Interaction of molecular beams of NaCl with (100) surfaces of KCl

Abstract

The interaction of molecular beams of NaCl with (100) surfaces of KCl is studied by measurements of the desorption fluxes of the monomers NaCl and KCl and the dimers Na 2Cl 2, NaKCl 2, and K 2Cl 2 and by static SIMS for crystal temperatures T from 619 to 866 K and for impinging sodium chloride fluxes from 1.3 × 10 10 to 5.8 × 10 14 molecules cm 2 · s. The results for undersaturation can be interpreted by an interplay of condensation and re-evaporation of Na + containing molecules and of bulk diffusion of Na + ions, respectively. From measurements of transient desorption fluxes the bulk diffusion coefficient of Na + in KCl D = 34.3( +121 −27) exp [ − (1.59 ± 0.10 eV) kT ] cm 2 s −1 with a corresponding jump energy E m = 0.34 eV is obtained. Determining the surface concentrations of K and Na by static SIMS the desorption fluxes can be interpreted in terms of activity coefficients. From these measurements and a rough model the ratio of the Na + K + exchange time with the terrace and the desorption time of the admolecules can be estimated. At supersaturation a solid solution layer grows on the surface with a surface composition changing continuously from KCl to NaCl with increasing thickness. The morphological development of the overlayers is studied by the electron microscopic decoration technique. At an intermediate stage multiple spirals corresponding to bunches of 2–9 screw dislocations are observed.