Thermal H/D exchange in polar ice - deuteron scrambling in space

TL;DR

This study investigates thermally induced proton/deuteron exchange in amorphous water ices, revealing a lower activation energy than previously thought, which impacts understanding of interstellar chemistry and the evolution of deuterium in space.

Contribution

It provides the first direct measurement of activation energy for H/D exchange in amorphous ices, showing faster exchange rates relevant for astrophysical environments.

Findings

Lower activation energy of 3840 K for H/D exchange.

Exchange process occurs at temperatures above 70 K within 10^4 years.

H/D exchange also occurs in water mixed with other hydrogen-bonded molecules.

Abstract

We have investigated the thermally induced proton/deuteron exchange in mixed amorphous H$_2$O:D$_2$O ices by monitoring the change in intensity of characteristic vibrational bending modes of H$_2$O, HDO, and D$_2$O with time and as function of temperature. The experiments have been performed using an ultra-high vacuum setup equipped with an infrared spectrometer that is used to investigate the spectral evolution of homogeneously mixed ice upon co-deposition in thin films, for temperatures in the 90 to 140 K domain. With this non-energetic detection method we find a significantly lower activation energy for H/D exchange -- $3840 \pm 125$ K -- than previously reported. Very likely this is due to the amorphous nature of the interstellar ice analogues involved. This provides reactive timescales ($\tau<10^4$ years at $T$ $>70$ K) fast enough for the process to be important in interstellar environments. Consequently, an astronomical detection of D$_2$O will be even more challenging because of its potential to react with H$_2$O to form HDO. Furthermore, additional experiments, along with previous studies, show that proton/deuteron swapping also occurs in ice mixtures of water with other hydrogen bonded molecules, in particular on the OH and NH moieties. We conclude that H/D exchange in ices is a more general process that should be incorporated into ice models that are applied to protoplanetary disks or to simulate the warming up of cometary ices in their passage of the perihelion, to examine the extent of its influence on the final deuteron over hydrogen ratio.