Simple interpretation of dephasing in absorption and resonance Raman theory

It is shown that the effects of ‘‘pure dephasing’’ in vibronic absorption and resonance Raman scattering may be simply derived by beginning with the Fermi Golden Rule. The medium introduces electronic dephasing by modulating the electronic energy gap of the molecule. In both absorption and Raman scattering the cross section depends upon a medium correlation function φ(t) whose form is identical to one that appears in vibrational relaxation theory. Explicit equations for the resonance Raman cross section are derived for an exponential medium correlation function. Also, φ(t) is derived for medium models which consist of a collection of classical and quantum mechanical oscillators.