Vibrational Predissociation in Hydrogen Bonded Dimers: The Case of (HF)2 and its Isotopomers

We use the dimer (HF???HF) as a model system to understand the dynamics in hydrogen-bonded systems. This particularly simple system has been widely used both in experimental and theoretical studies. Here we focus on the remarkable mode selectivity in vibrational predissociation processes which occur on time scales of picoseconds to nanoseconds. We have performed classical molecular dynamics (MD) calculations on the six-dimensional SO-3 potential energy surface (PES) of (HF)2[1] to estimate absorption spectra and predissociation lifetimes ?PD for various initial vibrational excitations involving HF stretching. Our calculations can qualitatively reproduce the mode selectivity in ?PD observed experimentally: Excitations involving the 'hydrogen-bonded' HF stretching mode give rise to shorter ?PD than those involving the 'free' HF stretching mode. Besides results concerning the HF dimer, this study offers the opportunity to check to what extent classical MD calculations on an accurate and realistic potential are suitable to study dynamical properties in such a molecular system.