Nitrogen recombination dynamics at Cu(111): Rotational energy release and product angular distributions

Abstract

Nitrogen atoms adsorbed on Cu(111) desorb thermally from an ordered Cu(100)−c(2×2)N phase in a sharp, zero order desorption feature near 700 K with an activation barrier of 143 kJ mol−1. Detailed N2 product rovibrational state distributions have been measured following recombinative desorption from a 700 K Cu(111) surface exposed to a N atom beam, with an equilibrium N coverage θN⩽10−2 ML. Although desorbing N2 is translationally and vibrationally hot, with a vibrational temperature of 5100 K and 4.2 eV of translational excitation perpendicular to the surface, rotation is excited with a temperature of just 910(±50) K for the vibrational ground state and 840(±250) K for (v=1). The energy released during recombinative desorption channels effectively into translational and vibrational motion, but not into rotational excitation. The angular distribution of recombinatively desorbed N2 is sharply peaked along the surface normal, P(θ)=cos(28±1) θ, indicating a mean energy release of 0.28 eV into translation parallel to the surface. This is inconsistent with 1D models of the translational energy release based on thermal motion parallel to the surface and a repulsive energy release directed along the surface normal. The dynamics can be described by a direct, repulsive model with a transition state at extended N2 separation, similar to the models developed for H2 dissociation on the same surface. We discuss the application of detailed balance to determine N2 sticking functions S(E,v,J) and, using a simple model for these functions, estimate a rotational efficacy of ∼0.23 for sticking of N2(v=0, J⩽24) and a vibrational efficacy of 0.7 for N2(v=1). The dynamics are compared to the models developed for H2 dissociation and the role of molecular chemisorption states and the local desorption site discussed.

The translational energy of desorbing products in NO and N2O decomposition on Pd (110)

“…24 However, the component of normally-collimated desorption found in the present study may not be attributed to this combinative process. This is because the dissociated N͑ad͒ rapidly reacts with NO͑ad͒ to produce N 2 O͑ad͒ around 500 K. In order to investigate this issue, we need further experiments of angular and velocity distributions of desorbing N 2 in a plane along the ͓110͔ direction.…”

Two-directional N2 desorption in thermal dissociation of N2O on Rh(110), Ir(110), and Pd(110) at low temperatures

“…Such repulsion was reported in the combinative desorption as 2N(a) →N 2 (g) on Ru͑001͒ and Cu͑111͒. [15][16][17] The force is exerted to the product along the surface normal because normally directed desorption was observed. However, the normally directed force in N 2 O dissociation is not expected to be as large as that in the combinative desorption because the location of N in N 2 O(a) is further from the surface than that of the adsorbed nitrogen atom.…”

Scattering of Hyperthermal Effusive N and N2 Beams at Metal Surfaces

“…For Ag(111) the knowledge is summarized in [37] and is limited. For Cu and Ru more is known [38][39][40][41][42][43]. In general, the N-atoms reside in three fold hollow sites and are almost inside the metal lattice.…”