Computer simulation of depolarized light scattered from clusters.

摘要

The homogeneous nucleation of a gas is its transition from vapor to liquid through an intermediate metastable state in which clusters form. The distribution of clusters characterizes the metastable state. I have studied light scattering as a method of measuring the cluster distribution. Computer models provided the nucleating systems studied, and simulated the light scattered by the systems.;I studied three models of argon gas: a hard core square well model, a hard core model, and a more realistic Lennard-Jones model. I used Gaussian quadrature and Monte-Carlo methods to calculate phase averages and molecular dynamics routines to calculate time averages. Time correlations were calculated using a mixture of phase and time average techniques.;The scattered light was described by a classical effective field dipole model. In this model the dipole moment of each particle results from its linear response to the local field; the local field is the sum of the incident field and the dipole fields. I analyzed the results by developing two series solutions of the local field equations, the multi-scattering expansion and the multi-particle expansion, to explain cluster effects on the scattered light.;I calculated the depolarized scattered spectra of a diatom at several temperatures by simulation. Each spectrum separated into classically bound and unbound pair contributions. The unbound component of the square well gas was similar to hard core gas spectrum. The bound component had peaks located at the natural frequencies of the bound pair. The spectra of Lennard-Jones diatoms were similar to the hard core square well spectra.;The light scattered from a triatom of square well gas was calculated in similar fashion. I used the multi-particle expansion to separate the two body and three body terms of the triatom spectrum. The three body term contained a diatom like term and terms characteristic of three body interactions. Trimers could be identified experimentally by separating the three body terms from the rest of the spectrum.;Finally, I discussed the application of the multi-particle expansion analysis to measurements of general cluster numbers.