Impact of oxygen chemistry on model interstellar grain surfaces

Temperature-programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS) are used to probe the effect of atomic and molecular oxygen (O and O-2) beams on amorphous silica (aSiO(2)) 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 O-2 is shown to result in different desorption energies of O-2 due to differences in O-2 film morphology when deposited on the aSiO2 surface. O-2 enthalpy of formation is dissipated into the aSiO(2) substrate without changes in the silica network. However, on the H2O surfaces, O-2 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 O-2 formation is, however, not enough to result in reactive desorption of O-2 or H2O under the current experimental circumstances. Further to this, O-2 formation on sub-monolayer quantities of H2O leads to enhanced de-wetting and a greater degree of H-bond reconnection in H2O agglomerates. Lastly, O-3 is observed from the O + O-2 reaction on all surfaces studied.