Abstract
Using the FEM method adsorption of CO on tungsten tips has been studied at adsorption temperatures of 300 K, 200 K and 80 K. A proposed model of adsorbed species has been used to analyze the observed FEM images. The low-temperature (below200 K) CO layer and the high-temperature (200 K-300 K) CO layer are shown to have no differences, except that weakly-bound CO species are additionally populated as the temperature of adsorption is lowered. At saturation the CO layer over a tungsten tip below 300 K is interpreted to be a superposition of seven states which are attributed to those seven intrinsic sites over a tip of a b.c.c. crystal which are given by considering not only the outermost atomic arrays of surface planes but also atomic arrays below the outermost ones. An interpretation of “pseudoclean pattern” is made that if all the above types of adsorption are completely surface-dependent the relative situation of the saturated tip is not different from that of the clean tip. The occupation at 80 K is found to be attained most completely, which seems to permit the image the most clever imitation of the clean pattern.
