New Rate Constants of Hydrogenation of CO on H 2 O‐CO Ice Surfaces

Abstract

The key route of the formation of solid H2CO and CH3OH on grain surfaces is the hydrogenation reactions. This route was confirmed recently by experiments carried out by Watanabe and his coworkers in 2002 and 2003. On the basis of these experimental results, we derive theoretical formulae and determine the rate constants of the formation of H2CO and CH3OH via the hydrogenation of CO molecules in amorphous H2O-CO ice. In order to reproduce the experimental results, we introduce a new model of the ice called the crack model. We find that the morphology of the ice plays a vital role for the calculations of the rate constants. The rate constants for hydrogenation of CO are k0nH = 0.58-0.52 minute-1 at temperature T = 10 K and 0.23-0.22 minute-1 at T = 15 K, where nH is the number density of hydrogen atoms. For H2CO, the rate constants are k2nH = 0.020-0.013 minute-1 at T = 10 K and 0.075-0.056 minute-1 at T = 15 K. At temperatures 10 and 15 K, the diffusion constant of hydrogen atoms into the amorphous H2O-CO ice is estimated to be 10-21 to 10-20 cm2 s-1, which has little effect on the calculations of the rate constant of the surface hydrogenation. We discuss the deviation of the theoretical results from the experimental ones at late times.