Formation of formaldehyde by the tunneling reaction of H with solid CO at 10 K revisited

The reaction of H atoms with the solid CO thin film was reinvestigated in the temperature region of 10 25 K. H(2)CO was found to be only the reaction product, and no other products such as CH(3)OH were detected. This indicates that the tunneling reactions of H with H(2)CO to form CH(3)OH are even slower than the slow reaction of H with CO to form H(2)CO. The CH(3)OH found in the envelopes of the dark clouds may have other sources for their production in addition to reactions 2H + H(2)CO --> CH(3)OH, e.g., reaction of O((1)D) with CH(4) trapped on the dust grains to form CH(3)OH. The yield of H(2) CO from the reaction H with solid CO showed a steep increase with a decrease of temperature from 25 to 10 K. This indicates that the dark clouds whose temperature is kept at as low as 10 K are the favorable place for the chemical evolution via tunneling reactions. The erosion of the solid CO film was not observed with the spray of the H atoms over the CO solid film in the temperature range of 10-25 K. This finding suggests that the contribution of the highly exothermic reaction 2H + H(2) to desorption of the grain mantle may not be as large as thought before.