Isotope Effects in Evaporation from Small Water Clusters

Abstract

The statistical decay of light and heavy water from ionic water clusters of 4 - 22 molecules was measured in vacuum. The rate constants were found to be increasingly favored towards evaporation of light water compared to the heavier isotopologues, with increasing cluster size. Calculations of the partition functions of H2O, HDO and D2O are presented and used to investigate the origin of the observed rate constants. Competing e ects were found between the higher partition function and the higher dissociation energy of the heavier isotopologues, leading to an overall favored H2O evaporation. The rate constants are found to depend exponentially on the deuterium concentration in the cluster, which may be explained by a second order indirect interaction between deuterium and neighboring hydrogen bonds in the cluster.