Oxygen Photolysis in Solid Molecular Hydrogen: Details Associated with the Production of Water

A common misconception is that outer space is too harsh of an environment to support the production of molecules. In fact, in the vast regions of space between solar systems, known as the interstellar medium, there are cold, dense clouds with dust grains coated in ice, and these dust grains serve as small reaction vessels for a variety of molecules. However, the chemistry that occurs at these low temperatures (as low as 10 K) is very different from the more traditional chemistry that occurs at elevated temperatures. In order to better understand this low temperature chemistry we simulate these reaction conditions in the laboratory. Specifically, we grow crystals of molecular hydrogen (H2) at temperatures between 2 to 4 K that contain small concentrations of oxygen molecules (O2). We then expose these H2 ices to UV radiation which dissociates the O2 molecules into oxygen atoms. Oxygen atoms are highly reactive species and they react with the H2 host allowing us to detect the production of water (H2O) molecules. In this study we specifically examine how the H2 ice rearranges around the nascent H2O molecule which is important for a detailed understanding of this chemistry. We have performed a number of kinetic studies aimed at understanding the time dependence of the relaxation of the H2 ice around the newly generated H2O molecule. These measurements allow us to develop and test chemical mechanisms for the types of cold chemistry that occur within the interstellar medium