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

A 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.