Substrate dependence of electron-stimulated O− yields from dissociative electron attachment to physisorbed O2

We present measurements of O− electron stimulated desorption yields obtained under identical experimental conditions from 0.15 monolayers (ML) of O2 deposited onto disordered substrates consisting of 4 ML of either Kr, Xe, C2H6, C2H4, N2O, CH3Cl, or H2O, all condensed on Pt (polycrystalline). The resulting O− yield functions, for incident electron energies below 20 eV, are compared to that obtained from the O2/Kr solid; this allows us to assess the order of magnitude effects of the local substrate environment on dissociative electron attachment (DEA) via the 2Πu and gas phase forbidden 2Σ+g,u resonances of O−2. We note that, in addition to electron energy losses in the substrate prior to DEA to O2 and post-dissociation interactions of the O− with the substrate molecules, charge or energy transfer from the O−2 transient anion to a substrate molecule, and capture of the incident electron into a dissociative anion resonance of the substrate molecule may contribute to a reduced O− yield from the physisorbed O2. In the case of O2 deposited on amorphous ice, we find that the O− signal from DEA to O2 is completely absent for electron energies below 14 eV; we attribute this to a complete quenching of the dissociative O−2(2Πu, 2Σ+) resonances by the adjacent water molecules.