A clean (110) tungsten surface can adsorb CO molecules in two different ways. Those striking bare surface have high sticking probability and are held firmly with possible rearrangement of the surfacehyphen;metal atoms. Molecules striking an already covered surface have a somewhat lower sticking probability and are held more weakly. Temperatures producing rapid desorption of the two components are 450deg; and 1100deg;K. The maximum coverage in the first layer is not pressure dependent. In the second layer the maximum coverage is about 0.8 of the firsthyphen;layer coverage at 10minus;7torr but decreases slightly at lower CO pressures. The structure of the first layer, as found by LEED observations, is very disorganized unless the crystal is heated; but a wellhyphen;ordered structure is produced by heating at 925deg;K and poorer order by heating at lower temperatures. The ordered surface mesh isC(9times;5). The detailed structure has not been determined. The intensities of various diffraction beams from a clean surface change in very complicated ways when the primary beam energy is altered; the variation with energy is much simpler when CO is adsorbed on the surface. A suggested distortion of the clean surface that accounts for these facts is presented.
展开▼
