In this second installment of a two-part series we present polarized Raman spectra and discuss the association of the symmetry
species of the normal vibrational mode and the depolarization ratio of Raman scattering. We discuss those aspects of molecular
symmetry and Raman polarization rules that can be applied with normal Raman instrumentation. Materials include liquids, single
crystals, and polycrystalline compounds.
The excitation in Raman spectroscopy is usually linearly polarized monochromatic light from a laser. The Raman scattered light
can be polarized parallel or perpendicular with respect to the incident laser polarization depending on the symmetry species
of the vibrational modes. Normally, Raman spectroscopy is performed without a polarization analyzer so that both polarizations
of the Raman scattered light are collected to maximize the signal. This is frequently a sensible choice. However, there are
instances for which the use of a Raman polarization analyzer can be helpful in both band assignment and the characterization
of molecular or solid-state structure.
In part I (1), we summarized and presented the most salient and beneficial aspects of group theory when applied to vibrational
spectroscopy in general and Raman spectroscopy in particular. Here, we apply that knowledge to Raman spectra obtained from
liquids, single crystals, and polycrystalline compounds. The treatment of polycrystalline compounds is a cautionary tale about
the importance of Raman sampling and sample grain size relative to that of the incident laser beam.
Polarized Raman Spectroscopy of Water
Recall that in the first part of this series we discussed the normal vibrational modes of water. We found that there were
three Raman active modes, two stretching and one bending. Furthermore, we found that the bending mode (ν2) and one of the stretching modes (ν1) are totally symmetric (A
1) and therefore polarized, whereas the antisymmetric stretch (ν3, B
2) is depolarized. The polarized Raman spectra of water shown in Figure 1 includes the depictions of the normal vibrational
modes that we presented in part I (1). Spectra are shown in the parallel and perpendicular polarized configurations and all
three of the normal vibrational modes can be assigned based on their energies and polarization responses. There are several
things to notice in these spectra. First, you see that the energy of the bending mode, which appears in the so-called fingerprint
region of the vibrational spectrum, is substantially less than that of the stretching modes. Second, note how much more intense
either the symmetric or antisymmetric stretching modes are relative to the angle bending mode. As a general rule, the symmetric
stretches of bonds tend to yield stronger Raman bands.
Figure 1: Polarized Raman spectra of H2O.
It is often said that Raman spectroscopy is the vibrational spectroscopy of choice for the analysis of analytes in aqueous
solutions. The Raman spectrum of water demonstrates why that statement has considerable merit. The angle bending mode at 1635
cm-1 is weak and produces little background or band interference in the fingerprint region. The presence of the weak angle bending
band leaves the entire fingerprint region without any substantial background, and this is the basis for the claim that Raman
spectroscopy can readily be applied to the analysis of aqueous solutions. The stretching modes beyond 3000 cm-1 are significantly more intense and broader. The substantial width of the stretching modes of water can be attributed to
H bonding. The entire OH stretching from two allowed bands covers the region from approximately 2900 cm-1 to 3700 cm-1! The Raman spectrum of water is a remarkable demonstration of the distribution of vibrational energy states created by H
bonding in the liquid state.