Stereodynamics in Dissociative Adsorption of NO on Si(111)

We report results of our study on the surface-temperature dependence of the steric effect in the dissociative adsorption of NO on $\mathrm{Si}(111)\mathrm{\text{−}}(7\times 7)$. Data presented here show that, at an incident energy of 58 meV, the reactive sticking probability for the N-end collision is larger than that for the O-end collision. Furthermore, this steric preference is quite sensitive to the surface temperature and the surface coverage. This study shows that the transient surface trapping into a shallow precursor well plays a key role in the stereodynamics of the dissociative adsorption at the low energy region.

Figure 1

(Left figure) Initial reactive sticking probability $S_0$ as a function of $E_i$ for the randomly oriented NO incidence at $T_s=400\mathrm{K}$. The full and dashed line represents the Baule-type and the HC models, respectively. (Right figure) Schematic interaction potential.

Figure 2

(a) Uptake curves of ${\theta }_{\mathrm{total}}={\theta }_{\mathrm{O}}+{\theta }_{\mathrm{N}}$ and ${\theta }_{\mathrm{O}}$ as a function of exposure time ($t$) for the incidence of 58 meV randomly oriented NO at $T_s=330$ (triangles), 400 (circles), and 600 K (squares). (b) ${\theta }_{\mathrm{N}}/{\theta }_{\mathrm{O}}$ ratio, estimated from (a), as a function of $t$.

Figure 3

Orientation dependence of the O-$1s$ (upper) and N-$1s$ (lower) XPS spectra for the 6 (left) and 8.5 (right) min exposures of 58 meV oriented NO beams at $T_s=400$ (left) and 600 (right) K. The N-end and O-end collisions are indicated by squares and triangles, respectively. Spectra correspond to ${\theta }_{\mathrm{total}}=\sim 0.41$ and $\sim 0.30\mathrm{ML}$ at $T_s=400$ and 600 K, respectively.

Figure 4

(a) Stereo-asymmetry $R$ as a function of ${\theta }_{\mathrm{total}}$ at $T_s=330$ (triangles), 400 (circles), and 600 K (squares). ${\theta }_{\mathrm{total}}$ for the exposure of the random-orientation NO is used for the plot of $R$. (b) Average reactive sticking probability $S_{\mathrm{av},i}$ as a function of the averaged coverage at $T_s=330$ (triangles), 400 (circles), and 600 K (squares). $S_{\mathrm{av},i}$ is plotted at ${\theta }_{\mathrm{Av}}=\frac{{\theta }_{\mathrm{total}}(t_1)+{\theta }_{\mathrm{total}}(t_2)}{2}$ obtained for the dose of random-orientation NO beam. The N-end or O-end collisions are indicated as full and open marks, respectively.