New Rate Constants of Hydrogenation of CO on H2O‐CO Ice Surfaces

The key route of the formation of solid H 2 CO and CH 3 OH 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 H 2 CO and CH 3 OH via the hydrogenation of CO molecules in amorphous H 2 O-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 k 0 n H = 0.58-0.52 minute -1 at temperature T = 10 K and 0.23-0.22 minute -1 at T = 15 K, where n H is the number density of hydrogen atoms. For H 2 CO, the rate constants are k 2 n H = 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 H 2 O-CO ice is estimated to be 10 -21 to 10 -20 cm 2 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.