UV photodissociation of H₂⁺ in interstellar radiation fields: Shape resonances and astrophysical implications

¹ School of Physics, Henan Normal University, Xinxiang 453000, China
² Institute of Environmental Science, Shanxi University, Taiyuan 030006, China
³ School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710119, China
⁴ Institute of Applied Physics and Computational Mathematics, PO Box 8009, Beijing 100088, China
⁵ School of Applied Science, Taiyuan University of Science and Technology, Taiyuan 030024, China

^★ Corresponding authors: wangkd@htu.edu.cn; wangy@tyust.edu.cn; song-bin.zhang@snnu.edu.cn

Received: 31 March 2025
Accepted: 12 June 2025

Abstract

Aims. Prior investigations into the photodissociation dynamics of the hydrogen molecular ion (H₂⁺) have frequently neglected the impact of shape resonances, which could potentially lead to inaccuracies in astrophysical modeling. This study systematically explores the photodissociation cross sections of H₂⁺ with a rigorous consideration of shape resonances. We aim to elucidate comprehensively the photodissociation mechanisms by accurately accounting for transitions from the electronic ground state 1²Σ_g⁺ to multiple electronically excited states. Our results provide updated, precise cross-sectional data essential for refining chemical evolution models of interstellar environments and for rectifying previous methodological oversights.

Methods. We employed high-level ab initio calculations based on the multireference single- and double-excitation configuration interaction (MRDCI) method to determine the electronic structure of the H₂⁺ ion accurately. The photodissociation cross sections were calculated under the assumption of local thermodynamic equilibrium (LTE) across photon wavelengths ranging from 25 nm to the dissociation threshold, incorporating contributions from the majority of rovibrational states of the ground electronic state. Particular attention was given to analyzing the effects of shape resonances, especially the significant role played by the 1²Π_u state near the spectral region of the Lyman α line.

Results. Our computed cross sections clearly demonstrate that shape resonances substantially influence the photodissociation dynamics of H₂⁺ near the Lyman α line. The contribution from the 1²Π_u excited state prominently shapes the spectral absorption features around the Lyman α region. These refined theoretical results offer substantial improvements over previous datasets, delivering the precise spectral information necessary for astrophysical simulations, modeling ultraviolet-driven chemical processes in interstellar media, and enhancing our understanding of photochemical dynamics in the early universe.