SATO Tetsuya University of Yamanashi, Clean Energy Research Center, Associate Professor, クリーンエネルギー研究センター, 助教授 (60252011)
WADA Akira University of Yamanashi, Clean Energy Research Center, Researcher, クリーンエネルギー研究センター, 研究員 (70377604)
曾越 宣仁 山梨大学, クリーンエネルギー研究センター, 博士研究員
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Light essentially cannot penetrate dense interstellar clouds, and the temperature inside dark clouds is kept at about 10K. More than 120 interstellar molecules have been observed by millimeter and submillimeter spectroscopy in such dark clouds. It is becoming increasingly clear that gas-phase ion-molecule reactions-the dominant processes in diffuse interstellar clouds-are less important in dense clouds. Instead, chemical reactions on cosmic dust grains play an important role. Interstellar ices, which grow on solid dust particles in cold, dense clouds also become the substrates for various chemical reactions. Recently, Infrared Space Observatory (ISO) has provided much information about interstellar and cometary ices.
Unfortunately, direct observations of the solid-phase chemistry of dense clouds are difficult to come by. In the absence of direct evidence, we have to resort to laboratory studies. A wealth of data has been gathered on gas-phase ion-molecule reactions, but investigations o
n solid-phase reactions of various molecules remain scarce, although there have been experimental studies of molecular hydrogen formation on the surface of comic dust analogs.
In this work, the simulation of the formation of interstellar molecules on cold dust grains at -10K has been made. Formaldehyde and methanol are among the most complex molecules for which the successive H-atom addition reactions have been proposed : CO→HCO→H_2CO→CH_3O→CH_3OH. In the reaction of H with solid CO film, we found a very small yield of H_2CO with no CH_3OH detected as a reaction product. This strongly suggests that there must be some another source for the formation of ubiquitous CH_3OH in the universe, e.g., cosmic ray, UV, etc. In order to solve the methanol paradox, the H_2O ice film containing a small amount of CH_4 was irradiated by 50-300 eV electron beam at 10K. It was found that methanol was formed at the very early stage of the electron irradiation as a major product with an exceptionally high yield. This result clearly indicates that the cosmic ray plays a very important role for the formation of interstellar molecules.
The reactions of H and D atoms with solid formaldehyde and methanol were also investigated in detail at -10K. From the reaction of H with solid H_2CO at 10K, CO, CH_3OH and paraformaldehyde (polyoxymethylene) were the major products with the yields of about 1% or less. The yields of these products decrease steeply with an increase of temperature and they become negligible at -25K. From the reaction of D with solid H_2CO at 10K, CO, HDCO, CH_2DOD, and paraformaldehyde were formed. The branching ratio of reaction [D+HCO→CO+HD] to [D+HCO→HDCO] was measured to be about 1:6. The ratio of the yield of CH_3OH from reaction [H+H_2CO] to that of CH_2DOD from reaction [D+H_2CO] is 7:1, i.e., a marked isotope effect on the addition reaction of H and D to H_2CO. In the reaction of H and D with CH_3OH, H_2CO was not detected as a reaction product. Less