Radiation-induced D/H Exchange Rate Constants in Aliphatics Embedded in
  Water Ice
  Gas-phase and solid-state chemistry in low-temperature interstellar clouds
and cores leads to a D/H enhancement in interstellar ices, which is eventually
inherited by comets, meteorites, and even planetary satellites. Hence, the D/ H
ratio has been widely used as a tracer for the origins of extraterrestrial
chemistry. However, the D/H ratio can also be influenced by cosmic rays, which
are ubiquitous and can penetrate even dense interstellar molecular cores. The
effects of such high-energy radiation on deuterium fractionation have not been
studied in a quantitative manner. In this study, we present rate constants for
radiation-induced D-to-H exchange for fully deuterated small (1-2 C)
hydrocarbons embedded in H2O ice at 20 K and H-to-D exchange for the protiated
forms of these molecules in D2O ice at 20 K. We observed larger rate constants
for H-to-D exchange in the D2O ice versus D-to-H exchange in H2O ice, which we
have attributed to the greater bond strength of C-D versus C-H. We find that
the H-to-D exchange rate constants are smaller for protiated methane than
ethane, in agreement with bond energies from the literature. We are unable to
obtain rate constants for the unsaturated and reactive hydrocarbons ethylene
and acetylene. Interpretation of the rate constants suggest that D/H exchange
products are formed in abundance alongside radiolysis products. We discuss how
our quantitative and qualitative data can be used to interpret the D/ H ratios
of aliphatic compounds observed throughout space.
