Formation and Deuterium Fractionation of Organic Molecules on Grain Surfaces

Abstract

A series of experiments on the surface reactions of hydrogen and deuterium atoms with solid CO, formaldehyde (H₂CO), and methanol (CH₃OH) has been performed. Successive hydrogenation of CO on surfaces at $\sim $10 K was found to proceed efficiently via tunneling to produce H₂CO and CH₃OH on dust grains under the typical conditions of molecular clouds. Formation rates are strongly dependent on the surface temperature and composition. The role of surface reactions in the formation of deuterated formaldehyde and methanol was investigated. The deuterium fractionation of methanol observed in molecular clouds was reproduced experimentally via H-D substitution in solid methanol at an accreting atomic D/H ratio of 0.05-0.1. This is the first evidence that grain-surface reactions can be responsible for fractionation. We have determined several effective rate constants for hydrogenation, deuteration, and H-D substitution to construct the surface reaction network for CO, H₂CO, CH₃OH, deuterated formaldehyde, and deuterated methanol.