Photolysis of hydrogen chloride embedded in the first argon solvation shell:: Rotational control and quantum dynamics of photofragments

Under standard conditions reaction yields are connected with terms like free energy differences and thermal distributions. However, many modern experimental techniques, such as supersonic beam expansion or matrix isolation, deal with cryogenic temperatures and isolated reactants in inert clusters or solid matrices. Under these conditions the photochemical reaction mechanism is in many cases strongly dependent on the shape of delocalized initial vibrational or rotational wave functions of the reactants which can be employed for an efficient reaction yield control. Here, we apply, using quantum molecular dynamics simulations, such a scheme to the rotational control of photolysis of the HCl molecule embedded in an icosahedral Ar-12 cluster. First, the HCl molecule is preexcited into a specific low lying rotational level. Depending on the rotational state, the hydrogen probability is enhanced in different directions within the cluster. In a second step, the HCl molecule is photolyzed by an UV pulse. The rapidly dissociating hydrogen atom then reaches primarily either the holes in the solvent shell or the argon atoms, depending on the rotational preexcitation. Starting either from the ground or from the first totally symmetric excited rotational states, the direct dissociation and the delayed process accompanied by a temporary trapping of the hydrogen atom have very different relative yields. As a consequence, differences up to a factor of 5 in the temporary population of the hydrogen atom inside the cluster after the first hydrogen-cage collision are observed. In the energy domain a significant difference in the structure of the kinetic energy distribution spectra, connected with the existence of short-lived vibrational resonances of the hydrogen atom, is predicted. (C) 1999 American Institute of Physics. [S0021-9606(99)51012-X].