The potential energy surface (PES) of NO on Pt(997) has been elucidated: the adsorption states and diffusion processes of NO on Pt(997) at low coverage were investigated by using infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM). When NO molecules adsorb on a surface at a low temperature (11？K), each molecule transiently migrates on the surface from the first impact point to a possible adsorption site. We found that there are four stable adsorption sites for NO on Pt(997): a bridge site of the upper step, an fcc- (or hcp-) hollow site of the terrace, an on-top site of the terrace, and an fcc-hollow site of the lower step. At higher temperatures above 45？K, NO molecules start to migrate thermally to more stable adsorption sites on a terrace, and they are finally trapped at the bridge sites of the step, which are the most stable among the four sites. 1. Introduction The elucidation of potential energy surface (PES) of adsorbed molecules on metal surfaces is quite important. In particular, surface diffusion of adsorbed atoms and molecules plays a vital role in various surface dynamical processes, such as heterogeneous chemical reactions, formation of self-assembled structures, and single atom manipulation by a scanning probe microscope [1]. Since an adsorbed molecule must encounter a reaction partner before a chemical reaction takes place, surface diffusion could be a rate-limiting process for surface chemical reactions. In particular, the adsorption of carbon monoxide (CO) and nitric monoxide (NO) on transition metal surfaces is a basic model for investigating the catalytic reaction on the automobile three-way catalyst, where the PES for CO and NO governs the migration of the molecules. Therefore, not only surface diffusion but also the adsorption states of CO and NO are important. So far, many studies of CO and NO on transition metal surfaces have been reported; NO adsorption on Pt(111) is one of the fundamental systems and there are many studies [2–10]. In addition, surface defects are often stable adsorption sites and active sites for NO dissociation [11–13]. Therefore, the adsorption states and diffusion process of NO on a stepped Pt surface are even more important than that on a flat Pt surface. In this review article, the adsorption states and the diffusion process of NO molecules on Pt(997) are discussed based on our recent investigations by infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM). Surface diffusion is classified into “thermal” and “transient” diffusion [1].
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