The mechanism and kinetics of evaporation from laser irradiated UO2 surfaces

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Abstract
A theoretical analysis of laser evaporation experiments for the determination of the high temperture saturated vapor pressure of uranium dioxide is presented. The interaction of laser radation with the condensed phase is discussed and the timescales involved in the energy equilibration processes are given. The mechanism of evaporation is described within the framework of the Terrace–Ledge–Kink model, where it is shown that by using symmetry properties of ionic crystal lattices, the surface Madelung potentials and binding energies of ions and neutral ionic units may be determined. Results are presented for NaCl, CsCl, and UO2 surfaces. Based on these calculations, general conclusions are drawn on the nature of the evaporating species. In addition, it is shown that the multispecies evaporation from uranium dioxide is fundamentally related to the amount of atomic and electronic disorder in the surface layer. The effects of the kinetics of surface reactions on the rate of evaporation are investigated. Parallel reaction paths such as direct evaporation from kink sites are treated in addition to surface diffusion followed by desorption series reactions. Finally, the possibility of producing superheated metastable surface layers under pulsed laser heating conditions is investigated using models for the kinetics of atomic and electronic disorder in the condensed phase.