H.J. Kreuzer

Desorption kinetics mediated by surface phonon modes

Abstract A quantum statistical theory of physisorption kinetics is formulated taking proper account of the surface phonon modes of a semi-infinite solid. Desorption times are shorter than those calculated with bulk phonons by a factor two to three. For mobile physisorption the rates are dominated by the Rayleigh mode, for localized physisorption by the bulk-like shear horizontal mode. An effective surface Debye temperature is deduced that is about 0.65 to 0.8 times the bulk Debye temperature.

Rate equations for positronium formation at metal surfaces

Abstract Processes for thermalized positrons deposited in a metal by a slow-positron beam are examined. These include diffusion back to the surface, trapping in vacancies and surface states, and positronium formation in the surface region. Coupled rate equations are solved in steady state. The resultant positronium fractions are in good agreement with experiments on silver.

Surface Debye temperature in desorption kinetics

Abstract Desorption times for gas-solid systems with localized or mobile physisorption are calculated with the desorption energy supplied by the surface phonon modes of a semi-infinite elastic solid. They are shorter by a factor 2 to 3 than those calculated using bulk phonons. A surface Debye temperature is deduced that is about .7 to .8 of the bulk Debye temperature.

Size effect in the magnetic field dependence of nonequilibrium electron tunneling

Abstract The electron bottleneck in normal metal-insulator-metal tunnel junctions due to nonequilibrium effects has been investigated in the presence of a magnetic field. The observed results are due to a new geometrical size effect and are explained in an extension of our nonequilibrium theory.

Hartree-Fock theory of multilayer physisorption

Abstract The two-body interaction between adsorbing gas particles is taken into account in finite temperature Hartree-Fock theory. Coverage, isosteric heat of adsorption, mean spacings between adlayers, binding energies and wavefunctions for adparticles are calculated for 3 He adsorbing on graphite.

Kinetic equations for physisorption; With some remarks on the compensation effect

Abstract Starting from a set of rate equations for the bound state occupation function for gas-solid systems in which the surface potential has many physisorbed bound states, we derive a master equation; its kernel is explicitly calculated for phonon-mediated adsorption and desorption in a Morse potential. We give the equivalent Smoluchowski-Chapman-Kolomogorov equation for which we find the Kramers-Moyal expansion. Identifying Van Kampens large parameter Ω for such gas-solid systems, we establish explicit criteria for the validity of a Fokker-Planck equation. The various kinetic equations are then used to calculate desorption times. The exact time evolution of the adsorbate as calculated from the set of rate equations shows that quasi-equilibrium is only maintained at low temperatures where perturbation theory of the master equation yields a simple analytic expression for the desorption time in weakly coupled gas-solid systems. At intermediate temperatures we derive another simple expression from the Fokker-Planck equation. Lower limits for the pre-exponential factor in the desorption time of the order 10 -16 s, proportional to the inverse of the heat of adsorption, are derived. We conclude with some remarks on the compensation effect.