NITROGEN RECOMBINATION DYNAMICS AT CU(111) - ROTATIONAL ENERGY-RELEASE AND PRODUCT ANGULAR-DISTRIBUTIONS

Nitrogen atoms adsorbed on Cu(lll) desorb thermally from an ordered Cu (100) - c(2 x 2)N phase in a sharp, zero order desorption feature near 700 K with an activation barrier of 143 kJ mol(-1). Detailed N-2 prod uct rovibrational state distributions have been measured following rec ombinative desorption from a 700 K Cu(lll) surface exposed to a N atom beam, with an equilibrium N coverage theta(N)less than or equal to 10 (-2) ML. Although desorbing N-2 is translationally and vibrationally h ot, with a vibrational temperature of 5100 K and 4.2 eV of translation al excitation perpendicular to the surface, rotation is excited with a temperature of just 910(+/- 50) K for the vibrational ground state an d 840(+/-250) K for (upsilon = 1). The energy released during recombin ative desorption channels effectively into translational and vibration al motion, but not into rotational excitation. The angular distributio n of recombinatively desorbed N-2 is sharply peaked along the surface normal, P(theta) = cos((28+/-1)) theta, indicating a mean energy relea se of 0.28 eV into translation parallel to the surface. This is incons istent with 1D models of the translational energy release based on the rmal motion parallel to the surface and a repulsive energy release dir ected along the surface normal. The dynamics can be described by a dir ect, repulsive model with a transition state at extended N-2 separatio n, similar to the models developed for H-2 dissociation on the sane su rface. We discuss the application of detailed balance to determine N-2 sticking functions S(E,upsilon,J) and, using a simple model for these functions, estimate a rotational efficacy of similar to 0.23 for stic king of N-2(upsilon = 0, J less than or equal to 24) and a vibrational efficacy of 0.7 for N-2(upsilon = 1) The dynamics are compared to the models developed for H-2 dissociation and the role of molecular chemi sorption states and the local desorption site discussed. (C) 1998 Amer ican Institute of Physics.