Energies and lifetimes of predissociative states of van der Waals molecules: Self-consistent field calculations for I₂(v)He, I ₂(v)Ne

The self-consistent field (SCF) approximation for coupled anharmonic vibrations is applied to the calculation of vibrational energy levels (predissociation resonances) of collinear models of I₂(v)He and I₂(v)Ne, with vibrational quantum number v between 5 and 30. The predissociation lifetimes of these same states are obtained from a distorted wave Born approximation calculation with self-consistent field states taken as the initial states of the complex, and with correlation between the modes taken as the interactions leading to decay. Although the binding energies of the van der Waals complex are very small (order of several cm^-1), the SCF eigenvalues are in remarkable agreement with the exact numerical values. The lifetimes obtained from the SCF-distorted wave Born approximation (DWBA) are compared with calculations in which the initial state is treated more simply, assuming separability of the modes involved. Results then show that the DWBA with SCF initial states is considerably more accurate than with the more primitive initial state choice. We conclude from these results that the self-consistent field method offers a very accurate description of large-amplitude vibrational motions in van der Waals clusters, with good quantitative results for both the energy levels and predissociation dynamics of these species.