Interaction of hydrogen with the Ag(110) surface.

Abstract

The interaction of atomic and molecular hydrogen with the (110) surface of silver has been studied using electron-energy-loss spectroscopy, thermal desorption spectroscopy, low-energy electron diffraction (LEED), and work-function measurements. No evidence for associative or dissociative chemisorption of ${\mathrm{H}}_{2}$ is observed at the substrate temperatures investigated (\ensuremath{\ge}90 K). However, at 100 K, atomic hydrogen bonds in the [1 1\ifmmode\bar\else\textasciimacron\fi{} 0] troughs of the surface in tilted-trigonal sites. As a function of concentration, a sequence of lattice-gas superstructures is observed with LEED including (1\ifmmode\times\else\texttimes\fi{}4), (1\ifmmode\times\else\texttimes\fi{}3), (2\ifmmode\times\else\texttimes\fi{}6), and (2\ifmmode\times\else\texttimes\fi{}2) patterns. At saturation coverage, the work function increases by 0.22 eV. However, this phase is metastable; upon annealing, hydrogen desorption is accompanied by an irreversible transition to a new bonding geometry in which LEED shows a dim (1\ifmmode\times\else\texttimes\fi{}2) superstructure. The desorption of molecular hydrogen is characterized by two overlapping peaks centered at \ensuremath{\sim}155 K [6.9 kJ/mol] and \ensuremath{\sim}180 K [9.9 kJ/mol] which obey first- and second-order kinetics, respectively. Various structural models and hydrogen site assignments are discussed in comparison with data for similar systems.