The surface structure and chemical reactivity of Rh(111)-(2 × 2)-3NO by HREELS and dynamical LEED analysis

The surface structure and chemical reactivity of nitric oxide on Rh(111) has been studied over the temperature range of 120-480 K by high-resolution electron energy loss spectroscopy (HREELS) and dynamical low-energy electron diffraction (LEED) analysis. HREELS results indicate that the adsorption sites of NO depend strongly on the surface coverage and adsorption temperature. At one-half monolayer coverage a c(4 × 2) LEED pattern is observed, with only bridge sites occupied (NO stretching frequency of 1590 cm ^-1). At saturation coverage and 120 K, only disordered bridge-site adsorption is observed (NO stretching frequency of 1635 cm ^-1). Saturation adsorption at a surface temperature between 250 and 350 K leads to NO occupation of top sites ( v(NO) = 1830 cm ^-1) in addition to bridge sites ( v(NO) = 1515 cm ^-1), in an ordered (2 × 2) lattice. A dynamical LEED intensity analysis of the (2 × 2) structure shows that each unit cell contains two molecules in near-top-sites and one in a bridge site. All the NO molecules are perpendicular to the surface with bond lengths of 1.15 Å. The RhN bond length is 2.07 Å at the near-top site and 2.05 Å at the bridge site. The NO surface structure is similar to a previously analyzed (2 × 2) structure at high CO coverage. This is the first LEED structural analysis for a pure molecular NO overlayer on a metal single crystal surface.