Direct vibrational excitation in gas-surface collisions of NO with Ag(111)

Abstract

Vibrationally excited NO molecules have been observed in the scattering of a molecular beam of vibrational ground state NO molecules from an Ag(111) surface. Specific vibrational-rotational states of the scattered molecules have been probed by resonance-enhanced laser ionization. This has allowed us to determine the rotational, angular and velocity distribution of the vibrationally excited molecules, and to estimate the excitation probability over a range of incidence kinetic energies and angles and as a function of surface temperature. These results reveal that vibrational excitation occurs in an essentially direct process. In particular, the NO(υ = 1) velocity, angular and rotational state distributions are found to depend on the incidence kinetic energy but are relatively insensitive to the surface temperature. Moreover, the velocity and rotational distributions of the vibrationally excited molecules are found to be highly non-Boltzmann, while angular distributions are found to be quasi-specular, in all cases resembling the corresponding distributions observed for the vibrationally elastic channel. The vibrational excitation probability for the process NO(υ = 0) → NO(υ = 1) is found to depend strongly on surface temperature, Ts, and relatively weakly on incident kinetic energy, E, reaching ~ 7 ± 5% at Ts = 760 K and E = 1.24 eV. Excitation of NO(υ = 2) is found to be < 0.5% under these conditions. Results are discussed in terms of an electronic mechanism for vibrational excitation and recent modeling of this process is extended to include the observed limit for the υ = 2 excitation probability. © 1987, Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division). All rights reserved.