscopy have wide range of applications in inorganic, organic, physical, pharmaceutical, forensic, polymer chemistry and have several biological, food and agricultural applications. It determines the atomic mass of the isotope. 45

46. THANK YOU 46