Threshold and cage effect for dissociation of H2O and D2O in Ar and Kr matrices

A threshold near 6.8 eV is observed for the dissociation of H2O (and D2O) via the repulsive Ã(1B1) potential in rare gas matrices. The threshold varies within 0.2 eV for exchange of Kr with Ar matrix, upon deuteration and with temperature. The observations are consistent with a prompt dissociation across matrix induced barriers of about 1.8 eV caused by the repulsive hydrogen–rare gas potentials. The increase of dissociation efficiency above threshold and its variation with matrix and isotope substitution seen in the experiments is rationalized by the mass dependence of energy exchange between fragment and matrix and the dependence of the accessible exit cone on kinetic energy. A strong increase in the dissociation efficiency with temperature is observed which is different for Ar and Kr.