Electron impact ro-vibrational transitions and dissociative recombination of H₂⁺ and HD⁺

Rate coefficients and astrophysical implications

¹ Laboratoire Ondes et Milieux Complexes, UMR 6294 CNRS and Université Le Havre Normandie, 25 rue Philippe Lebon, BP 540, 76058 Le Havre, France
² Department of Physical Foundation of Engineering, Politehnica University of Timişoara, Blvd. Vasile Pârvan 2, 300223 Timişoara, Romania
³ Department of Physics, West University of Timişoara, Blvd. Vasile Pârvan 4, 300223 Timişoara, Romania
⁴ LPF, UFD Mathématiques, Informatique Appliquée et Physique Fondamentale, University of Douala, PO Box, 24157 Douala, Cameroon
⁵ Istituto per la Scienza e Tecnologia dei Plasmi, CNR, Bari, Italy
⁶ Department of Comput. Informat. Technol. Engineering, Politehnica University of Timişoara, Bv. Vasile Pârvan 2, 300223 Timişoara, Romania
⁷ Université Paris-Saclay, CNRS, CentraleSupélec, Structures Propriétés et Modélisation des solides, Gif-sur-Yvette, France
⁸ Université Paris-Saclay, CentraleSupélec, Laboratoire de Génie des Procédés et Matériaux, Gif-sur-Yvette, France
⁹ LSAMA, Department of Physics, Faculty of Science of Tunis, University of Tunis El Manar, 2092 Tunis, Tunisia
¹⁰ Department of Chemistry, Università degli Studi di Bari Aldo Moro, 70125 Bari, Italy
¹¹ INAF Osservatorio Astrofisico di Arcetri, 50125 Firenze, Italy
¹² HUN-REN Institute for Nuclear Research (ATOMKI), Bem Sqr. 18/c, 4026 Debrecen, Hungary
¹³ LAC CNRS-UMR 9025, Université Paris-Saclay, ENS Cachan, Campus d’Orsay, Bât. 505, 91405 Orsay, France

^★ Corresponding author: riyad.hassaine@univ-lehavre.fr

Received: 19 July 2025
Accepted: 6 October 2025

Abstract

Context. Molecular hydrogen and its cation H₂⁺ are among the first species formed in the early Universe, and play a key role in the thermal and chemical evolution of the primordial gas. In molecular clouds, H₂⁺ ions formed through ionization of H₂ by particles react rapidly with H₂ to form H₃⁺, triggering the formation of almost all detected interstellar molecules.

Aims. We present a new set of cross sections and rate coefficients for state-to-state ro-vibrational transitions (RVT) of the H₂⁺ and HD⁺ ions, induced by low-energy electron collisions. The study includes the major electron-impact processes relevant for low-metallicity astrochemistry: inelastic and superelastic scattering, and dissociative recombination (DR).

Methods. The electron-induced processes involving H₂⁺ and HD⁺ were treated using the multichannel quantum defect theory (MQDT). Results. The newly calculated thermal rate coefficients show significant differences compared to those used in previous studies. When introduced into astrochemical models, particularly for shock-induced chemistry in metal-free gas, the updated DR rates produce substantial changes in the predicted molecular abundances.

Conclusions. These data provide updated and improved input for the modeling of hydrogen-rich plasmas in environments where a high abundance of free electrons is expected, such as planetary nebulae, HII regions, and the ionospheres of giant planets.