Formation of Formaldehyde by the Tunneling Reaction of H with Solid CO at 10 K Revisited

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Abstract
The reaction of H atoms with the solid CO thin film was reinvestigated in the temperature region of 10-25 K. H2CO was found to be only the reaction product, and no other products such as CH3OH were detected. This indicates that the tunneling reactions of H with H2CO to form CH3OH are even slower than the slow reaction of H with CO to form H2CO. The CH3OH found in the envelopes of the dark clouds may have other sources for their production in addition to reactions 2H + H2CO → CH3OH, e.g., reaction of O(1D) with CH4 trapped on the dust grains to form CH3OH. The yield of H2CO 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 + H2 to desorption of the grain mantle may not be as large as thought before.