Room temperature observation of nitric oxide on Ir(1 1 1) by scanning tunneling microscopy

Room temperature (RT) adsorption of nitric oxide (NO) on Ir(1 1 1) was studied by scanning tunneling microscopy (STM). At low exposures, NO molecules can not be imaged by STM, because at RT the diffusion of NO is much faster than the STM scanning speed. At high exposures near the saturation coverage, however, a well-ordered 2 × 2 structure is observed. The coverage of the major 2 × 2 species is 0.25 and they can be assigned to the NO molecules adsorbed on the Ir ontop sites. A small number of less bright spots are assigned to nitrogen atoms produced by dissociation. Their number increases by annealing the NO-saturated surface at 380 K. A small number of another dissociation product, oxygen, are observed as black lines, indicating that the diffusion of oxygen atoms is fast. Scratch-like noise features were also detected by the STM, which suggests that a mobile precursor state exists, which was clearly shown by the effects of electron irradiation from the STM tip. These results are consistent with the previous molecular beam studies. Hopping of the 2 × 2 ordered NO species was frequently observed at the anti-phase domain boundaries and edges of the 2 × 2 islands.