Tunneling chemical exchange reaction $\textrm{D}+\textrm{HD}\rightarrow\textrm{D}_{2}+\textrm{H}$ in solid HD and D$_2$ at temperatures below 1$\,$K

TL;DR

This study investigates the tunneling exchange reaction D+HD→D₂+H in solid hydrogen matrices at sub-Kelvin temperatures, confirming quantum tunneling as the reaction mechanism and suggesting similar behavior for H+H₂ reactions at ultra-low temperatures.

Contribution

First direct measurement of exchange tunneling reaction rates in solid HD and D₂ matrices at temperatures below 1.5 K, demonstrating temperature independence and quantum tunneling dominance.

Findings

Reaction rates are nearly temperature-independent below 1.5 K.

Exchange tunneling reactions can occur at temperatures approaching absolute zero.

Recombination reactions are suppressed at ultra-low temperatures due to resonance tunneling violations.

Abstract

We report on a study of the exchange tunneling reaction D+HD$\rightarrow$D$_{2}$+H in a pure solid HD matrix and in a D$_{2}$ matrix with 0.23$\%$ HD admixture at temperatures between 130$\,$mK and 1.5$\,$K. We found that the exchange reaction rates, $k_{exHD}\sim3\times10^{-27}$cm$^{3}$s$^{-1}$ in the pure HD matrix, and $k_{exD_{2}}=9(4)\times10^{-28}$cm$^{-3}$ in the D$_{2}$ matrix are nearly independent of temperature within this range. This confirms quantum tunnelling nature of these reactions, and their ability to proceed at temperatures down to absolute zero. Based on these observations we concluded that exchange tunneling reaction H+H$_{2}\rightarrow$H$_{2}$+H should also proceed in a H$_{2}$ matrix at the lowest temperatures. On contrary, the recombination of H atoms in solid H$_{2}$ and D atoms in solid D$_{2}$ is substantially suppressed at the lowest temperatures as a result of increasing of violation for resonance tunneling of atoms when they approach each other.