1. Raman Spectroscopy BY
Dr. Bhawna

2. SPECTROSCOPY
Spectroscopy is that branch of science which deals with the study of interaction of electromagnetic radiations with matter

3. ELECTROMAGNETIC SPECTRUM
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.

4. Main interactions with matter spectrum
Region of the
Main interactions with matter
spectrum
Collective oscillation of charge carriers in
Radio
bulk material (plasma oscillation). An
example would be the oscillation of the
electrons in an antenna.
Microwave through
Plasma oscillation, molecular rotation
far infrared
Molecular vibration, plasma oscillation (in
Near infrared
metals only)
Molecular electron excitation (including
Visible
pigment molecules found in the human
retina), plasma oscillations (in metals only)

5. Excitation of molecular and atomic valence
Ultraviolet
electrons, including ejection of the
electrons (photoelectric effect)
X-rays
Excitation and ejection of core atomic
electrons
Energetic ejection of core electrons in
Gamma rays
heavy elements, excitation of atomic
nuclei, including dissociation of nuclei
Creation of particle-antiparticle pairs. At
High energy
very high energies a single photon can
gamma rays
create a shower of high energy particles
and antiparticles upon interaction with matter.

6. ABSORPTION AND EMISSION SPECTROSCOPY
A molecule has a number of energy levels which are quantized. The transitions take place only between these energy levels according to certain rules, called selection rules.
The transition may take place from lower energy level to higher energy level by absorbing energy. It is then called absorption spectroscopy and the result obtained as a result of a number of such transitions is called ``absorption spectrum.``
The transition may take place from higher energy level to a lower energy level thereby emitting the excess energy as a photon. It is then called emission spectroscopy and the result obtained as a result of a number of such transitions is called ``emission spectrum.``
E2 – E1 = hv = hc/ λ

8. TYPES OF MOLECULAR SPECTRA
A molecule has different types of quantized energy level i.e. translational, rotational, vibrational and electronic. The molecular spectra arise due to transitions taking place between these energy levels. The energy absorbed for any transition is equal to the difference in the energies of the two levels involved. It is found that these energies for transition are in the order
E t <<E r <<E v <<E e*
In fact, the difference between the successive translational energy levels is so small (~ J mol-1) that it cannot be observed experimentally. For this reason, for practical purposes, translational energy is considered as continuous and we do not observe any translational spectrum. The different types of molecular spectra generally observed, depending upon the energies absorbed and the regions of the electromagnetic spectrum in which they are observed, are shown in Fig. and are briefly explained below:

9. (1) Pure rotational (Microwave) spectra.
If the energy absorbed by the molelcule is so low that it can cause transition only from one rotational level to another within the same vibrational level, the result obtained is called the rotational spectrum. These spectra are, therefore, observed in the far-infra-red region or in the microwave region whose energies are exceedingly small (v = cm-1). The spectra obtained is, therefore, also called microwave spectra.

10. (2) Vibrational rotational spectra
If the exciting energy is sufficiently large so that it can cause a transition from one vibration level to another within the same electronic level, then as the energies required for the transitions between the rotational levels are still less, both types of transitions will take place. The result is, therefore, a vibration-rotational spectrum. Since such energies are available in the near infra-red region, these spectra are observed in this region (v = cm-1) and are also called infra-red spectra.

11. (3) Electronic Band spectra
If the exciting energy is still higher such that it can result in a transition from one electronic level to another, then this will also be accompanied by vibrational level changes and each of these is further accompanied by rotational level changes. For each vibrational change, a set of closely spaced lines is observed due to rotational level changes. Such a group of closely spaced rotational lines is called a band. Thus for a given electronic transition, a set of bands is observed. This set of bands is called a band group or a band system. Each electronic transition gives a band system. The complete set of band systems obtained due to different electronic transitions gives the electronic band spectrum of the gaseous molecule. Thus whereas atoms give line spectra, molecules give band spectra. As such high excitation energies are available in the visible and ultraviolet regions, these spectra are observed in the visible region (12,500-25,000 cm-1) and ultraviolet region (25,000-70,000 cm-1).

