CHEMISORPTION OF HYDROGEN ON THE AG(111) SURFACE

The interaction of atomic hydrogen/deuterium with the Ag(111) surface was investigated with high-resolution electron loss spectroscopy (HREELS), thermal desorption spectroscopy, low-energy electron diffraction (LEED) and the results correlated with the work function change and an absolute coverage measurement. Dissociation of molecular hydrogen is an activated process on Ag(111) but atomic hydrogen chemisorbs and at a temperature of approximately 185 K, recombines and desorbs as H-2. Upon the adsorption of atomic hydrogen at 100 K, a sharp (2X2) LEED pattern is observed in the coverage range 0.25m approximately 0.5 ML and is followed by combined (2X2)+(3X3) patterns with an increase in coverage of 0.1 ML. The saturation coverage is determined to be THETA(sat)=0.6+/-0.1 ML. In contrast to other H/fcc(111) systems, the intensities of the fractional order diffraction beams are quite large, suggesting an H-induced reconstruction. The work function increases monotonically with H-exposure to a maxImum value of DELTApsi(max)=+0.32 eV at saturation. Specular HREELS vibrational data show a single loss at 106 meV for hydrogen (76 meV for deuterium), similar to the normal vibrational mode observed for the H/Ag(110) system. A rather low activation energy (8.7+/-1.0 kcal/mol) for recombinative desorption is obtained from the low coverage thermal desorption spectra (THETA<0.1 ML).