Distribution of Weakly Interacting Atoms and Molecules in Low-Temperature Amorphous Solid Water

Understanding the distribution and mixing of atoms and molecules in amorphous solid water (ASW) at low temperatures is relevant to the exploration of the astrochemical environment in the interstellar medium (ISM) that leads to the formation of new complex molecules. In this study, a combination of temperature programmed desorption (ΔP-TPD) experiments and Ne⁺ ion sputtering is used to determine the extent of mixing and distribution of guest atoms and molecules within thin ASW films deposited at 35 K on a Ru(0001) substrate, prior to sputtering. The mixing of krypton atoms and methyl chloride molecules within thin ASW films is directed by the physical properties of the respective species and the nature of their interaction with the host water molecules. While the Kr-H₂O interaction may be described as a weak van der Waals attraction, the CD₃Cl-H₂O interaction can be characterized as weakly hydrophobic in nature. This leads to differences in the level of homogeneity in mixing and distribution of the guest species in the ASW film. Both krypton atoms and methyl chloride molecules reveal a propensity to migrate toward the ASW-vacuum interface. The krypton atoms migrate through both diffusion and displacement by incoming H₂O molecules, while the methyl chloride molecules tend to move toward the vacuum interface primarily via displacement. This behavior results in more homogeneous mixing of Kr in ASW at 35 K compared to the dipole moment containing molecule CD₃Cl. As a general outcome of our study, it is observed that mixing in ASW at low temperatures is more homogeneous when the guest atom/molecule is inert and does not possess a constant dipole moment.