Autoionization of H2 induced by a doubly excited triplet state

Spectrum of Field-Ionized Triplet Gerade Rydberg States of H2 and Comparison to Multichannel Quantum-Defect Theory

A spectrum of field-ionized triplet Rydberg states of gerade symmetry H₂ has been measured, excited from the υ″ = 0, N″ = 1-3 rovibrational levels of the metastable c³Π_u^-2pπ state in a 6 keV fast molecular beam. The field-ionized spectrum is kinetic energy labeled in order to separate it from the well-studied υ⁺ ≥ 1 autoionization spectrum. The spectrum consists of both ns and nd Rydberg series with n between 10 and 28 converging to the υ⁺ = 0, N⁺ = 1-3 levels of the X^+ 2Σ_g⁺ ground state of H₂⁺. Transitions with changes in vibration and/or rotation are also identified. The spectral positions of 59 transitions in the field ionization spectrum are identified and assigned quantum numbers using the predictions of multichannel quantum-defect theory (MQDT). The transition energies and subsequent effective quantum defects are compared between the experiment and theory. Most of the observed transitions, within the experimental uncertainty of 0.2 cm^-1, agree with the energies predicted by MQDT.

Spectral identification of diffuse resonances in H2 above the n = 2 dissociation limit

The resonance structure in molecular hydrogen above the n = 2 dissociation limit is experimentally investigated in a 1 XUV + 1 VIS coherent two-step laser excitation process, with subsequent ionization of H(n = 2) products. Diffuse spectral features exhibiting widths of several cm(-1) in the excitation range of 118,500-120,500 cm(-1) are probed. Information on angular momentum selection rules for parallel and crossed polarizations, combination differences, the para-ortho distinction, extrapolation from rovibrational structure in the bound region below the n = 2 threshold, and mass-selective detection of H(2)(+) parent and H(+) daughter fragments is used as input. This allows for an assignment of the diffuse resonances observed in terms of (1)Σ(g)(+), (1)Π(g), and (1)Δ(g) states, specified with vibrational and rotational quantum numbers.

Predissociation and autoionization of triplet Rydberg states in molecular hydrogen

We present single-photon spectroscopy in molecular hydrogen starting from the metastable c3Piu- state to a number of triplet nd-Rydberg states (v = 0 - 4, n = 12 - 20). Using fast beam spectroscopy both the autoionization channel and the predissociation channel are quantified, field free, as well as with small electric fields. Coupling with the i3Pig state is assumed to be responsible for field-free predissociation of the v = 0 Rydberg levels. The stronger observed predissociation channel of the v = 1 Rydberg levels is due to the nonadiabatic interaction with the h3Sigmag+ state in combination with l mixing due to an external electric field. No direct evidence is found for possible electric field induced predissociation of the gerade Rydberg states by low lying ungerade states. The competition between autoionization and predissociation is discussed in terms of possible consequences for dissociative recombination involving low energy electron collisions with the H2+ molecular ion.