“Condon” and “non‐Condon” Nonlinear Spectroscopy of Polyatomic Molecules in Solutions: Solvation Dynamics Study

Abstract

AbstractA theoretical basis for the solvation dynamics study of complex molecules in solutions by resonance four‐photon spectroscopy has been developed. A non‐Markovian theory of four‐photon spectroscopy of electronic transitions in complex molecules has been generalized for the “non‐Condon” tensor case. It has been shown that for definite conditions the cubic susceptibility χ(3), describing four‐photon interaction, can be expressed as a product of “Condon” and non‐Condon parts. The latter describes the mixing effect of different electronic molecular states by nuclear motions (Herzberg‐Teller effects). The Condon part depends on the excitation frequency, and the non‐Condon one depends on the polarization state of the pump and the probe beams. It has been shown that the Condon resonance transient grating spectroscopy is a good method for the investigation of ultrafast solvation dynamics.The dependence of the non‐Condon part of χ(3) on the polarizations of the interacting beams permits one to formulate the principles of a new method for the investigation of Herzberg‐Teller effects on electronic molecular transitions by transient polarization four‐photon spectroscopy.