Very high resolution study of photoabsoprtion, photoionization, and predissociation in H2

Abstract

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