Molecular simulation of static hyper-Rayleigh scattering: A calculation ofthe depolarization ratio and the local fields for liquid nitrobenzene

Molecular dynamics (MD) simulation is used to assess the hyper-Rayleigh sca ttering (HRS) depolarization ratio of liquid nitrobenzene subject to vertic ally polarized light. In contrast to previous theoretical work, we have qua ntified both incoherent and coherent scattering arising from positional and orientational inhomogeneities in the molecular distribution. Although cohe rent scattering is shown to be much less important than in the case of Rayl eigh scattering, it can not be neglected. Therefore, our analysis supports the current practice of working with dilute solutions (for which coherent c ontributions to HRS are truly negligible) to extract the first molecular hy perpolarizability from HRS measurements. In cases where experiments with pu re liquids can not be circumvented, our analysis may be used to separate co herent and incoherent signals. Our work, which uses as input static "gas-ph ase" (hyper)polarizabilities obtained from ab initio calculations, also pro vides information on the orientations and magnitudes of the local electric fields experienced by the individual molecules in the liquid. For nitrobenz ene it is found that the local fields are largely determined by specific di polar alignment between neighboring pairs of molecules, with consequences o n the HRS signal. (C) 1999 American Institute of Physics. [S0021-9606(99)51 143-4].