George Wolken

A model potential for chemisorption: H2+W(001)

A potential function to describe the interaction of a diatomic molecule with a solid surface is formulated as a modified LEPS potential. The surface is approximated by a rigid background potential, periodic in the plane of the surface. It is assumed that the individual atom–atom and atom–surface potentials are known and can be reasonably approximated by Morse functions. H2+W (001) is presented as an example. One adjustable Sato parameter is introduced and by varying this parameter a wide range of potentials can be generated. The functional form is sufficiently simple so that classical trajectory studies of the reaction dynamics are feasible. Several illustrative trajectories are presented.

Atomic recombination dynamics on solid surfaces: Effect of various potentials

A model potential for gas–solid interactions has been used to investigate the dynamics of recombination of two atoms initially adsorbed on a solid surface. In the spirit of Polanyi’s investigation into the effect of the potential energy surface on the dynamics of gas‐phase reactions, a range of gas–solid potential energy surfaces has been constructed. Classical trajectories have been used to study the dynamics of reactions on those surfaces. It has been found that many of the rules postulated by Polanyi for energy requirements and disposal mechanisms for gas‐phase systems are applicable also to the case of recombination of adsorbed atoms to form a gas‐phase molecule. Previous work assumed a rigid surface providing a static background potential in which the adsorbed atoms moved. An extension of this model is described in which the rigid surface restriction is relaxed and one or more surface atoms are allowed to move interacting with the adsorbed atoms. Using this potential the rigid surface model is shown to be a good approximation for describing many aspects of recombination dynamics.

Atomic recombination dynamics on a solid surface: H2+W(001)

A model potential to describe the interaction of a diatomic molecule with a rigid solid surface has been described previously. Using this interaction potential, a series of classical trajectory calculations have been carried out, designed to simulate the recombination dynamics of two hydrogen atoms initially adsorbed on a tungsten (001) solid surface to form a gas‐phase hydrogen molecule. The vibrational and rotational state distributions of the desorbed hydrogen molecules are discussed in terms of a simple statistical model. The angular and speed distributions for the desorbed atoms and molecules are presented. The angular distributions are found to be substantially noncosine in form and peaked towards the surface normal, in qualitative agreement with experiment.

Rotational transitions in molecule–solid scattering

A quantum mechanical calculation of the collision of H2 with the (001) face of LiF has been carried out and compared with recent experimental data in which rotational transitions were resolved. A simple model potential accounts quantitatively for many of the observed effects. Refined potential parameters are extracted and the effects of some of these parameters on rotational transitions are discussed.

Dynamics of atom-adsorbed atom collisions: Hydrogen on tungsten

A model potential to describe the interaction of a diatomic molecule with a solid surface has previously been derived and applied to the case of H 2 interacting with a tungsten (001) surface. Using this potential we have computed a series of classical trajectories designed to simulate the collision of a gas-phase hydrogen atom with an adsorbed hydrogen atom. Comparisons are made with the results of recombination studies of two adsorbed atoms and with the collision of an atom with a bare surface.

Recombination dynamics of hydrogen on a tungsten surface

The results of a series of classical trajectory calculations designed to simulate the recombination of two hydrogen atoms adsorbed on tungsten to form a gas phase H2 molecule are reported. (AIP)

Theoretical study of gas–solid energy transfer: An isotropic Debye model

We present a model calculation for gas–solid energy transfer in which the solid is approximated as an isotropic Debye solid. For a given initial state of the solid (described by phonon occupation numbers), only states differing from this arbitrary initial state by one phonon (created or destroyed in an arbitrary state) are included in the basis set describing the solid. Using this basis set, a close‐coupling calculation is carried out describing energy transfer during a gas–solid collision. Since the scattering calculation does not employ perturbation, all states in the basis set are coupled. Averages over phonon momenta and directions are carried out before the calculation, which results in considerable simplification. Comparisons are made with He+Ag(111) experimental results. Qualitative agreement is good, and quantitative agreement is within a factor of 2. Our model tends to underestimate inelastic effects. The importance of kinematic and geometric considerations is stressed.

Quantum mechanical studies of collision induced dissociation: Discretization theory

The collinear collision of an atom with a diatomic molecule A +  (B,C) is formulated quantum mechanically using the close coupling approach. Neglecting rearrangement collisions but including electronically adiabatic inelastic and dissociative processes, it is shown that the resulting continuously infinite set of coupled differential equations can be reformulated as a discrete set of coupled differential equations. The technique is analogous to a procedure developed to treat gas–solid energy transfer. However, the boundary conditions involving three free particles in the final state has no analog in the gas–solid formulation of the problem. These are formulated also in terms of the discretized equations, and various simplifications and symmetries of the equations are discussed.

Dynamics of adsorption: Model calculations for several interaction potentials

A model potential to describe the interaction of a diatomic molecule with a rigid solid surface has been derived previously. Using this interaction potential, classical trajectory calculations have been carried out to study the dynamics of adsorption on a solid surface. Several forms of the interaction potential were employed so as to observe the effect of different types of potential surfaces on adsorption dynamics, and both H2 and HD were used in these model calculations in order to study the isotope effects on adsorption. Sticking probabilities for the different cases are reported and the validity of the rigid surface model is discussed.

Dynamics of adsorption on covered surfaces

A previous model for the interaction of a diatomic molecule with a solid surface is extended to allow the treatment of three atoms interacting with the solid. The effect of an adsorbed atom on the diatom–solid surface potential is examined. The dynamics of adsorption of a hydrogen molecule in the presence of an adsorbed hydrogen atom is studied. For the potential function used, the dissociative sticking probability of the incident molecule decreases for closer collisions with the adsorbed atom.