Akihiro Nagaoka

Hydrogenation of CO on pure solid CO and CO-H2O mixed ice

Using a cold (30 K) atomic hydrogen beam, hydrogenation of CO on pure solid CO and CO-H2O mixed ice is investigated at temperatures below 20 K. Hydrogenation proceeds efficiently on both pure solid CO and CO-H2O mixed ice below 12 K, but the rate of reaction on pure CO decreases significantly at 15 K compared to CO-H2O mixed ice. Hydrogenation of CO at 12 K and above is found to be promoted by H2CO and H2O molecules on the CO surface.

H-D Substitution in Interstellar Solid Methanol: A Key Route for D Enrichment

Deuterium enrichment of interstellar methanol is reproduced experimentally for the first time via grain-surface H-D substitution in solid methanol at an atomic D/H ratio of 0.1. Although previous gas-grain models successfully reproduce the deuterium enrichments observed in interstellar methanol molecules (D/H of up to 0.4, compared to the cosmic ratio of ~10-5), the models exclusively focus on deuterium fractionation resulting from the successive addition of atomic hydrogen/deuterium on CO. The mechanism proposed here represents a key route for deuterium enrichment that reproduces the high observed abundances of deuterated methanol, including multiple deuterations.

Conversion of H2CO to CH3OH by Reactions of Cold Atomic Hydrogen on Ice Surfaces below 20 K

The conversion of formaldehyde (H2CO) to methanol (CH3OH) by successive hydrogenation on H2O ice was measured at 10, 15, and 20 K using atomic hydrogen beams of 30 and 300 K. The conversion rates and CH3OH yields under the 30 K beam are very similar to those under the 300 K beam at all ice temperatures, demonstrating that the reaction is independent of beam temperature. The dependence of the conversion rates on ice temperature is consistent with that for previous experiments on CO hydrogenation. The conversion rate for H2CO → CH3OH at 15 K was found to be about half that for CO → H2CO. The dependence of the reactions on the initial thickness of H2CO was also measured. More than 80% of H2CO was converted to CH3OH for H2CO layers of less than 1 monolayer in average thickness. Irradiation of CH3OH with H atoms did not produce H2CO, demonstrating that the reverse process, CH3OH → H2CO (H abstraction), is minor compared to the forward process.

Laboratory simulation of competition between hydrogenation and photolysis in the chemical evolution of H2O-CO ice mixtures

Hydrogenation and photolysis of H2O-CO binary ice mixtures at 10-50 K have been revisited in order to quantitatively evaluate their relative importance in the chemical evolution of interstellar dust icy mantles. The dominant product of photolysis was CO2, with lower yields of formaldehyde, methanol, and formic acid, while only formaldehyde and methanol were obtained by hydrogenation reactions. Hydrogenation has higher formation efficiencies and yields of formaldehyde and methanol than photolysis. However, the contribution of photolysis should not be negligible for the formation of these molecules in molecular clouds. The simultaneous irradiation of binary ice mixtures with hydrogen atoms and UV photons resulted in relative abundances of CO2, formaldehyde, methanol, and formic acid that are consistent with the observed abundances. Our results show that the composition and structure of ice are crucial in the chemical evolution of ice mantles, as much as the temperature and the type of irradiation.

Efficient Formation of Deuterated Methanol by H‐D Substitution on Interstellar Grain Surfaces

We performed an experiment in which pure solid CO was simultaneously exposed to cold H and D atoms at 10 K, and observed efficient formation of H2CO, CH3OH and deuterated isotopologues HDCO, D2CO, CH2DOH, CHD2OH, and CD3OH. The D/H ratios obtained for formaldehyde and methanol are in good agreement with previously reported observations. We found that the H‐D substitution reaction in methanol is a key process in the formation of deuterated methanol, especially multi‐deuterated isotopologues CHD2OH and CD3OH.