Two-dimensional vibrational spectroscopy. VIII. Infrared optical Kerr effect and two-color infrared pump-probe measurements

The infrared optical Kerr effect (IR-OKE) and two-color infrared pump-probe spectroscopies, which can be directly used to study intermolecular vibrati onal energy relaxation pathways, are theoretically studied. The IR-OKE meth od involves an intense IR pulse with controlled frequency to build up large population on a specific intramolecular vibrationally excited state. After a finite mixing time, which is also experimentally controlled, the time ev olution of the nonequilibrium density matrix is detected by using the femto second optical Kerr effect measurement. Due to the nonequilibrium populatio n distribution initially induced by the intensive infrared pulse, the measu red OKE signal differs from that measured for a molecular system initially in thermal equilibrium state. Particularly, it is found that the deviation from the harmonic response can be specifically measured by using this metho d. By following the same procedure developed in this paper, the two-color I R pump-probe absorption spectroscopy is also considered and the correspondi ng nonequilibrium IR response function is theoretically investigated with a direct comparison with complementary IR-OKE response function. By using th e perturbation theory, the corresponding response functions are obtained in terms of the molecular properties such as linear and nonlinear spatial der ivatives of dipole and polarizability and cubic and quartic anharmonic coef ficients. Also, it is emphasized that this method is conceptually quite sim ilar to the transient nuclear Overhauser effect nuclear magnetic resonance spectroscopy. (C) 2001 American Institute of Physics.