Photogeneration of atomic hydrogen in rare gas matrices

Photodissociation of HCl and HBr upon excitation on their repulsive A (1)Pi states is studied in low-temperature Ar, Kr, and Xe matrices at photon energies of 5.0 and 6.4 eV. The dissociation is followed by fluorescence spectroscopy and electron paramagnetic resonance. In Ar matrix dissociation can be considered as a local event with simple first-order kinetics and 100% conversion efficiency of the precursor into isolated hydrogen atoms. In Kr matrix the conversion efficiency varies from 18% in 1:500 matrix to 100% in 1:8000 matrix. In Xe matrix the obtained H atom number density is extremely low and prevents detailed analysis of the photogeneration dynamics. The observed behavior is ascribed to long-range dissociation followed by efficient bimolecular reactive loss channels, and thus supports the previous findings by LaBrake, Ryan, and Weitz [J. Chem. Phys. 102, 4112 (1995)]. Molecular dynamics simulations based on a simplified model for dissociation are carried out. The initial 2.6 eV excess kinetic energy of the excited H atom is relaxed as local heating in Ar matrix, whereas in Kr and Xe matrices the excess energy is directed to long-range mobility with flight distances up to 40 Angstrom. (C) 1999 American Institute of Physics. [S0021-9606(99)00316-5].