Oxygen diffusion and reactivity at low temperature on bare amorphous olivine-type silicate

Abstract

The mobility of O atoms at very low temperatures is not generally taken into account, despite O diffusion would add to a series of processes leading to the observed rich molecular diversity in space. We present a study of the mobility and reactivity of O atoms on an amorphous silicate surface. Our results are in the form of reflection absorption infrared spectroscopy and temperature-programmed desorption spectra of O2 and O3 produced via two pathways: O + O and O2 + O, investigated in a submonolayer regime and in the range of temperature between 6.5 and 30 K. All the experiments show that ozone is formed efficiently on silicate at any surface temperature between 6.5 and 30 K. The derived upper limit for the activation barriers of O + O and O2 + O reactions is ∼150 K/kb. Ozone formation at low temperatures indicates that fast diffusion of O atoms is at play even at 6.5 K. Through a series of rate equations included in our model, we also address the reaction mechanisms and show that neither the Eley-Rideal nor the hot atom mechanisms alone can explain the experimental values. The rate of diffusion of O atoms, based on modeling results, is much higher than the one generally expected, and the diffusive process proceeds via the Langmuir-Hinshelwood mechanism enhanced by tunnelling. In fact, quantum effects turn out to be a key factor that cannot be neglected in our simulations. Astrophysically, efficient O3 formation on interstellar dust grains would imply the presence of huge reservoirs of oxygen atoms. Since O3 is a reservoir of elementary oxygen, and also of OH via its hydrogenation, it could explain the observed concomitance of CO2 and H2O in the ices.

Water deuteration and ortho-to-para nuclear spin ratio of H2in molecular clouds formed via the accumulation of H I gas

“…Recently Dulieu et al (2013) found that desorption probabilities of these reactions are very high, >90% and 60%, on a silicate substrate, while the probabilities are much lower on an ice substrate. Minissale & Dulieu (2014) found that the desorption probability of O + O → O 2 becomes smaller with the increasing ice coverage. Motivated by these experiments, we use the coverage dependent desorption probabilities for these two reactions and their deuterated analogues:…”

The importance of OH radical–neutral low temperature tunnelling reactions in interstellar clouds using a new model

“…We have considered only thermal hopping, however, because most recent experiments do not indicate the existence of quantum mechanical tunneling for diffusive motion [36][37][38]. Minissale et al [39], on the other hand, have interpreted the rapid diffusion of oxygen on olivine at low temperatures as due to tunneling. In addition to the diffusive, or Langmuir-Hinshelwood, mechanism, surface reactions can occur via the Eley-Rideal mechanism, in which a gas-phase species collides with an adsorbate, leading to reaction.…”

Hydrogenation at low temperatures does not always lead to saturation: the case of HNCO

“…Recent experimental studies using FORMOLISM have highlighted the significance of the chemical desorption mechanism in the monolayer regime at low temperatures (Dulieu et al 2013;Minissale et al 2014;Noble et al 2011). It has been shown that up to 90% of molecules (on a graphite surface) formed by radical-molecule or radical-radical surface reactions sublimate from the surface after formation because of the inability of the surface to quench the energy released during the reaction.…”