MOLECULAR-DYNAMICS SIMULATIONS OF THE HOMOGENEOUS NUCLEATION OF UF, AND SF, MOLECULES - EFFECTS OF THE INTRAMOLECULAR VIBRATIONAL RELAXATIONS ON THE NUCLEATION RATES

The effects of flexibility in the homogeneous nucleation processes of UF6 and SF6 molecules from vapor phase were investigated by classical molecular dynamics (MD) simulations. We performed MD simulations using a flexible-molecule model and compared the results with those obtaine d from a rigid-molecule model. We took into account the flexibility of molecules in MD simulations by a harmonic intramolecular potential. W e found that the nucleation rate in the flexible model of the UF6 mole cule was about twice as large as that in the rigid model of UF6. This acceleration in nucleation rate was attributed to the flow of the cond ensation heat into the intramolecular vibrations. On the other hand, t he nucleation rates in rigid and flexible models of SF6 were almost th e same because the flow of the condensation heat into the intramolecul ar vibrations in the flexible model of SF6 was negligibly small. In or der to confirm the reliability of the classical intramolecular vibrati onal model in the present work, we estimated the intramolecular vibrat ional relaxation times of the flexible UF6 and the flexible SF6 molecu les in the gas phase using the same MD simulation technique as used fo r the nucleations. The intramolecular vibrational relaxation times obt ained from the MD simulations were in good agreement with those from t he experimental data in cases of both UF6 and SF6 molecules. (C) 1998 American Institute of Physics.