Theory of resonance Raman scattering. An improved formulation of the vibronic expansion method

Publisher Website
Google Scholar
Abstract
The vibronic expansion theory of Raman intensities in polyatomic molecules is reconsidered so as to improve its accuracy and extend its validity into the resonance region. Several fundamental improvements are introduced: (i) Terms which couple adiabatic Born−Oppenheimer states are shown to be potentially important sources of Raman intensity even when the electronic level spacing is large. (ii) The conventional asymptotic expansion of the energy denominators is replaced by an expansion which remains convergent in the resonance region and allows direct summation over the intermediate vibrational states. (iii) The vibrational linewidth is shown to be an important parameter in the theory and is fully taken into account by treating the energy denominator consistently as a complex quantity. The over−all treatment results in analytic formulas expressed in terms of spectroscopic parameters. A preliminary comparison with general spectroscopic properties is presented. Far from resonance the new results are shown to reduce to the conventional ones if the appropriate approximations are introduced. A special effort is made to present the results in such a way that they clarify the relation between resonance fluorescence, ordinary Raman, and resonance Raman scattering.