Resonances in the photodissociation of HCl in the Ar-HCl van der Waals complex: How prominent are they?

We investigate the cage effect in the ultraviolet (UV) photodissociation of the Ar⋯-HCl van der Waals complex, especially the possibility of resonance structures caused by trapping of the hydrogen atom between its heavy partners as recently highlighted by Garcia-Vela and Gerber [J. Chem. Phys. 98, 427 (1993)]. The dynamics is described by solving the time-dependent Schrödinger equation employing the standard Jacobi coordinates which are routinely used for triatomic systems. Due to the large size of the required grid, exact three-dimensional (3D) wave packet calculations are extremely time consuming and could be followed up to 20 fs only. This time is sufficient for calculating the absorption spectrum, but too short for determining the final kinetic energy distributions of the fragment atoms. Therefore, the photodissociation dynamics is mainly treated in a vibrationally sudden approximation, in which the dynamical calculations are performed for a range of fixed ArCl bond distances, and the results averaged over this bond length. 3D classical trajectory calculations show that the energy transfer out of the dissociative HCl mode is very weak (∼5% of the total energy), supporting the application of the sudden approximation. In this approximation, both the absorption spectrum and the kinetic energy distribution associated with the dissociating HCl motion exhibit very weak diffuse structures (resonances) which, following the work of Garcia-Vela and Gerber, can be assigned to the transient vibrational motion of hydrogen between Ar and Cl. However, in our calculations these structures are much less pronounced than in the work of Garcia-Vela and Gerber. The very small amplitudes of the resonance features indicate that trapping in the dissociation of HCl in Ar⋯-HCl is marginal, and much less, important than suggested by the previous studies of Garcia-Vela et al. Furthermore, in contrast to the work reported by Garcia-Vela et al., we do not find any evidence for the narrow, irregular features superimposed on the resonance structures.