About the degree of dissociation of admolecules on surfaces

The characteristics of the thermal desorption spectra obtained for isotopic exchange reactions have been related to the degrees of dissociation introduced on a phenomenological basis by Matsushima [Surf. Sci. 157 (1985) 297; 217 (1989) 155]. This formulation leads to an underdetermined system of equations. Indispensable simplifications are analysed by means of kinetic models with one-site (or bridged) molecular adsorption precursor states. As a result, we tentatively propose an experimental test about the nature of these adsorption precursor states as well as subsequent experimental procedures to reach reliable values for the degrees of dissociation of the admolecules.