Probing difference in diffusivity of chloromethanes through water ice in the temperature range of 110-150 k

Abstract

In this work, we have examined the diffusive mixing of chloromethanes in different molecular solids H₂O, D₂O, and CH₃OH by monitoring their chemical sputtering spectra due to the impact of Ar⁺ ions in the collision energy range of 3-60 eV, focusing on amorphous solid water. The chemical sputtering spectra have been monitored over the temperature window accessible by liquid nitrogen, and the coverages of the molecules of interest and ice have been varied from one to several hundred monolayers. Instrumentation and sensitivity of the technique have been discussed. It is found that while the diffusion of CCl₄ in the molecular solids investigated is hindered, other choloromethanes such as CHCl₃ and CH₂Cl₂ undergo diffusive mixing over the same temperature range. Quantitatively, while ~4 monolayers (ML) of ice are found to block CCl₄ diffusion, the numbers are ~250 and ~600 ML for CHCl₃ and CH₂Cl₂, respectively. Crystallinity of ice does not have any effect on the diffusivity of water molecules when it is deposited below the chloromethanes. The effect of substrate was insignificant, and the rise in temperature increased diffusive mixing wherever the process was observed at a lower temperature.