ABSTRACT
The subject of Raman scattering has already been investigated in several chapters. Here, we want to tie together various theoretical approaches and introduce the topic of resonance Raman scattering, in which the frequency of the exciting radiation falls within an electronic absorption band. The selection rules and polarization properties of Raman scattered light are considered for both off-resonance and resonance Raman experiments. We also introduce the very interesting topic of surface-enhanced Raman spectroscopy (SERS). We are concerned here with vibrational Raman scattering in the absence of resolved rotational subbands, but we do consider the influence of rotational motion and vibrational dynamics on the Raman lineshape. In effect, Raman spectroscopy is a two-dimensional experiment, in that the intensity depends on both the incident and scattered light frequency. The Raman spectrum is represented by the intensity of scattered light as a function of the frequency shift Δν = ν0 − νs between the incident (0) and scattered (s) radiation. Peaks in this spectrum correspond to transitions between vibrational levels within the ground electronic state. On the other hand, the Raman excitation profile (REP) is the intensity of a particular normal mode as a function of the incident frequency ν0. The dependence of the Raman intensity on the incident and scattered light frequency is illustrated in Figure 12.1. As the incident frequency is tuned, the intensities of bands in the Raman spectrum vary, but the frequency shifts remain the same. The intensity profiles of the various Raman bands, as a function of ν0, reflect dynamics taking place in the excited electronic state.
