DISSOCIATIVE SCATTERING OF N-2(+) FROM CU(111)

We have scattered 2.6-5.2 keV N-2(+) and N+ beams under glancing angles of incidence (1-5-degrees) from a Cu(111) surface. Neutralization occurs with high efficiency. The dissociation probability is a strong function of the incidence energy and incidence angle. The energy loss of non-dissociated molecules is independent of the crystal azimuth at small incidence angles, while the dissociation probability is not. The kinetic energy released in the dissociation is much larger than that expected from a consideration of the momentum transfer in elastic atom-atom collisions. We tentatively propose that the fast motion of the beam parallel to the surface creates electron-hole pairs, which explains the translational energy loss, and that vibrational excitation and ultimately dissociation occurs via coupling of this electron-hole pair excitation to the vibrational coordinate. We suggest that this may occur through partial occupation of negative ion-like resonances of N2 via the mechanism suggested by both Rettner et al. [Phys. Rev. Lett. 55 (1985) 1904] and Newns [Surf. Sci. 171 (1986) 600].