Experimental investigation of O2 diffusion and entrapment in interstellar amorphous solid water (ASW)

Abstract

Interstellar ices are mainly composed of amorphous solid water (ASW) containing small amounts of hypervolatiles, such as O2, whose diffusion-limited reactions play a key role in space chemistry. Although O2 is an important precursor molecule present during the early stages of ice formation, its surface diffusion in ASW remains poorly constrained. In this study, we experimentally investigate the surface diffusion and the entrapment efficiency of O2 in porous ASW under astrophysically relevant conditions. Experiments were conducted in an ultra-high vacuum chamber and monitored using infrared (IR) spectroscopy and quadrupole mass spectrometry. Diffusion coefficients were extracted through a novel approach applicable to IR-inactive molecules, by fitting the mass spectrometer signal during the isothermal phase with a Fickian model. These coefficients were then used to derive the diffusion energy barrier of O2 in ASW. Entrapment efficiencies were measured by analyzing the subsequent temperature-programmed desorption phase. We measured the surface diffusion coefficients at different temperatures (35 K, 40 K, 45 K) and water ice coverages (40 ML, 60 ML, 80 ML), yielding values on the order of 1E-16 to 1E-15 cm^2 s^-1. From these values, we derived a diffusion energy barrier of ED = 10 +/- 3 meV (116 +/- 35 K), corresponding to a chi ratio of about 0.1. Entrapment measurements revealed that a residual amount of about 20% of O2 remains trapped in the ASW matrix at the highest temperatures investigated. This work demonstrates that the surface diffusion of IR-inactive molecules can be experimentally quantified using mass spectrometry. Our findings show that O2 exhibits a low diffusion barrier, indicating high mobility in interstellar water ices. Moreover, we suggest these water ices likely retain a residual fraction of hypervolatiles entrapped within their structure.