Threshold electron-photoion coincidence mass spectrometric study of CH4, CD4, C2H6, and C2D6

An experiment is reported which directly determines the kinetics of dissociation of a molecular ion as a function of the internal energy of that ion. The experiment involves photoionization, followed by detection of only those mass analyzed ions which are produced in coincidence with photoelectrons of zero initial kinetic energy. Under these conditions the internal energy of a parent ion is equal to the photon energy minus the adiabatic ionization potential of the molecule. With this technique, in contrast to previously used techniques, the kinetics of dissociation can be studied independently of assumptions concerning threshold laws, i.e., variation in cross section above threshold, and autoionization. Fragmentation as a function of internal energy is reported for CH4+, CD4+, C2H6+, and C2D6+. The results are compared with previous photoionization, charge exchange and photoelectron-photoion coincidence mass spectrometry, and with fragmentation calculations based on quasiequilibrium theory of unimolecular dissociation. The major discrepancies of earlier experiments with the predictions of the quasiequilibrium theory are shown to be in error. The new experimental results are in good qualitative agreement with the predictions of the theory. Although this experiment was not designed to determine the average kinetic energy release in unimolecular reactions, it is sensitive to this energy. An analysis of the instrument and the data indicate average fragmentation energies which are of the same order as those obtained by other workers. The implications of these data on the thermochemistry of the molecules studied is discussed.