It is shown that under the usual conditions of zero-electron-kinetic-energy, pulsed field ionization (ZEKE–PFI) spectroscopy the lifetimes of very high-lying Rydberg states are increased by at least approximately the factor n (in addition to the expected factor of n3), the principal quantum number, due to strong l mixing by the Stark effect. Additional factors may increase lifetimes by still another factor of approximately n. Pulsed field ionization under typical conditions is shown as likely to be predominantly diabatic and the effect on resolution is assessed. Factors affecting rotational line intensities are also discussed.

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Relative photoabsorption and photoionization cross sections for H2 (para and ordinary) have been measured at 78°K from 715 to 805 Å for para-H2 and from 745 to 805 Å for ordinary H2, with a wavelength resolution of 0.016 Å. This resolution represents a factor of 3 improvement over the previous data reported from this laboratory, and in addition, the new data have significantly improved statistics. This enables observation and identification of the R (0) npσ and npπ Rydberg series in para-H2 to principal quantum numbers of approximately 40 for series converging to H2+(2Σg+, v=1–6). Linewidths and relative intensities were measured for a large number of these levels and the results are compared to calculations using a quantum defect theory (QDT) approach. Two-channel QDT is used to assign nearly all the prominent structure in the para-H2 spectrum. The strengths and limitations of the simple two-channel theory and the necessity for extension to multichannel calculation for a complete understanding of the spectrum are discussed. The ionization efficiency for these Rydberg states is always close to unity for states which autoionize with Δv=−1. Decay by predissociation and/or emission compete to varying degrees with autoionization for those states which cannot autoionize with Δv=−1. For states which decay to a detectable extent by fluorescence, the measured autoionization efficiencies yield estimates of rates for highly suppressed autoionization processes which are in good agreement with theoretical calculations. Similarly, comparison of autoionization and predissociation rates yields some estimates of the latter rates for slowly decaying states

The interpretation of appearance potential data on diatomic molecules should take account of possible effects caused by predissociation, emission of light and autoionization. In the case of complex polyatomic molecules, the kinetics of predissociation and the internal thermal energy of the molecules become especially important. The intensities of the parent, fragment, and metastable ions produced by photoionization of n-propylamine, n-propanol, and methyl ethyl ketone are studied as a function of photon energy. The excess kinetic energies of the fragment ions are found to be negligibly small. The data are interpreted in terms of Rosenstock's quasi-equilibrium theory of unimolecular decomposition and indicate that the theory is qualitatively correct for the dissociative processes investigated. However, the theory is shown to be quantitatively inadequate at least in the energy range near threshold. In this region the rate constant for dissociation varies much more rapidly with energy than the theory predicts. Some of the assumptions of the theory are examined and compared to deductions from the data. The meaning of appearance potential data is examined in the light of these results. The effects of both the kinetics of dissociation and of internal thermal energy on ionization efficiency curves are significant. Most of the methods used to determine appearance potentials tend to minimize these effects and there is probably some cancellation of errors. A new method for the determination of appearance potentials is described. Experimental methods which can yield more detailed information concerning dissociation processes of complex molecular ions are suggested.