A novel low-temperature adsorption state of nitric oxide on a Rh(111)-p(2 x 2)-CCH₃ surface studied by reflection-absorption infrared spectroscopy and auger electron spectroscopy

Adsorption of nitric oxide (NO) at a low surface temperature (120 K) onto a well-ordered ethylidyne (CCH3) adlayer chemisorbed on Rh(111) was studied by reflection-absorption infrared spectroscopy (RAIRS) and Auger electron spectroscopy (AES). The study revealed that adsorption of NO takes place as a second adlayer on top of the CCH3 adlayer with the NO intermolecular axis lying parallel to the surface. This is manifest by the strong perturbation of the ethylidyne methyl group by the NO molecule resulting in the observation of the asymmetric C-H stretch mode, which is normally forbidden in RAIRS by the metal surface selection rule. Remarkably, under those conditions, no RAIRS features of the NO molecule could be observed even when exposing the surface at a dynamic NO pressure of 1 x 10(-6) Torr. In addition, this mechanism appears to be independent from adsorption of NO at room temperature in which the molecule adsorbs onto the metal surface in defected regions of the ethylidyne overlayer and, therefore, supporting the description of the NO adsorption as a second surface layer at low surface temperature. Auger electron spectroscopy was used to confirm the presence of NO species adsorbed as a second adlayer on top of a CCH3 adlayer.