LEED STUDIES OF THE ADSORPTION OF CO2 ON THIN EPITAXIAL NACL(100) FILMS

The adsorption of CO2 on the NaCl(100) surface was studied with a high-resolution LEED-system. Measurements without charging up at low electron energies and without damage by the e-beam could be performed by using ultrathin epitaxial films on a conducting Ge(100) substrate. The adsorption behavior was recorded as a function of time and pressure at constant substrate temperatures of 78 and 83 K and CO2 partial pressures from 4 x 10(-8)-2 X 10(-3) pa. The adsorption system shows a first-order two-dimensional phase transition to a (2 x 1) superstructure including glide planes (herringbone-like structure) at p = 7.2 x 10(-8) Pa (T = 78 K). The condensation of the CO2 Solid is starting at p = 1.5 x 10(-4) Pa (T = 78 K). The LEED-pattern shows in this case a c(2 x 2) superstructure, which corresponds to the pyrite-like structure of the CO2 solid. Both observed superstructures are commensurable with the NaCl(100) surface. Observation of island growth shows that the domains of the (2 x 1) superstructures have already at coverage of 5% of a monolayer an average lateral size of at least 200 angstrom.