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

Relative photoionization efficiency curves were determined for the production of O+2, O+, and O− from molecular oxygen in the wavelength region of the helium continuum. The photoionization efficiency curve for O+2, determined at a wavelength resolution (FWHM) of 0.07 Å throughout most of the region from 1030–580 Å, shows rotationless autoionization structure which is broadened due to the processes of predissociation and/or autoionization. The photoionization effciency curves for fragment ions from ion pair formation and from dissociative ionization were determined at wavelength resolutions of 0.07 and 0.15 Å, respectively. The data show considerable autoionization structure and provide information about the mechanisms for these dissociation processes. The ion pair photoionization efficiency curve shows steplike structure at threshold resulting from direct ionization from individual rotational levels of the ground state of O2. A value of the electron affinity of atomic oxygen of 1.462±0.003 eV was determined from the wavelength corresponding to ionization from the O2(3Σ−g, v=0, J=1) ground state.

High-resolution and high-sensitivity HeI photoelectron spectra (PES) are reported for the first four valence levels of N2O. The vibrational structure, including many new peaks, is completely assigned for the X 2Π, A 2Σ+, and C 2Σ+ electronic states, and in all three cases excitation of a single quantum of the bending vibration is observed. The bending mode appears as a result of vibronic coupling within and between different electronic states. The Renner–Teller splitting is resolved in the (0,1,0) band of the transition to the X 2Π state. The relative intensities of the vibrational bands in the X 2Π and A 2Σ+ states are compared to the intensities determined from the threshold photoelectron spectra (TPES) of Frey et al. [Chem. Phys. Lett. 54, 411 (1978)]. In both states, autoionization of intermediate neutral states that are nearly degenerate with the molecular ion state to which they decay increases the relative intensities of the higher vibrational bands in the TPES. This resonant autoionization process can enhance bands selectively, and, in particular, the higher quanta of the symmetric stretch are systematically more intense in the TPES than in the HeI PES. New relative photoionization cross sections for N2O and its fragment ions are presented which show that even weak autoionization features can have a substantial effect on the intensities in the TPES.

A new photoionization mass spectrometer which is significantly superior to previous instruments in both wavelength resolution and ion intensity is described. It has been used in the study of photon-induced ion-pair formation at 78 °K in para-H2, ordinary H2 and D2 (the ortho-para equilibrium mixtures), and HD in the wavelength region from 718 to 700 Å, at a wavelength resolution (FWHM) of 0.035 Å for para-H2, 0.07 Å for ordinary H2 and D2, and 0.15 Å for HD. The threshold for formation of ion pairs from para-H2 occurs at 715.753+0.046−0.008 Å (17.3223+0.0002−0.0011 eV) and this value together with the accurately known values of the dissociation energy of H2 and the ionization potential of atomic hydrogen yields a value of the electron affinity of the hydrogen atom of 0.7542 −0.0004 +0.0013 eV in excellent agreement with the theoretical value of 0.75421 eV calculated by Pekeris. The observed thresholds for ion-pair formation in D2 and HD are also in excellent agreement with the calculated positions. For para-H2 the region within 0.5 Å of the threshold has a continuumlike appearance contains window resonances which are interpreted as predissociating members of a Rydberg series converging to H+2(v=9, N=2). The continuum results from closely spaced predissociating Rydberg states converging to H+2(v=9 N=0) which were not resolved in the present experiment. A number of other tentative Rydberg state assignments were made for para-H2 based on the results of an analysis using multichannel quantum defect theory. No Rydberg state identifications were attempted for the other systems due to the greatly increased complexity of their spectra. For HD which can dissociate to either H++D− or H−+D+, the H−/D− ratio is ∼2 just above threshold and appears to decrease with decreasing wavelength; however, this is due, at least in part, to discrimination by the detection system against the more energetic H− ions. Some conclusions and inferences regarding the mechanism for predissociation in H2 and HD are discussed, particularly the possible importance of the 4fσ 1Σ+u state.

The photoelectron spectra of Ar2 and Kr2 were determined at 584 Å with resolutions of 18 and 21 meV, respectively, using an apparatus which combines a supersonic molecular beam source with a hemispherical photoelectron spectrometer. Dissociation energies were determined for the weakly bound B 2Π(3/2)g and C 2Π(1/2)u excited states of the dimer ions, and were found to average approximately 30% greater than those predicted by recent theoretical calculations. Optimum potentials were determined for the Ar+2 and Kr+2 ground states by fitting calculated Franck–Condon distributions to the observed photoelectron peaks. As in the case of Xe+2, the optimum Rg+2 potentials retain the shapes determined by theoretical calculations, but they are modified by scaling their dissociation energies to the experimentally determined values. The Morse function is shown to be an inadequate potential form for the Rg+2 ground states, since it is too attractive at large internuclear distance.