Nonresonant Selective Spectroscopy of Intermolecular Dynamics in Liquid in the Terahertz Frequency Range

A new method of nonresonant selective spectroscopy was proposed for studying the low-frequency dynamics of molecules in liquids. The method is based on the nonresonant two-pulse laser excitation of the ultrafast optical Kerr effect. The first pulse induces anisotropy of the medium exclusively due to the orientational response of molecules. Theoretical calculations showed that, under these conditions, one can send a second pulse, which excites other vibrational and rotational responses, but suppresses the induced orientational anisotropy. It was shown that this scenario corresponds to conventional single-pulse excitation of the ultrafast optical Kerr effect, which does not include the contribution from orientational anisotropy. This allows for accurate determination of the spectral characteristics of intermolecular dynamics.