A UNIFIED APPROACH TO RAMAN AND ELECTRONIC ABSORPTION-EMISSION SPECTROSCOPY

The talk will deal with the interaction of molecules with weak visible and ultraviolet light as in normal Raman or electronic spectroscopy. First, we note that there are two parts of the radiation field, that which is already there (the decay photon field for spontaneous emission) and that which the experimentalist adds (the light source). The point of view to be taken is that instead of using time-dependent perturbation theory in a basis of ``naked” molecular states and the pure radiation field, it is physically more reasonable to partly ``clothe” the discrete molecular states by their interaction with neighboring continua, the decay photon field plus any nonradiative molecular continua (e.g., vibronic, dissociative, or ionization continua). The new basis set is therefore composed of mixed states, the purely molecular component of which distributes itself over the entire energy spectrum. An absorption spectrum would reveal a series of resonances with widths $\\geq$ natural linewidth. Normal Raman and Rayleigh scattering simply become extensions of electronic absorption-emission spectroscopy in the faintly absorbing ``tails” of these resonances, but with their major time-dependent component exactly following the temporal growth and decay of the light source. The theory is ``tunable” in the sense that a smooth passage is possible between off-resonance experiments using a narrow frequency exciting line (Raman) and on-or near-resonance excitation using either a narrow (resonance or pre-resonance Raman) or broad (absorption-emission) exciting line, and intermediate cases. Damping near a resonance comes in automatically, but is frequency dependent in the most general case. Appropriate approximations give back the usual equations of second order perturbation theory.