Absorption by dissociative continua in condensed matter: H 2O in rare gas and ice matrices

Abstract

The absorption spectrum of H 2O in Ne, Ar, Kr, Xe and ice matrices in the region of the first à 1B 1 continuum is reported. Substantial blue gas-to-matrix shifts are observed for the maximum, which increase with decreasing matrix cage size. The absorption thresholds show much weaker shifts. These features are interpreted in terms of Rydbergization of the (3s/4a 1) molecular orbital correlated to the à state. The photochemistry is strongly affected by the environmental effects on the excited state potential surface. An energy threshold to photodissociation is confirmed, consistent with a potential barrier due to the cage and a prompt exit of the H fragment. The photodissociation quantum yield shows a linear dependence with excess kinetic energy in Ar and Xe matrices, in agreement with recent MD simulations also favouring prompt exits as the dominant mechanism to permanent dissociation.