Investigation of condensation and evaporation of alkali halide crystals by molecular beam methods: VIII. Molecular beam pulse experiments with lithium flouride

Abstract

Experiments with molecular beam pulses allow the determination of the relaxation times of monomeric, dimeric, and trimeric molecules of lithium fluoride on (100) surfaces of LiF as functions of the crystal temperature T and of the molecular flux j on impinging onto the surface. At high T and low j on the relaxation times increase exponentially with 1/ T. The corresponding activation energies are 0.79, 0.77, and 1.14 eV for the monomers, dimers, and trimers. At lower T and higher j on they become independent of T but increase with decreasing j on. The experiments were performed with different lithium isotopes: 6Li for the impinging molecular beam and 7Li for the crystal. Measurements with dimers and trimers of differing isotopic composition show that the measured relaxation times are independent of the isotope exchange on the surface. Therefore, the Li exchange times are immeasurably small ( < 10 μs).