Impact of oxygen chemistry on model interstellar grain surfaces

Abstract

Temperature-programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS) are used to probe the effect of atomic and molecular oxygen (O and O₂) beams on amorphous silica (aSiO₂) and water (H₂O) surfaces (porous-amorphous solid water; p-ASW, compact amorphous solid water; c-ASW, and crystalline solid water; CSW). Altering the deposition method of O₂ is shown to result in different desorption energies of O₂ due to differences in O₂ film morphology when deposited on the aSiO₂ surface. O₂ enthalpy of formation is dissipated into the aSiO₂ substrate without changes in the silica network. However, on the H₂O surfaces, O₂ formation enthalpy release is dissipated into the H-bonded matrix leading to morphological changes, possibly compacting p-ASW into c-ASW while CSW appears to undergo amorphisation. The enthalpy release from O₂ formation is, however, not enough to result in reactive desorption of O₂ or H₂O under the current experimental circumstances. Further to this, O₂ formation on sub-monolayer quantities of H₂O leads to enhanced de-wetting and a greater degree of H-bond reconnection in H₂O agglomerates. Lastly, O₃ is observed from the O + O₂ reaction on all surfaces studied.