Molecular dynamics simulation of the homogeneous nucleation of UF₆ molecules: Configurations and infrared spectra of the excited hot clusters

The temperature, potential energy, and configurations of the clusters produced in the homogeneous nucleation of UF₆ molecules from the supercooled (supersaturated) vapor phase were determined by classical molecular dynamics (MD) simulations. We observed two phenomena which demonstrate that the nucleation process occurs in the state far from thermal equilibrium. First, the excited hot clusters, the temperature of which is much higher than that of the monomer, were produced and continued to exist during the nucleation process. Second, the relationship between the potential energy and temperature of the clusters depends on the monomer temperature, that is, the potential energy at a temperature decreases with the increase in monomer temperature. In the simulations, various types of cluster configurations were observed: prolate, oblate, spherelike, and confeitolike. The confeitolike cluster is composed of one core and a few horns, and it was found predominantly in the hotter clusters. The infrared spectra of the UF₆ clusters were calculated using a dipole-dipole and dipole-induced dipole interaction model and compared with those measured in a supercooled state by Tanimura et al. [J. Chem. Phys. 107, 7096 (1997)]. As a result, we found that the spectra measured in the supercooled state can be attributed to the excited hot clusters, the configuration of which are confeitolike.