An analysis of isotope effects on molecular ionization yields (efficiencies) is given on the basis of a model that extends the Platzman discussion of the decay of superexcited states as a simple competition between preionization and neutral-product formation. The model is presented in terms of several parameters and the implications of changes of these parameters are investigated. The parameters include the quantum yields of direct ionization and of a new process, noncompetitive neutral-product formation (neutral products formed from states in which preionization is absent or slow in comparison with neutral-product formation). This new process has only a moderate influence on isotope effects unless it is producing nearly all of the neutral products. Another parameter ρ gives the isotope effect on the competition of preionization and neutral-product formation, and allows for a general mass dependence. The magnitude of ρ is discussed and a detailed analysis of deuterium-isotope effects observed in the photoionization of methyl bromide in the energy range of 10.5-11.4 eV provides estimates of ρ for this example.

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