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84 CHAPTER 2

Smekal predicted a radically different type of scattering in 1923. He suggested
that if a beam of light were incident on a solution, the scattered light would contain
(apart from the elastically scattered light of unchanged frequency) two more frequen-
cies, one greater and one lesser than the frequency of the incident beam. These would
result from a collision of a photon with a molecule in which the photon would indeed
(in contrast to Rayleigh’s idea) exchange energy with the molecule it struck and bring
about a transition in the molecule’s vibrational band. Smekal thought that two kinds
of energy exchange would occur. The photon might excite the molecule to higher states
and the resulting frequency of the scattered light would be lower than the incident
frequency (because some energy had been removed from the light). Alternatively, the
molecule might give energy to the photon and the emerging photon would have a
higher frequency than that of the initial beam. These nonelastic scattering effects
predicted by Smekal in 1923–1924 were first experimentally observed by the Indian
scientists Raman and Krishnan in 1928.
Thus, in the Raman spectra, it is a frequency shift, a that is observed. The
value of such shifts in the frequency of nonelastically scattered light is not dependent
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on the exciting frequency, but on the structure of the molecule with which the photon
interacts. Thus, the Raman shifts reflect vibrational transitions, as do the IR spectra.
One other aspect of Raman spectra must be explained before one can understand
how information on structures around ions in solution can be extracted from positive
and negative Raman shifts. In Rayleigh scattering, the oscillating dipoles may radiate
in all directions with the frequency of the exciting beam. However, in Raman spectra,
the radiation depends on the polarizability of the bond. In some molecules, polarizabil-
ity does not have the same value in all directions and is called nonisotropic. Let it emit
light in the x-y plane only. Then the intensity ratio (light in the xy plane)/(light in the
incident beam) is called the depolarization factor. For Raman scattering, this depo-
larization factor gives information on structure, for example, on the degree of symme-
try of the entity in the solution, the Raman spectra of which are being observed (Tanaka
et al., 1965).

2.11.5. Raman Spectra and Solution Structure

One type of Raman study of solutions concentrates on water–water bonding as it
is affected by the presence of ions. Hydrogen bonds give Raman intensities, and the
variation of these with ionic concentration can be interpreted in terms of the degree
and type of structure of water molecules around ions. The ion has often been
used in Raman studies to illustrate structure-breaking effects because it is a relatively
large ion.
In studies of the spectra of intramolecular water and how they are affected by ions,
new Raman peaks can be interpreted in terms of the model of solvation suggested by

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This must, however, have a frequency greater than that of any of the transitions envisaged, but less than
that which would cause an electronic transition.