Theory of Field Desorption

The theory of field desorption proposed previously has been extended by a more detailed quantum-mechanical calculation of the rate constants of desorption for various cases, including an attempt to evaluate matrix elements for the electronic transition when the latter is slow. A simple, generalized model of chemisorption on metals is presented and used in these calculations. It is found that two-electron transitions lead to somewhat smaller rate constants than one-electron transitions, but that the effect is not very large. In general, a one-dimensional theory leads to rate constants with essentially normal pre-exponential terms, except under conditions where adsorbate tunneling becomes important. A tentative explanation for the compensation effect, i.e., decreases in pre-exponential term with decreasing activation energy observed experimentally in a number of different cases, is given in terms of changes of reaction path with temperature.