Laboratory simulation of competition between hydrogenation and photolysis in thechemical evolution of H2O-CO ice mixtures
N. Watanabe1, O. Mouri1, A. Nagaoka1, T. Chigai1, A. Kouchi1, and V. Pirronello2
1

Institute of Low Temperature Science, Hokkaido University, JAPAN2
D.M.F.C.I., Universita` di Catania, Italy

Photolysis and hydrogenation of ice dust are important processes of chemical evolution inmolecular clouds. Phyotolysis has been studies experimentally by many researchers using
astrophysically-relevant ice-mixture and found to yield various molecules [e.g. 1,2].
Hydrogenation has been first proposed theoretically since 1980s [3] as a main formation routeof formaldehyde and methanol from a primordial CO molecule on ice dust. Recently, theseries of our experiments [4] have confirmed that this process efficiently proceeds due totunneling effect under the condition of quiescent molecular cloud where the temperature is as
low as 10 K. Experiments of photolysis and hydrogenation were performed separately bydifferent groups with their own experimental conditions. In addition, kinetic data such as rate
constants obtained are still not enough. It is very difficult not only to measure those data forsurface reactions but also to construct the models because reactions strongly depend on thesurface composition and morphology. Therefore, the relative importance of photolysis andhydrogenation in molecular clouds was not examined. To do so, the simulation experiments
for those processes under well controlled conditions were very desirable.
In the present study, photolysis and hydrogenation of H2O-CO binary ice mixtures (as theanalogues of the most primordial ice composition) at 10–50 K are investigated in order toquantitatively evaluate their relative importance in the chemical evolution of interstellar dust
icy mantles [5]. The dominant product of photolysis was CO2, with lower yields offormaldehyde, methanol, and formic acid, while only formaldehyde and methanol wereobtained by hydrogenation reactions. Hydrogenation has higher formation efficiencies andyields of formaldehyde and methanol than photolysis. However, the contribution of photolysis
should not be negligible for the formation of these molecules in molecular clouds, in contrast
to the previously reported results [2]. The simultaneous irradiation of binary ice mixtures withhydrogen atoms and UV photons resulted in relative abundances of CO2, formaldehyde,
methanol, and formic acid that are consistent with the observed abundances. Our results showthat 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.

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