EXPERIMENTS OF WATER FORMATION ON WARM SILICATES

When dust grains have a higher temperature than they would have in dense clouds, and when H, H-2, and O-2 have a negligible residence time on grains, the formation of water should still be possible via the hydrogenation of OH and Eley-Rideal-type reactions. We determined that the OH desorption energy from an amorphous silicate surface is at least 143 meV (1656 K). This is 400 K higher than the value previously used in chemical models of the interstellar medium and is possibly as high as 410 meV (4760 K). This extends the temperature range for the efficient formation of water on grains from about 30 K to at least 50 K, and possibly over 100 K. We do not find evidence that water molecules leave the surface upon formation. Instead, through a thermal programmed desorption experiment, we find that water formed on the surface of an amorphous silicate desorbs at around 160 K. We also measured the cross-sections for the reaction of H and D with an O-3 layer on an amorphous silicate surface at 50 K. The values of the cross-sections, sigma(H) = 1.6 +/- 0.27 angstrom(2) and sigma(D) = 0.94 +/- 0.09 angstrom(2), respectively, are smaller than the size of an O-3 molecule, suggesting the reaction mechanism is more likely Eley-Rideal than hot-atom. Information obtained through these experiments should help theorists evaluate the relative contribution of water formation on warm grains versus in the gas phase.