Energy Threshold for D+H2→DH+H Reaction

Abstract

the surface states depend on the field rather than the crystal lattice.The conventional model of a surface, with an atom near that surface, in an electric field is shown in Fig. 1. Low-energy electrons tunneling into the roughly triangular well outlined in the figure have a certain probability of being reflected by the metal surface. 2 That such reflectivity exists can be empirically demonstrated by the familiar difficulty of collecting lowenergy electrons; special means such as a Faraday cage must be used for quantitative collection. If the reflectivity is unity and tunneling back to the atom is unlikely, well-defined energy levels will exist within the well. Lower reflectivities will result in short-lived states and consequent level broadening.Alternately, the problem may be described in terms of the matching of wavefunctions at the boundaries of the three regions of interest: the atom, the triangular well, and the surface. The probability amplitudes in the region of linearly increasing potential are given in terms of Airy functions 3 and are initially unspecified in amplitude and phase. These two parameters are fixed by equating logarithmic derivatives at the boundaries and result in small amplitudes in the well except at energies near the eigenvalues of the simple infinitely high triangular well. Consequently, low tunneling rates are expected for atomic levels not aligned with the well levels. The presence of the atomic potential and associated image force acts as a small perturbation on the well levels. A calculation using square potentials verifies this picture; a square well open on one side to a higherenergy continuum demonstrates resonances at energies very near to the closed-well eigenvalues.Eigenvalues E,. of the infinite triangular well are given by the relationwhere m is the electronic mass, e the electronic charge, F the field, and a,. the nth root of the particular Airy function which is finite for all real values of its argument. To complete a rough correspondence with our picture, we must add to En a quantity Eo given by the difference between the Fermi energy and the energy at which the extrapolated field curve intersects the surface. Results of this calculation and an observed distribution are compared in Fig. 1. Good agreement is not expected for the first level because of the influence of the image potential which is not included in the above calculation and because of the sensitivity of this level to the assumed surface configuration. The calculated variation of peak spacings with electric field agrees closely with experimental values and, over the energy range observable, is very nearly indistinguishable from a first-power field dependence.In addition to the main structure explained above, the experiments show further detail in peak structures indicative of the nature of the surface and surface reactions. It is expected that this type of analysis will be useful in surface studies. For example, it is to be noted that a pure surface reaction such as H 3 + formation exhibits no secondary st...

Photochemical decomposition of methanethiol. II. The wavelength dependence of the hydrogen atom energy

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