Formaldehyde and methanol were produced efficiently by the hydrogenation of CO in H 2 O-CO ice at 10 K in an atomic hydrogen beam experiment. The relative yields to the initial CO were of the order of 10%, 3 orders of magnitude larger than those reported previously. This reveals for the first time experimentally that successive hydrogenation of CO is most likely to produce formaldehyde and methanol on the surface of icy grains in molecular clouds as suggested by theoretical models.

Formation through H-Atom Recombination 8794 3.3.2. Energy Partitioning in H 2 Formation through Recombination 8796 4. Quantum-Tunneling Reactions on Surfaces 8797 4.1. Water Formation 8798 4.1.1. O 2 + H (Sequential H-Atom Addition to O 2 ) 8800 4.1.2. OH + H 2 8800 4.1.3. Other Reactions 8805 4.2. Formaldehyde and Methanol Formation and Deuteration 8805 4.2.1. Hydrogen and Deuterium Addition to CO 8805 4.2.2. Deuterium Enrichment of CH 3 OH and H 2 CO 8807 4.3. Carbon Dioxide Formation 8811 4.3.1. CO + O 8812 4.3.2. CO + OH 8813 5. Nuclear-Spin Modifications of Molecules 8814 5.1. orthoand para-H 2 Molecules and Their Roles in Interstellar Chemistry 8814 5.1.1. Nascent Ortho-to-Para Ratio of H 2 Molecules Formed on Ice Mantles 8815 5.1.2. Nuclear-Spin Conversion of H 2 Molecules 8816 5.2. Ortho and Para States of Other Abundant Molecules (H 2 O, etc.) 8819 5.2.1. Nuclear-Spin Conversion of Polyatomic Molecules by Collisions 8820 5.2.2.

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The reactions of cold H atoms with solid O 2 molecules were investigated at 10 K. The formation of H 2 O 2 and H 2 O has been confirmed by in-situ infrared spectroscopy. We found that the reaction proceeds very efficiently and obtained the effective reaction rates. This is the first clear experimental evidence of the formation of water molecules under conditions mimicking those found in cold interstellar molecular clouds. Based on the experimental results, we discuss the reaction mechanism and astrophysical implications.