We carried out molecular dynamics (MD) simulations for a dilute aqueous solution of pyrimidine in order to investigate the mechanisms of field-induced molecular alignment in a liquid phase.An anisotopically polarizable molecule can be aligned in a liquid phase by the interaction with a nonresonant intense laser field.We derived the effective forces induced by a nonresonant field on the basis of the concept of the average of the total potential over one optical cycle.The results of MD simulations show that a pyrimidine molecule is aligned in an aqueous solution by a linearly polarized field of light intensity I approx 10~(13) W/cm~2 and wavelength X=800 nm.The temporal behavior of field-induced alignment is adequately reproduced by the solution of the Fokker-Planck equation for a model system in which environmental fluctuations are represented by Gaussian white noise.From this analysis,we have revealed that the time required for alignment in a liquid phase is in the order of the reciprocals of rotational diffusion coefficients of a solute molecule.The degree of alignment is determined by the anisotropy of the polarizability of a molecule,light intensity,and temperature.We also discuss differences between the mechanisms of optical alignment in a gas phase and a liquid phase.
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