Experimental study of the adsorption of lithium fluoride on the (111) surface of CaF 2

Abstract

The interaction of molecular beams of the lithium fluoride monomers LiF and dimers Li 2F 2 with the (111) surface of CaF 2 crystals has been studied by the method of desorption transients for crystal temperatures between T=777 and 985 K and for total lithium fluoride fluxes j on=1×10 10–1×10 14 cm −2s −1. The studies show that the monomers are adsorbed at the monatomic steps on the CaF 2 surface. The steps have a limited capacity of the order of 10 12 cm −2 for an adsorption direct at them; further monomers are adsorbed in their immediate neighborhood beside already adsorbed monomers. The adsorption times derived from transient desorption fluxes result as τ 0 (s)=1.3 (+9.5, −1.15)×10 −15 exp[(2.70±0.17) eV/ kT] for an adsorption direct at the steps and as τ 1 (s)=7.6 (+30, −6.0)×10 −15 exp[(2.44±0.11) eV/ kT] beside them. For the dimers, only a portion α d, 0.5< α d( T)<0.85, of the impinging dimer flux reaches the steps, while the remainder desorbs quickly. From α d( T), the ratio of the mean diffusion length x d s on the terrace to the mean step distance, λ, is determined as x d s/ λ=(2.9±0.9)×10 −2 exp[(0.5±0.05) eV/2 kT]. With λ≈8×10 −6 cm as derived from atom force microscopic studies jump distances for the dimers in the order of the lattice constant of CaF 2 are obtained corresponding to a surface diffusion model with jumps between neighboring lattice sites. Dimers at the steps are completely dissociated into monomers. For larger monomer concentrations at the steps also a formation of dimers and their subsequent desorption is observed. By a comparison of experimental desorption transients with a simple theoretical model, the above adsorption times as well as a constant k (cm s −1)=8.0(+42, −6.8) exp[−(1.63±0.16) eV/ kT] describing the formation and desorption of the dimers are determined.