Letter to the Editor

Experimental confirmation of barrierless reactions between HeH⁺ and deuterium atoms suggests a lower abundance of the first molecules at very high redshifts

¹ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
² Laboratoire Univers et Particules de Montpellier, Université de Montpellier, UMR-CNRS 5299, 34095 Montpellier Cedex, France
³ IPR – Université de Rennes Bât 11b, Campus de Beaulieu, 263 Avenue du Général Leclerc, 35042 Rennes Cedex, France
⁴ I. Physikalisches Institut, Justus-Liebig-Universität Gießen, D-35392 Gießen, Germany
⁵ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
⁶ Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain, Louvain-la-Neuve B-1348, Belgium

^⋆ Corresponding author: holger.kreckel@mpi-hd.mpg.de

Received: 28 April 2025
Accepted: 16 June 2025

Abstract

The HeH⁺ ion was the first molecule to form in the early Universe, and its comparatively large dipole moment renders it a potential coolant, relevant during the epoch of first star formation. The main destruction mechanisms under primordial conditions are recombination with free electrons and chemical reactions with hydrogen atoms. The latter process was believed to be slow at low temperatures, owing to a barrier forming along the reaction coordinate. Here we present a joint experimental and theoretical study of the reaction HeH⁺ + D → HD⁺ + He that confirms the very recent proposition that the reaction is, in fact, barrierless and fast at low collision energies. The present evidence suggests that previous studies underestimated the low-temperature rate coefficient significantly because of an artifact in a widely used potential energy surface, and calls for a reassessment of the helium chemistry in the early Universe.