Universality in the cold and ultracold dynamics of the barrierless D$^{+}$+ H$_2$ reaction

TL;DR

This study investigates the quantum dynamics of the barrierless D$^{+}$+ H$_2$ reaction across a wide energy range, revealing a nearly constant reaction rate and confirming universality in ion-molecule reactions at ultracold temperatures.

Contribution

The paper introduces an improved hyperspherical quantum scattering method that accurately includes long-range interactions for ion-molecule reactions, demonstrating universality in reaction rates over many orders of magnitude.

Findings

Reaction rate remains nearly constant over ten orders of magnitude in energy.

The low-energy reaction rate matches the classical Langevin value.

Results support the universality predicted by quantum defect theory.

Abstract

We have calculated quantum reactive and elastic cross-sections for D$^{+}+$ para-H$_2$($v$=0, $j$=0) $\rightarrow$ H$^+$ + HD collisons using the hyperspherical quantum reactive scattering method [Chem. Phys. Lett. 1990,169, 473]. The H$_{3}^{+}$ system is the prototype of barrierless ion-molecule reactions, apart from its relevance in astrochemistry. The considered collision energy ranges from the ultracold regime, where only one partial wave is open, up to the Langevin regime, where many of them contribute. At very low kinetic energies, both an accurate description of the long-range (LR) region in the potential energy surface (PES), and long dynamical propagations, up to distances of 10$^{5}$ a0, are required. Accordingly, calculations have been carried out on the PES by Velilla et al. [J. Chem. Phys. 2008, 129,084307] which accurately reproduces the LR interactions. Hyperspherical methodology was recently modified in order to allow the accurate inclusion of LR interactions while minimizing the computational expense. Such implementation is shown particularly suitable for systems involving ions, where the $R^{-4}$ behavior largely extends the range of the potential. We find a reaction rate coefficient which remains almost constant in a kinetic energy range of more than ten orders of magnitude. In particular, the value reached in the Wigner regime, where only one partial wave is open, is paradoxically the Langevin classical value (within a few percent) expected at high energies. Results are discussed in terms of {\em universality} and related to the recently published quantum defect theory [Phys. Rev. Lett. 2013, 110, 213202]. Since the system has small exothermicity and a low number of channels, it is possible to test such model in a case where the loss probability at short range is appreciably far from unity.