High-Resolution Photoionization Study of the H2 Molecule near Threshold

Abstract

Relative photoionization and absorption cross sections of H2 (ordinary and para) have been measured from 745–810 Å at 300 and 78°K, with a resolution width of 0.04 Å. The photoionization data show the presence of extensive structure due to autoionization of vibrationally excited Rybderg states. Analysis of the data on the D–X(6, 0) band in the region of the ionization threshold leads to the conclusion that the ionization potential of H2 lies between 124 418.2 and 124 393.5 cm−1 and is probably very near the former value. Analysis of the data on the B″–X(4, 0) band requires either that the ionization potential lies below 124 407.2 cm−1 (which is rather unlikely) or that autoionization of the R(2) and R(3) lines occurs in violation of one of the autoionization selection rules proposed by Beutler and Jünger. Some Rybderg states that lie just below their ionization limits and do not autoionize spontaneously can be made to ionize in weak electric fields and by collision with unexcited H2 molecules. Direct ionization of H2(J = 0) appears to begin about 23 cm−1 below the expected threshold, probably as a result of collisional ionization of Rydberg states lying just below the ionization threshold. A series of peaks appearing in a short region immediately above threshold apparently coincides with a series which Herzberg has observed in absorption and identified as the Rydberg series converging to the rotationally excited K = 2 state of the H2+ ion. The autoionization of these Rydberg states must occur by conversion of rotational to electronic energy. An intensity anomaly of this series is ascribed to channel interaction with a vibrationally excited Rydberg state. A large number of the autoionization peaks have been identified in the spectrum of parahydrogen at 78°K. By comparison of the absorption cross section with the photoionization cross section, it was found that all states that can autoionize with Δυ = 1 are strongly autoionized (∼40%–100%) while the autoionization probabilities of states that autoionize with Δυ > 1 vary widely. The dominance of autoionization with Δυ = 1 over predissociation extends to the very highest Rydberg states and leads to the absence of observable thresholds for formation of vibrationally excited H2+ ions. Some peaks that autoionize with Δυ = 1 have widths larger than the instrumental width, and the autoionization rates have been estimated. They are in fair quantitative agreement with recent calculations of Nielsen and Berry. The widths decrease rapidly with principal quantum number as Bardsley predicts. The autoionization rate decreases by about a factor of 10 or more when the Δυ of the transition changes from 1 to 2. For larger values of Δυ, the autoionization rate appears to decrease still more but not necessarily as a monotonic function of Δυ.