Hydrogen adsorption and diffusion on amorphous solid water ice

Abstract

Results of classical trajectory calculations on the adsorption of H atoms to amorphous solid water (ASW) ice, at a surface temperature T s of 10 K are presented. The calculations were performed for incidence energies E i ranging from 10 to 1000 K, at random incidence. The adsorption probability P s can be fitted to a simple decay function: P s = 1.0e −E i (K )/300 . Our calculations predict similar adsorption probabilities for H atoms to crystalline and ASW ice, although the average binding energy E b of the trapped H atoms calculated for ASW of 650 ± 10 K is higher than that found for crystalline ice of 400 ± 5 K. The binding energy distributions were fitted to Gaussian functions with full width half-maximum of 111 and 195 K for crystalline and amorphous ice surfaces, respectively. The variation of the H atom binding sites in the case of the ASW surface leads to broadening of the distribution of E b compared to that of crystalline ice. We have also calculated the 'hot-diffusion' distance travelled by the impinging atom over the surface before being thermalized, which is found to be about 30 Å long at E i = 100 K and increases with E i . The diffusion coefficient D of thermally trapped H atoms is calculated to be 1.09 ± 0.04 × 10 −5 cm 2 s −1 at T s = 10 K. The residence time τ of H atoms adsorbed on ASW is orders of magnitude longer than that of H atoms adsorbed on crystalline ice for the same ice T s , suggesting that H 2 formation on crystalline and non-porous ice is quite limited compared to that on porous ice. This is in good agreement with the results of experiments on H 2 formation on porous and non-porous ASW surfaces. At low T s , the long values of τ , the high values of D and the large hot distance travelled on the ASW surface before trapping the impinging H atom ensure that Langmuir-Hinshelwood and hot-atom mechanisms for H 2 formation will be effective. The data presented here will be important ingredients for models to describe the formation of H 2 on interstellar ices and reactions of H atoms with other species at the ice surface.

Comprehensive Study of Thermal Desorption of Grain-surface Species by Accretion Shocks around Protostars

“…This value is the same as that listed in the UMIST database RATE12 (McElroy et al 2013). However, E des is not represented by a single (average) value, especially on the surfaces of amorphous substrates, as suggested by previous temperature-programed desorption (TPD) experiments (Kimmel et al 2001;Amiaud et al 2006;Fillion et al 2009) and molecular dynamics calculations (Al-Halabi et al 2004;Al-Halabi & van Dishoeck 2007). As the interstellar grains are also amorphous (Kemper et al 2004;Henning 2010), E des should be broadly distributed.…”

Structural effects of ice grain surfaces on the hydrogenation of CO at low temperatures

“…In other words, if the desorption rate is significantly high, H atoms desorb before the hydrogenation reaction occurs. The adsorption energy of H atoms to ice surface was reported to be about 350-650 K [34,35], while that from solid CO has not been reported. However, several experiments have strongly suggested a lower adsorption energy for solid CO [21,23].…”

ANALYTICAL FORMULAE OF MOLECULAR ION ABUNDANCES AND THE N2H+RING IN PROTOPLANETARY DISKS

“…Desorption energies of atomic hydrogen, CO, and N 2 are set to be 600, 1150, and 1000 K, respectively; they are the values on water-ice substrates (Garrod & Herbst 2006;Al-Halabi & van Dishoeck 2007). The sublimation temperatures of CO and N 2 are then ∼23 and ∼19 K, respectively, when the gas density is 10 6 cm −3 .…”