Competition between electron detachment and monomer evaporation in the thermal destruction of hydrated electron clusters

Abstract

We have examined the competition between electron detachment and monomer evaporation in the thermal destruction (dissociation plus detachment) of hydrated electron clusters by monitoring the products in a selected ion flow tube apparatus as (H2O)−n clusters, 14≤n≤24, were heated over the temperature range 100 to 150 K. The destruction of the smaller clusters is dominated by electron detachment, and the detachment occurs over the narrow temperature range 120–145 K. The larger clusters initially undergo sequential evaporation of neutral monomer units, forming smaller and smaller ionic clusters. As the temperature increases, the electron detachment channel begins to compete with monomer evaporation, and the smaller ions eventually decay by electron detachment. Second-order rate constants and activation energies were obtained for the thermal destruction of clusters 14≤n≤17 and 23≤n≤24. The activation energies for the destruction of the larger clusters, n≥17, are nearly constant at ∼0.34 eV, which is close to the energy required to evaporate a single water molecule from the clusters, ∼0.40 eV. The difference indicates we are in the low-pressure limit of thermal dissociation. The activation energy for the smaller cluster sizes, n<16, is significantly smaller than the monomer evaporation energy, and since the primary thermal destruction channel for these clusters’ is electron detachment, the activation energies determined here are a measure of the clusters adiabatic electron affinity. The estimated electron affinities for n=14 and n=15 are 0.12 and 0.23 eV, respectively. The electron affinities are in accord with that predicted by the dielectric continuum model. A model reported by Klots considering the temperature- and size-dependent kinetics for the evaporation of particles from van der Waals clusters is in accord with the experimentally observed competition between these two cluster thermal decay processes.