PRESTAGES AND INITIAL-STAGES OF EPITAXY IN ALKALI-HALIDE SYSTEMS .1. INTERACTION OF MOLECULAR-BEAMS OF KCL WITH (100) SURFACES OF NACL

The interaction of molecular beams of KCl with (100) surfaces of NaCl is studied by mass spectrometry of transient and steady state desorption fluxes and by static SIMS, LEED, and the electron microscopic decoration technique. The results for undersaturation are interpreted by an interplay of condensation, bulk diffusion and re-evaporation of KCl molecules and K+ ions, respectively. From the time constants of the transients the bulk diffusion coefficient of K+ in NaCl D = 35(+32/-17) exp[-(2.04 +/- 0.04) eV/kT] cm2 s-1 with a corresponding jump energy E(m) = 0.98 eV is obtained. The results for supersaturation are interpreted by the formation of a solid-solution layer growing on the surface with a surface composition changing continuously from pure NaCl to nearly pure KCl with increasing thickness. Different activity coefficients of KCl and NaCl for this mixed-crystal layer are interpreted kinetically, i.e., by the different bulk diffusion coefficients of Na+ and K+ ions.