Why ${\mathrm{N}}_2$ Molecules with Thermal Energy Abundantly Dissociate on W(100) and Not on W(110)

Low-energy ${\mathrm{N}}_2$ molecules easily dissociate on W(100) but not on W(110). In this Letter, the six-dimensional potential energy surface for the dissociation of ${\mathrm{N}}_2$ molecules on W(110) has been determined by density functional calculations. Results are compared to those of ${\mathrm{N}}_2$ dissociation on W(100). The difference in reactivity between the two faces is shown to arise from the characteristics of the potential energy surface far from the surface ($>3\AA$) and not from the properties of a precursor well or those of the final atomic adsorption sites.

Figure 1

Experimental sticking coefficient $S_0$ for ${\mathrm{N}}_2$ on W(110) (left panel) and W(100) (right panel) as a function of impact energy for normal incidence. Data are extracted from Refs. 12, 25 (100) and 13, 18 (110). Surface temperature is 800 K (black squares and diamonds), 300 K (red circles), and 100 K (blue triangles). Lines are drawn to guide the eye.

Figure 2

Dissociative sticking probability $S_0$ for $N_2$ molecules on W(110), as a function of impact energy $E_i$ (in eV). Incidence angles with respect to the surface normal are ${\Theta }_i=0°$ (left panel) and ${\Theta }_i=60°$ (right panel). Small red circles: our theoretical results (line is shown to guide the eye). Experimental results are from Refs. 13 (squares) and 18 (diamonds).

Figure 3

Potential (in meV) as a function of time (in picoseconds) along a trajectory leading to dissociation, for ${\mathrm{N}}_2$ on W(110). The geometry of the system at some points of the trajectory is represented in the upper panels. For each geometry, the PES is also plotted as a function of radial coordinate (horizontal axis) and distance from the surface (vertical axis). The energy difference between contour lines is 0.2 eV. Distances are in Å.

Figure 4

Probability for ${\mathrm{N}}_2$ molecules to reach a given distance $Z$ from the W surface, as a function of normal impact kinetic energy (in meV) for 5000 trajectories. Up-pointing triangles and circles correspond to $Z=3.25\AA$ and $Z=2\AA$, respectively. Squares show the final value of the sticking coefficient $S_0$. Lines are drawn to guide the eye: (red) solid lines refer to W(100) and (black) dotted lines to W(110).