A theoretical study of resonance Raman scattering from molecules

Abstract

An expression for the resonance Raman scattering (RRS) cross section from large molecules is derived by means of a time independent Green function method. This expression is obtained in the adiabatic basis set, and both the radiative and nonradiative damping effects are explicitly considered, assuming the statistical limit in the excited vibronic states. Expanding the nuclear coordinate dependence of the transition moment, an analytical expression for the RRS cross section is derived for a displaced potential energy surface model. The expression obtained is applicable to the resonance Raman scattering not only in the weak coupling but also in the strong coupling case, and consists of the sum of the nth order vibrational transitions. It is shown that in the weak coupling case, the fundamental cross section may be explained as a succession of the absorption to the lowest level in the excited electronic state followed by the emission from it. Multimode effect on the RRS cross section which has not been reported before is studied and the analytical expression for the RRS cross section including this effect is obtained. The dimensionless displacement Δ dependence on the excitation energy profile is calculated. The effect of the nonadiabatic coupling (or the Born–Oppenheimer coupling) on the RRS cross section is also investigated. It is shown that using the adiabatic basis set, the BO coupling appears only in the high order approximations in most cases.