Lyman-α driven molecule formation on SiO2 surfaces-connection to astrochemistry on dust grains in the interstellar...

As a model for silicate dust grains in the interstellar medium, we have used high area amorphous SiO(2) as a surface on which to carry out Lyman-α (10.2 eV) photodecomposition of adsorbed N(2)O at 71 K and at a coverage of ∼0.3 monolayer. The N(2)O molecules are adsorbed by hydrogen bonding to surface Si-OH groups. Transmission IR spectroscopy measurements permit the observation of the consumption of adsorbed N(2)O and the production of various photoproducts. It is observed that in comparison to N(2)O consumption, the relative rate of formation of the products NO(2) and N(2)O(4) made by combination reactions is enhanced significantly on the SiO(2) surface. Reactions between photogenerated radicals themselves or between radicals and parent N(2)O on the SiO(2) surface exceed the relative rates observed in the gas phase by factors of up to ∼20. As the complexity of the combination product increases, its relative production rate, compared to the gas phase, increases due to the involvement of multiple surface-combination elementary steps. It is proposed that the enhancement of combination reactions on the SiO(2) surface is due to the surface's ability to absorb excess energy evolved during the chemical-bond-forming events on the surface. This principle is probably significant on grain surfaces supporting photochemical processes of astrochemical interest, and indeed is expected. The cross section for adsorbed N(2)O photodecomposition on the porous SiO(2) surface is about 7 × 10(-20) cm(2) and the quantum yield for the adsorbed molecule decomposition is about 0.006, compared to a quantum yield of 1.46 in the gas phase. This decrease in photon efficiency is attributed to absorption and scattering of Lyman-α radiation by the SiO(2) particles.