The Femtosecond Fifth-Order Nonlinear Response of Nuclear Motion in Liquids

Abstract

The femtosecond fifth-order nonlinear response of CS₂ benzene and toluene is measured using nonresonant six-wave mixing¹. This method provides information on the dephasing mechanisms (homogeneous/inhomogeneous) of the coherently excited nuclear motion that is not accessible in third-order experiments. From fs optical Kerr effect and transient phase-grating scattering experiments it is well known that rotational and translational motion of small molecules in liquids is inertial on a sub-100 fs time scale, i.e. the molecules cannot follow the impulsive excitation of the fs laser pulses immediately. These experiments provide information analogous to the free-induction decay in a resonant two-level system. Only higher-order nonlinear experiments alow to characterize the dephasing mechanisms of the nuclear motion. The interpretation of the results depends crucially on the coordinate dependence of the polarizability (non-Condon effects).