12. (4) Nuclear Magnetic Resonance (NMR) spectra.
This type of spectrum arises from the transitions between the nuclear spin energy levels of the molecule (involving reversal of nuclear spin) when an external magnetic field is applied on it. The energies involved in these transitions are very high which lie in the radio frequency regions (5-100 MHz). The method is based upon applying such frequencies on the sample so that it is in resonance with the applied frequency.

13. (5) Electron Spin Resonance (ESR) spectra.
This type of spectrum arises from the transitions between the electron spin energy levels of the molecule (involving reversal of electron spin) when an external magnetic field is applied on it. These involve frequencies corresponding to microwave region ( MHz). Frequencies of this range are applied on the sample to bring the sample in resonance condition.

14. (6) Raman spectra.
This is also a type of vibrational-rotational spectrum but is based on scattering of radiation and not on the absorption of radiation by the sample. It is based upon the principle that when a sample is hit by monochromatic radiation of the visible region and scattering is observed at right angles to the direction of the incident beam, the scattered radiation have frequency equal to that of the incident beam (called Rayleigh scattering) as well as frequencies different (higher as well as lower) than that of the incident beam (called Raman scattering). The difference in the frequencies of the incident beam and that of the scattered beam (called Raman frequencies) are similar to those needed for the vibrational and rotational transitions. However, by suitably adjusting the frequency of the incident radiation, Raman spectra are observed in the visible region (12,500-25,000 cm-1).

16. RAMAN SPECTRA GENERAL INTRODUCTION
It is a special type of spectroscopy which deals not with the absorption of electromagnetic radiation but deals with the scattering of light by the molecules. It is observed that 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 observed at right angles to the direction of the incident light, the scattered light is found to have the same frequency as that of the incident light. This type of scattering is called Rayleigh scattering.

17. Prof. C.V.Raman of Calcutta University, however, observed in 1928 that when a substance (gaseous, liquid or solid) is irradiated with monochromatic light of a definite frequency v, 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. 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 vi is the frequency of the incident light and vs' that of particular scattered line, the difference v = vi - vs is called Raman frequency or Raman shift.

18. Thus the Raman frequencies observed for a particular substance are characteristic of that substance. The various observations thus made by Raman constitute what is called Raman effect and the spectrum observed is called Raman spectrum. Thus in a simple way, Raman spectrum may be represented as shown in Fig.

20. EXPLANATION FOR OBSERVING RAYLEIGH LINE AND RAMAN LINES
When the incident photon hits the molecule, this collision may be elastic or inelastic. Elastic collision means that the colliding particles will return with the same energy. Thus the scattered photon has the same frequency as that of the incident photon. This explains Rayleigh scattering or Rayleigh line. However, if the collision is inelastic, still the law of conservation of energy will hold good i.e. total energy before collision and after collision must remain the same. Thus some energy may be transferred from the incident photon to the molecule so that the scattered photon has lower energy and hence lower frequency than that of the incident photon.

21. This explains the occurrence of Stocks' lines
This explains the occurrence of Stocks' lines. Alternatively, some energy may be transferred from the incident photon to the molecule so that the scattered photon has higher energy and hence higher frequency than that of the incident light. This explains of the occurrence of anti-Stoke's lines. This explanation may be further elaborated as follows:
When the photon strikes the molecule, the energy is absorbed by the molecule and it gets excited to some higher energy level. Now if it returns to the original level, it will emit the same energy as absorbed and thus we have Rayleigh scattering. However, in most of the cases, the excited molecule does not return to the original level. It may return to a level higher than the original level thereby emitting less energy than absorbed.

22. This explains the occurrence of Stokes' lines
This explains the occurrence of Stokes' lines. Thus a part of the energy of the incident photon remains absorbed by the molecule (so that molecule has higher energy than before). Alternatively, the excited molecule may return to a level lower than the original level. Thus more energy is emitted than absorbed. This explains the occurrence of anti-Stokes' lines. In this case, the molecule has less energy than before. The different cases may be represented diagrammatically as shown in Fig.

