Cumulant expansion for fast estimate of non-Condon effects in vibronic transition profiles.

When existing, cumulants can provide valuable information about a given distribution and can in principle be used to either fully reconstruct or approximate the parent distribution function. A previously reported cumulant expansion approach for Franck-Condon profiles [Faraday Discuss., 150, 363 (2011)] is extended to describe also the profiles of vibronic transitions that are weakly allowed or forbidden in the Franck-Condon approximation (non-Condon profiles). In the harmonic approximation the cumulants of the vibronic profile can be evaluated analytically and numerically with a coherent state-based generating function that accounts for the Duschinsky effect. As illustration, the one-photon 11 Ag  → 11 B2u UV absorption profile of benzene in the electric dipole and (linear) Herzberg-Teller approximation is presented herein for zero Kelvin and finite temperatures.