Single-molecule cold chemistry: Isotopic branching ratios of the $\mathrm{H_2O^+ + HD}$ reaction at cryogenic temperatures

Isotopic branching in ion-molecule reactions influences interstellar deuterium fractionation, yet its dynamics remain poorly understood for many important systems at astrophysically relevant temperatures. Here, we investigate H- and D-atom transfer in the $\mathrm{H_2O^+ + HD}$ reaction using single sympathetically cooled water ions in a cryogenic radiofrequency trap. Nondestructive identification of the products from 441 individual reaction events at a mean collision energy of 7 K reveals an $\mathrm{H_3O^+/H_2DO^+}$ product branching ratio of $1.50 \pm 0.15$ demonstrating a preferential H-atom transfer. The experiments are rationalized using quasi-classical-trajectory calculations which indicate a transition from direct dynamics at higher energies to a complex-mediated reaction under the present experimental conditions and point to an important role of intramolecular vibrational redistribution affecting the isotopic branching in this regime. The present results extend measurements into an astrochemically relevant energy regime and establish cryogenic single-ion chemistry as a new platform for resolving competing reaction pathways.

Source: arXiv:2610.32904v1 - http://arxiv.org/abs/2610.32904v1 PDF: https://arxiv.org/pdf/2610.32904v1 Original Link: http://arxiv.org/abs/2610.32904v1