A rotationally resolved two-dimensional photoelectron spectroscopic study of vibrational autoionization in H2

Two-dimensional photoelectron spectroscopy, in which photoelectron yield is measured as a function of both photon and electron energy, has been used to investigate rotational transitions associated with vibrational autoionization in molecular hydrogen, following excitation by synchrotron radiation. The energy resolution achieved in this study was sufficient to separate individual rotational transitions and hence determine the change in rotational quantum number between the initial, neutral and final, ion states. Rather than concentrate on the rotational decay routes of particular autoionizing Rydberg states, advantage has been taken of the comprehensive nature of the two-dimensional photoelectron spectra (2DPES) to perform a more general type of analysis. Constant transition energy spectra (CTES) were extracted from the 2DPES corresponding to specific rotational transitions J''-J = 1-1, 2-2, 3-3 and ΔJ = + 2, between the initial, neutral and final, ionic states. The ΔJ = 0 spectra proved to be broadly similar, once an allowance was made for the different amounts of rotational energy involved, and the comparison highlighted areas in which differences occurred. In many cases low-n high-υ interlopers, Rydberg states converging on higher vibrational ionic thresholds than members of the main series, were found in these spectral regions, suggesting that these Rydberg states have a significant effect on the rotational spectrum. Analysis of the ΔJ = + 2 CTES revealed a tendency for these low-n high-υ interlopers to feature strongly in these spectra and this has been tentatively associated with the relatively long lifetime of these states against autoionization.