Rotationally Resolved Vacuum Ultraviolet Pulsed Field Ionization-Photoelectron Vibrational Bands for H2+ (X2g+,v+=0-18)

Abstract

We have obtained a rotationally resolved vacuum ultraviolet pulsed field ionization-photoelectron (VUV-PFI-PE) spectrum of H2 in the energy range of 15.30-18.09 eV, covering the ionization transitions {H2}+(X2g+, v+=0-18, N+=0-5)H2(X1g+, v=0, J=0-4). The assignment of the rotational transitions resolved in the VUV-PFI-PE vibrational bands for {H2}+(X2g+, v+=0-18) and their simulation using the Buckingham-Orr-Sichel (BOS) model are presented. Only the N=N+ - J=0 and 2 rotational branches are observed in the VUV-PFI-PE spectrum of H2. However, the vibrational band is increasingly dominated by the N=0 rotational branch as v+ is increased. The BOS simulation reveals that the perturbation of VUV-PFI-PE rotational line intensities by near-resonance autoionizing Rydberg states is minor at v+ 6 and decreases as v+ is increased. Thus, the rotationally resolved PFI-PE bands for {H2}+(v+ 6) presented here provide reliable estimates of state-to-state cross sections for direct photoionization of H2, while the rotationally resolved PFI-PE bands for {H2}+(v+ 5) are useful data for fundamental understanding of the near resonance autoionizing mechanism. On the basis of the rovibrational assignment of the VUV-PFI-PE spectrum of H2, the ionization energies for the formation of {H2}+(X2g+, v+=0-18, N+=0-5) from {H2}+(X1g+, v=0, J=0-4), the vibrational constants (e, ee, !US!eye, and !US!eze), the rotational constants (Bv+, Dv+, Be, and e), and the vibrational energy spacings G(v++1/2) for {H2}+(X2g+, v+=0-18) are determined. With a significantly higher photoelectron energy resolution achieved in the present study, the precisions of these spectroscopic values are higher than those obtained in the previous photoelectron studies. As expected, the spectroscopic results for {H2}+(X2g+, v+=0-18) derived from this VUV-PFI-PE study are in excellent agreement with high-level theoretical predictions.