Rate Constants for the Unimolecular Decomposition of Excited Molecular Species Near Threshold

The theoretical formulation of the unimolecular decomposition of excited (active) molecules developed by Marcus is applied to the numerical calculation of rate constants for such processes in the threshold region. The Marcus formulation for calculating numbers of vibrational energy states and densities of such states is briefly investigated. Threshold-region rate constants are investigated as a function of threshold energy and of molecular size. Decompositions of excited molecule ions in the ion source of a mass spectrometer are discussed in terms of these rate constants. It is found that this more exact theory of unimolecular dissociations removes some of the difficulties found in an earlier approach to the theory of mass spectra. Some interesting predictions of the present formulation are pointed out.