DIFFUSION AND CLUSTERING OF N2O MOLECULES IN ARGON CLUSTERS - A THEORETICAL APPROACH BY MOLECULAR DYNAMICS SIMULATIONS

摘要

Results of molecular dynamics simulations of small van der Waals clusters composed of one argon cluster of size 147 or 125 (incomplete outer-layer cluster) and from one to four N2O molecules deposited at thermal relative collision energy on the argon cluster are presented. The potential energy is calculated through the semiempirical Claverie method. We discuss here the necessity and the practical application of fitting some of the potential parameters in order to reproduce the N2O experimental dipole moment value as well as the experimentally observed N2O...N2O and N2O...Ar equilibrium geometries. We first show that, as in the case of atomic projectiles, a very efficient capture by collision of the N2O molecules by the argon clusters is observed, independently of the initial molecular orientation. Studying trajectories over tens of nanoseconds then gives evidence that the N2O molecules move independently on the surface of the argon clusters, and that the molecules migrate randomly through jump displacements on the surface of the clusters. We observe a very high N2O mobility and we explain the influence of the argon cluster outer-layer structure on mobility. Collisions of the N2O molecules on the surface of the argon clusters result in a sequential and fast clustering. The geometries of the energetically stable (N2O)(m) microclusters have been characterized. Using the formula of Perrin, we calculate and interpret single N2O diffusion coefficients and (N2O)(m) microcluster diffusion coefficients, whose values are 1 to 100 times lower than in the liquid state. Finally, we extend our results to larger argon clusters, such as Ar-1000, through a random walk model taking place on the surface of a sphere, which enables us to calculate mean encounter times between particles. We thus interpret the fluorescence quenching that occurs in chemical reactions taking place on finite-size argon clusters. (C) 1997 American Institute of Physics. [References: 50]