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