COMPETITION BETWEEN ELECTRON DETACHMENT AND MONOMER EVAPORATION IN THE THERMAL-DESTRUCTION OF HYDRATED ELECTRON CLUSTERS

We have examined the competition between electron detachment and monom er evaporation in the thermal destruction (dissociation plus detachmen t) of hydrated electron clusters by monitoring the products in a selec ted ion flow tube apparatus as (H2O)(n)(-) clusters, 14 less than or e qual to n less than or equal to 24, were heated over the temperature r ange 100 to 150 K. The destruction of the smaller clusters is dominate d by electron detachment, and the detachment occurs over the narrow te mperature range 120-145 K. The larger clusters initially undergo seque ntial evaporation of neutral monomer units, forming smaller and smalle r ionic clusters. As the temperature increases, the electron detachmen t channel begins to compete with monomer evaporation, and the smaller ions eventually decay by electron detachment. Second-order rate consta nts and activation energies were obtained for the thermal destruction of clusters 14 less than or equal to n less than or equal to 17 and 23 less than or equal to n less than or equal to 24. The activation ener gies for the destruction of the larger clusters, n greater than or equ al to 17, are nearly constant at similar to 0.34 eV, which is close to the energy required to evaporate a single water molecule from the clu sters, similar to 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 d etermined here are a measure of the clusters adiabatic electron affini ty. 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 evap oration of particles from van der Waals clusters is in accord with the experimentally observed competition between these two cluster thermal decay processes. (C) 1995 American Institute of Physics.