3-DIMENSIONAL THEORY OF ELECTRON-STIMULATED DESORPTION FROM PHYSISORBED LAYERS - ANGLE-RESOLVED KINETIC-ENERGY DISTRIBUTIONS OF DESORPTION PRODUCTS FOR THE ANTONIEWICZ AND THE WAVE-PACKET-SQUEEZING MODELS

Three-dimensional quantum theory of electron-stimulated desorption of neutral particles from physisorbed layers on metals is formulated. The theory accounts for the effects associated with the time evolution in all spatial directions of the adsorbed particle wave function in the electronically excited metastable state of the system. The one-dimensional Antoniewicz model and the recently proposed wave-packet-squeezing (WPS) model emerge as particular limiting cases. A simple expression for the angle-resolved kinetic-energy distribution of desorbing neutral particles is derived in the approximation of the position-independent electronic deexcitation rate. The distributions for the three-dimensional versions of the Antoniewicz and WPS scenarios of desorption are numerically compared and their dependence on the details of the wave-packet time evolution in the directions parallel to the surface plane (i.e., in the lateral directions) are studied. The system parameters used are those for N2O on Ru(001) for which the best agreement with the experimental data on the kinetic-energy distributions for particles desorbing in the direction of the surface normal was obtained in the past. The angular dependence of the desorption yields are very similar in both desorption scenarios and, generally, the particles desorb within a wider polar cone for systems with surface potentials narrower in the lateral directions. Preferential desorption in polar directions away from the surface normal may occur for systems in which the lateral extent of the surface potential gets strongly modified by the initial electronic excitation which triggers desorption. A multipeak structure of the kinetic-energy distribution of the products desorbing in the direction of the surface normal is possible for the Antoniewicz scenario (but not for the WPS) as a result of fast evolution of the wave function in the lateral direction. Similar effects are expected to affect the distributions for chemisorbed species desorbing according to the Menzel-Gomer-Redhead scenario.