Laboratory studies of interstellar ice analogs

This thesis presents investigations of various processes responsible for the origin and evolution of the icy mantles on dust grains present in the interstellar medium (ISM). The physical state and the chemical composition of the icy mantles are thought to depend strongly on the environment that harbors the dust grains. The mantles are continually altered by interactions with the interstellar radiation and cosmic rays that pervade the ISM. We grew thin ice films on cold substrates in an ultrahigh vacuum environment and subjected them to particle bombardment to mimic conditions that exist in the ISM. We studied the influence of growth conditions and subsequent heating on the morphology of ice films and the dependence of ice porosity on radiation processing. We extrapolate our results to estimate lifetime of porous ices that coat the dust grains. We also explored CO2 synthesis via surface reactions between solid CO and neutral O since this process had been suggested to occur on the surface of the dust grains. Finally, we studied the effects of ultraviolet irradiation on thin solid O2 and O3 films. Both these molecules are predicted to exist on interstellar ice mantles and photolysis from ambient interstellar ultraviolet radiation field could produce reactive O atoms which could initiate rich chemistry in the ice mantles. The results presented in this thesis can be compared with results of astronomical observations and used to identify/explain the physical state, composition and abundance of different molecules detected in the interstellar ices. Furthermore, these experiments can help test hypotheses and constrain models developed to understand many physio-chemical processes that occur in the ISM.