Applications of the interaction between light and anisotropic molecular polarizabilities: Enhanced diffraction patterns of gas-phase polyatomic molecules and analysis of optical Kerr effect spectra for a molecular liquid

Nonresonant interaction between light and anisotropic molecular polarizabilites can induce molecular alignment in fluid systems. We present two applications of molecular alignment with regards to gas-phase molecular diffraction and short time liquid dynamics. First, we found that, once molecules of a gas-phase sample are oriented or aligned in space, its diffraction signals become intensified strongly enough to distinguish between the effects of different vibrational modes on signal patterns. We assess the feasibility of observing the mode-specific signal behavior, which we call the vibrational signature, from general poly-atomic molecules. The simulation of diffraction signals of an 8-atomic cyclic molecule, s-tetrazine, illustrated that, when molecular alignment is achieved with the use of a laser intensity on an order of TW/cm2, the vibrational signatures, which are hardly observable in conventional diffraction experiments, can be enhanced noticeably. We discuss in great detail how using a laser field as "optical goniometer" in diffraction experiments can help elucidate both molecular structures and dynamics reflected on signal patterns. In order to characterize a time-domain signal of optical Kerr effect spectroscopy of liquid benzene, which features the short time (subpicosecond) molecular response to aligning laser pulses, we performed molecular dynamics simulation and employed the instantaneous-normal-mode liquid theory. We found that, while most of the short-time signal originates from the librational motion of individual molecules, there is another kind of molecular librational motion highly correlated with very delocalized translational dynamics of the system. We suggest that the latter librational motion is responsible for the controversial low-frequency feature in the Fourier-transform of the signal.