Dynamics of the desorption of D2 and H2 from Cu(111)

Abstract

We have determined rotational and vibrational distributions for D2 and H2 desorbed from Cu(111) and have determined velocity distributions of the desorbed molecules for a wide range of vibrational and rotational states. The quantum state populations display strong deviations from a Boltzmann distribution, and the velocity distributions are highly supersonic with properties that depend strongly on quantum state. Molecules desorbed at 925 K were detected in a state-specific manner using laser ionization. Velocity distributions were obtained by measuring the flight times of D+2 ions in a field-free region. Vibrational populations of D2(v=1), D2(v=2), and H2(v=1) were found to be a factor of 20, 79, and 18 above that expected for equilibration at 925 K, respectively, relative to v=0 populations. Boltzmann plots of the rotational state distributions display distinct curvature, particularly for molecules in the ground vibrational state. H2 and D2 molecules in the ground vibrational state have mean kinetic energies of ∼0.6 eV for molecules in low rotational states. This energy was found to increase slightly with increasing J at low J, pass through a maximum at a rotational energy of ∼0.15 eV, and then fall as rotational energy is further increased. H2(v=1), D2(v=1), and D2(v=2) molecules have mean energies of ∼0.30, 0.45, and 0.25 eV, respectively. In each of these cases, the mean energy also varies significantly with rotational state. The velocity distributions of molecules in v=0 have a speed ratio of ∼0.35, which decreases with both increasing rotational and vibrational energy. Results are discussed in terms of a simple dynamical picture of desorption via an anisotropic potential energy barrier.