Interaction of Nitric Oxide with Molecular Adlayers Adsorbed on Rh(111)

Abstract

The interaction of nitric oxide (NO) with well-ordered carbon monoxide (CO) and ethylidyne (CCH3) overlayers chemisorbed on Rh(111) has been studied. State-resolved rotational and translational energy distributions of NO scattered from the CO and CCH3 adlayers on Rh(111) at 300 K were measured using (1+1) resonance-enhanced multiphoton ionization. To compliment these studies, the adsorption of NO onto the same surfaces at 120 K was investigated, using a combination of reflection−absorption infrared spectroscopy, temperature-programmed desorption, and collision-induced desorption. Rotational state and velocity distributions indicate that the NO dynamics show a dominance of scattering via trapping desorption over direct inelastic processes, even at the relatively high incident translational energy (47 kJ mol-1) and the surface temperature (300 K) used. This is in stark contrast with the scattering dynamics of the same molecule from the same adlayers adsorbed on Pt(111). The study of the static interaction of NO with the same adlayers at 120 K revealed that adsorption of NO takes place in an unusual second adlayer on top of both CO and CCH3 overlayers. In particular, it was found that the NO molecule adsorbs with its internuclear axis parallel to the surface. The potential well depth of this particular gas−surface system has been estimated from desorption experiments to be 36 kJ mol-1 for both adlayers. However, the CO adlayer showed a greater propensity for the accommodation of the NO molecule in comparison with the CCH3 adlayer. This is also consistent with the conclusions from the dynamical studies.