Predissociation and autoionization of triplet Rydberg states in molecular hydrogen

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.

Dynamics of dissociative recombination versus electron ejection in single rovibronic resonances of BH

Optical-optical-optical triple resonance spectroscopy isolates transitions to vibrationless Rydberg states of BH with principal quantum numbers from n=7 to 50. Corresponding resonances appear in the excitation spectrum of excited boron atoms produced by the dissociative relaxation of these states. The decay to neutral products occurs on a nanosecond time scale. Yet, corresponding resonances show Fano coupling widths that approach 1 cm-1. Above threshold, spontaneous ionization dominates, but line shapes match for resonances with the same electron orbital quantum numbers built on v+=0 and v+=1 cores. This striking feature-for-feature similarity in predissociation and autoionization line shapes affirms that inelastic electron-cation scattering pathways leading to electron ejection and dissociative recombination proceed through a common continuum.