Deuterium enrichment of the interstellar grain mantle
  We carry out Monte-Carlo simulation to study deuterium enrichment of
interstellar grain mantles under various physical conditions. Based on the
physical properties, various types of clouds are considered. We find that in
diffuse cloud regions, very strong radiation fields persists and hardly a few
layers of surface species are formed. In translucent cloud regions with a
moderate radiation field, significant number of layers would be produced and
surface coverage is mainly dominated by photo-dissociation products such as,
C,CH_3,CH_2D,OH and OD. In the intermediate dense cloud regions (having number
density of total hydrogen nuclei in all forms ~ 2 x 10^4 cm^-3), water and
methanol along with their deuterated derivatives are efficiently formed. For
much higher density regions (~ 10^6 cm^-3), water and methanol productions are
suppressed but surface coverage of CO,CO_2,O_2,O_3 are dramatically increased.
We find a very high degree of fractionation of water and methanol.
Observational results support a high fractionation of methanol but surprisingly
water fractionation is found to be low. This is in contradiction with our model
results indicating alternative routes for de-fractionation of water. Effects of
various types of energy barriers are also studied. Moreover, we allow grain
mantles to interact with various charged particles (such as H^+, Fe^+,S^+ and
C^+) to study the stopping power and projected range of these charged particles
on various target ices.
