Structure and dynamics of H$_2^+$ near the dissociation threshold: a combined experimental and computational investigation

TL;DR

This study combines experimental spectroscopy and computational modeling to explore the structure and dynamics of H$_2^+$ near the dissociation threshold, revealing detailed energy levels, shape resonances, and the effects of various corrections.

Contribution

It provides a comprehensive analysis of H$_2^+$ energy levels and shape resonances near dissociation, incorporating advanced calculations and experimental data for the first time.

Findings

Identification of bound levels and shape resonances of H$_2^+$ near dissociation

Quantitative assessment of adiabatic, nonadiabatic, relativistic, and radiative effects on energy levels

Observation of slow oscillations in dissociative-ionization yield linked to Franck-Condon factors

Abstract

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of H$_2$ has been recorded in the vicinity of the dissociative-ionization threshold following three-photon excitation via selected rotational levels of the B $^1\Sigma_u^+$ ($v=19$) and $\bar{\rm H}$ $^1\Sigma_g^+$ ($v=11$) intermediate states. The spectra consist of transitions to bound levels of the X$^+$ $^2\Sigma_g^+$ state of H$_2^+$ with $v^+$ in the range 14-19 and $N^+$ in the range 0-9, of the A$^+$ $^2\Sigma_u^+$ state with $v^+=0$ and $N^+=0-2$, and of shape resonances corresponding to the X$^+\ (v^+=17,N^+=7)$ and X$^+\ (v^+=18,N^+=4)$ quasibound levels. Calculations of the level structure of H$_2^+$ have been carried out and the influence of adiabatic, nonadiabatic, relativistic and radiative corrections on the positions of these levels, and in the case of the shape resonances also on their widths, has been investigated. Different methods of calculating the widths and profiles of the shape resonances have been tested for comparison with the experimental observations. Slow oscillations of the dissociative-ionization yield have been observed and reflect, in first approximation, the Franck-Condon factors of the $\bar{\text H}$ $\rightarrow$ X$^+$, A$^+$ bound - free transitions.