Abstract :
The nonlinear optical response of a molecular system in the condensed phase subjected to a series of five off-resonant femtosecond laser pulses has been studied using a quantum Fokker–Planck equation. This equation can treat a molecular system with any shape of potential coupled to a Gaussian–white noise-bath. The third- and fifth-order response functions, which are equivalent to the second- and third-order correlation functions of the molecular coordinate were obtained from the equation of motion. Assuming the potential surface of a cesium dimer [Cs2], which is modeled by a Morse potential, and considering both the linear and nonlinear coordinate dependence of the polarizability, we calculated the third- and fifth-order response functions for various temperatures and heat-bath couplings. The temporally two-dimensional (2D) profiles of the fifth-order signal are affected by both the shape of potential and the coordinate dependence of the polarizability, even at strong damping. The nonlinearities caused by the anharmonic potential and by the nonlinear polarizability have different temperature dependence. This indicates that fifth-order two-dimensional spectroscopy carried out for a different temperature allows us to access information of the potential and the polarizability.
