Cage dynamics in the third-order off-resonant response of liquid molecules: A theoretical realization

It is generally believed that the ultrafast initial spectroscopic response from the molecules in the condensed phase originates from small amplitude i nertial motions within the cage formed by the nearest-neighbor solvent mole cules surrounding the probe, or the cage effect. However, no quantitative e stimate of this dynamics has been available for the currently popular exper iments which measure the third-order off-resonant response. In this work, w e fill this gap by a microscopic approach and demonstrate that the cage dyn amics alone can produce the initial rise in the subpicosecond (ca. 200 fs) range in the third-order response. A simple analytical expression for the i nitial Gaussian time constant relevant to various kinds of the third-order off-resonant experiments is presented, which is found to be rather strongly dependent on the temperature. Connection with the non-polar solvation dyna mics is also discussed.