The Dependence of H 2 CO and CH 3 OH Formation on the Temperature and Thickness of H 2 O-CO Ice during the Successive Hydrogenation of CO

The formation of formaldehyde (H 2 CO) and methanol (CH 3 OH) by the successive addition of hydrogen atoms to CO molecules in H 2 O-CO mixed ice was measured at 10, 15, and 20 K. The maximum yield of CH 3 OH was obtained at 15 K, with the next highest yield at 10 K. At 20 K, the reactions proceeded very slowly, and the resulting yield of CH 3 OH was significantly smaller than at 10 and 15 K and was considered to be due to a fall in the sticking probability of hydrogen at this temperature. The observed temperature dependence indicates that the abundances of H 2 CO and CH 3 OH in a molecular cloud are strongly dependent on the temperature of dust as well as the flux of atomic hydrogen. The diffusion of hydrogen atoms in ice is also discussed based on measurements of the dependence of CO hydrogenation on ice thickness in the range of less than 30 monolayers.