Photodissociation of HBr in and on Ar_n clusters: the role of the position of the molecule

Abstract

Photodissociation experiments are carried out for single HBr molecules which are embedded in the interior or absorbed on the surface of large Arn clusters. For the embedded case the size dependence is measured for the average sizes n=51, 139, 230, 290 and 450. For the surface case and the average size n=139 the source temperature is varied from T=163 K to 263 K. The measured kinetic energy of the H atom fragments exhibits peaks at zero and 1.3 eV which mark completely caged and unperturbed fragments, respectively. The results are compared with Molecular Dynamics simulations which account for the quantum librational delocalization of the HBr molecule. The location of the molecule in/on the cluster is obtained from a trajectory study of the pick-up process under realistic conditions. For the embedded case corresponding to a co-expansion experiment, three argon layers are sufficient to completely hinder the H atom, in perfect agreement with the calculations. For the pick-up experiment, the large change of the source temperature leads to very small changes of cluster temperature dependent properties. Events starting from the second shell have a higher exit probability than those coming from the surface