Kr and Xe on NaCl(100): pure phases and their miscibility

Using LEED spot profile analysis (SPA-LEED) and thermal programmed desorpti on spectroscopy (TDS), the pure Kr/NaCl(1 0 0) system and its mixture with Xe has been studied. As substrate thin epitaxial NaCl films on a Ge(1 0 0) substrate have been used. Kr in the monolayer (MC) regime forms a faint (2 x 1)pmg structure with glide plane symmetry, whereas the second layer forms a quadratic lattice with a lattice constant close to the Kr bulk value. Fr om the phase diagram for Kr condensation into the first and second monolaye r heats of condensation of 9.5 +/- 2 and 9.0 +/- 2 kJ/mol have been derived , respectively. These are significantly lower than those obtained from TDS. Mixtures of Kr with preadsorbed Xe have been studied for preadsorbed Xe co verages 0 less than or equal to Theta (Xe) less than or equal to 3 ML under isobaric Kr pressures between 10(-7) less than or equal to p(Kr) less than or equal to 10(-6) mbar and at surface temperatures between 35 less than o r equal to T less than or equal to 60 K. Emphasis is put on investigations of the first monolayer. The phase transition of Kr condensation into the fi rst monolayer is shifted to higher transition temperature and broadens for the mixtures. Whereas no new ordered structures have been observed compared with the pure Xe and Kr systems, atomic mixture in the first layer is conf irmed by the observation of a continuously varying average lattice constant in one direction between the (2 x 1)pmg and the distorted hexagonal struct ure of Xe. A miscibility gap depending on T, pK, and Ex, is found. At T, of Kr condensation in the first layer, the solubilities are s(Kr) similar to 0.25 and s(Xe) similar to 0.3. For p(Kr) > 5 x 10(-7) mbar the miscibility gap closes prior to the transition to a Kr three-dimensional layer. The mix ture of Kr into the first layer of Xe is even observed for preadsorbed Xe c overages above one monolayer. (C) 2001 Elsevier Science B.V. All rights res erved.