24. POLARIZABILITY OF MOLECULES AND RAMAN SPECTRA
The Raman effect arises on account of the polarization (distortion of the electron cloud) of the scattering molecules that is caused by the electric vector of the electromagnetic radiation. The induced dipole moment depends upon the strength of the electric field E and the nature of the molecules. We write
μ= αE

25. where the quantity depends upon the nature of the molecules
where the quantity depends upon the nature of the molecules* and is called polarizability of the molecule. Thus polarizability is the ratio of the induced dipole moment to the strength of the electric field.
In case of atoms or spherically symmetrical molecules (spherical rotors) such as CH4' SF6 etc same polarizability is induced whatever be the direction of the applied electric field. They are said to be isotropically polarizable. Such molecules are said to be isotropic molecules.

26. In case of all diatomic molecules (homonuclear or heteronuclear) or non-spherical molecules (non-spherical rotors), the polarizability depends upon the direction of the electric field. For example, in case of H2 molecule, the distortion produced is more when the electric field is applied parallel to the bond than when it is applied perpendicular to it and we write α11>α1
Such molecules are, therefore, said to anisotropically polarizable.

27. Types of molecules showing Rotational Raman Spectra.
A molecule scatters light because it is polarizable. Hence the gross selection rule for a molecule to give a rotational Raman spectrum is that the polarizability of the molecule must be anisotropic i.e. the polarizability of the molecule must depend upon he orientation of the molecule with respect to the direction of the electric field. Hence all diatomic molecules, linear molecules and non-spherical molecules give Raman spectra i.e. they are rotationally Raman active. On the other hand, spherically symmetric molecules such as CH4' SF6 etc. do not give rotational Raman spectrum i.e. they are rotationally Raman inactive. (These molecules are also rotationally microwave inactive).

28. In fact, this is one of the reasons for importance of rotational Raman spectroscopy over infrared spectroscopy because the latter requires that the molecule must have a permanent dipole moment whereas the former only requires that polarizability of the molecule must change during vibration.

29. PURE ROTATIONAL RAMAN SPECTRA OF DIATOMIC MOLECULES
The selection rules for pure rotational Raman spectra of diatomic molecules are
ΔJ = 0 , + 2
The selection rule ΔJ = 0 corresponds to Rayleigh scattering whereas selection rule
ΔJ = + 2 gives rise to Raman lines as explained .

30. INTENSITIES OF LINES OF THE PURE ROTATIONAL RAMAN SPECTRA
As explained earlier, the intensities of lines depend upon the population of initial level from where the molecules are excited or de-excited to the final level. Since the population of rotational energy levels is as shown in Fig. therefore the intensities of the Stokes' and anti-Stokes' lines vary in a similar manner.

32. APPLICATION OF PURE ROTATIONAL RAMAN SPECTRA
From the pure rotational Raman spectra, noting the separation between the lines, the value of B can be obtained from which the moment of inertia and the bond length of the diatomic molecules can be calculated, as explained earlier.

33. ROTATIONAL-VIBRATIONAL RAMAN SPECTRA OF DIATOMIC MOLECULES
For large molecules, the lines obtained due to rotational transitions are so weak that they are beyond resolution. Hence we have pure vibrational Raman spectra for which the selection rules are same as for pure vibrational spectra i.e. Δ v = + 1, + 2, ….. However in case of diatomic gaseous molecules, the resolution of rotational fine structure is sufficient and can be studied. Thus diatomic gaseous molecules give rotational-vibrational Raman spectra which can be explained as follows:

35. ADVANTAGES OF RAMAN SPECTROSCOPY OVER INFRA-RED SPECTROSCOPY
Raman spectroscopy has a number of advantages over infra-red spectroscopy as briefly explained below:
(i) Since Raman frequencies are independent of the frequency of the incident radiation, hence by suitable adjusting the frequency of the incident radiation, Raman spectra can be obtained in the visible spectrum range where they can be easily observed rather than the more difficult infra-red range.

36. (ii) Raman spectra can be obtained even for molecules such as O2' N2' C12 etc. which have no permanent dipole moment. Such a study has not been possible by infra-red spectroscopy.
(iii) 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. Because of the ease with which the Raman spectra can be studies, the molecular structure of a large number of compounds have been determined by Raman spectroscopy. However infra-red method still continues to remain popular.

38. THANKS