Eley-Rideal Reactions with N Atoms at Ru(0001): Formation of NO and ${\mathrm{N}}_2$

Forward-directed NO molecules with large translational energies are formed upon exposure of an O-covered Ru(0001) surface to a nitrogen ($\mathrm{N}+{\mathrm{N}}_2$) beam. This is an unequivocal experimental demonstration of the Eley-Rideal reaction for a “heavy” (i.e., nonhydrogenated) neutral system. The time dependence of prompt NO formation exhibits an exceptionally fast decay as a consequence of shifting reaction pathways and probabilities over the course of the exposure. Prompt production shuts down as the O coverage decreases due to competition from more favorable Eley-Rideal production of ${\mathrm{N}}_2$.

Figure 1

NO angular intensity distributions produced by the nitrogen beam incident on $\mathrm{Ru}(0001)-\mathrm{O}(2\times 1)$ at ${\theta }_i=50°$ (circles), 60° (squares), and 70° (triangles). Intensities are normalized to the N atom intensity of the incident beam. Data points are fitted with a shifted cosine functions to the power of $\sim 8.2$, $\sim 7$, and $\sim 7$, respectively. The dashed line is a normal cosine distribution. This function has been scaled to match the integrated area of the cosine fit to the ${\theta }_i=60°$ data.

Figure 2

Energy profiles of N atoms in the incident beam (solid line), and NO molecules formed during nitrogen beam exposure of $\mathrm{Ru}(0001)-\mathrm{O}(2\times 1)$ at ${\theta }_i=60°$, ${\theta }_f=25°$ in TOF mode (dashed line; note the scaling factor). Inset: Time dependency of the NO TOF integrated area at ${\theta }_i=60°$, ${\theta }_f=25°$ The solid line is a single exponential decay fitted to the data.

Figure 3

(a) Comparison of QMS $m/z=28$ (${\mathrm{N}}_2$, dashed) and $m/z=30$ (NO, solid; note the scaling factor) responses during two sequential full beam exposures of an initial $\mathrm{Ru}(0001)-\mathrm{O}(2\times 1)$ surface (${\theta }_i=60°$). The up and down arrows indicate beam flag openings and closings, respectively. (b) Initial NO response during full beam exposure of as-prepared $\mathrm{Ru}(0001)-\mathrm{O}(2\times 1)$ (dashed) and the O-Ru surface after a 180 s TOF exposure (solid). (c) Corresponding missing ${\mathrm{N}}_2$ signals derived from the ${\mathrm{N}}_2$ responses during full beam exposure.

Figure 4

${\mathrm{N}}_2$ (filled circles) and N (open circles) angular intensity distributions formed by the nitrogen beam incident on a N-Ru surface at ${\theta }_i=60°$. The NO distribution at ${\theta }_i=60°$ from Fig. 1 (squares) is reproduced. The lines are to guide the eye.