Published on: Jul 24, 2025
Experimental confirmation of barrierless reactions between HeH+ and deuterium atoms suggests a lower abundance of the first molecules at very high redshifts
New experimental and theoretical results show the primordial reaction HeH⁺ + D → HD⁺ + He is barrierless and fast at low temperatures, implying less HeH⁺ (and different early-universe chemistry) than earlier models predicted.
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.
DOI: 10.1051/0004-6361/202555316
Key Takeaways
HeH⁺ matters because it is expected to be among the first molecules in the early universe and a potential coolant during the first-star era.
A key destruction pathway, HeH⁺ + D → HD⁺ + He, is confirmed to be fast at low collision energies, contrary to older assumptions of a low-temperature barrier.
The work argues prior low-temperature rate estimates were biased by an artifact in a widely used potential energy surface, meaning legacy models likely underestimated the reaction rate.
The result triggers a need to reassess helium chemistry in primordial environments and revisit predicted abundances of early molecules at high redshift.
Practically, it strengthens the case that even "simple" astrochemical networks can hinge on accurate microphysics, and that a single rate correction can propagate into star-formation era predictions.
Institutions
Columbia Astrophysics Laboratory, Columbia University
New York, New York, United States
I. Physikalisches Institut, Justus-Liebig-Universität Gießen
Gießen, Germany
Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain
Louvain-la-Neuve, Belgium
IPR – Université de Rennes
Rennes, France
Laboratoire Univers et Particules de Montpellier, Université de Montpellier
Montpellier, France
Max-Planck-Institut für Kernphysik (MPIK)
Heidelberg, Germany
Journal
EDP Sciences
