  Laboratory studies of the radiation chemistry occurring in astrophysical ices
have demonstrated the dependence of this chemistry on a number of experimental
parameters. One experimental parameter which has received significantly less
attention is that of the phase of the solid ice under investigation. In this
present study, we have performed systematic 2 keV electron irradiations of the
amorphous and crystalline phases of pure CH3OH and N2O astrophysical ice
analogues. Radiation-induced decay of these ices and the concomitant formation
of products were monitored in situ using FT-IR spectroscopy. A direct
comparison between the irradiated amorphous and crystalline CH3OH ices revealed
a more rapid decay of the former compared to the latter. Interestingly, a
significantly lesser difference was observed when comparing the decay rates of
the amorphous and crystalline N2O ices. These observations have been
rationalised in terms of the strength and extent of the intermolecular forces
present in each ice. The strong and extensive hydrogen-bonding network that
exists in crystalline CH3OH (but not in the amorphous phase) is suggested to
significantly stabilise this phase against radiation-induced decay. Conversely,
although alignment of the dipole moment of N2O is anticipated to be more
extensive in the crystalline structure, its weak attractive potential does not
significantly stabilise the crystalline phase against radiation-induced decay,
hence explaining the smaller difference in decay rates between the amorphous
and crystalline phases of N2O compared to those of CH3OH. Our results are
relevant to the astrochemistry of interstellar ices and icy Solar System
objects, which may experience phase changes due to thermally-induced
crystallisation or space radiation-induced amorphisation.
