The investigation on low-temperature tunneling reactions and its application

光基本上不能穿透浓密的星际云，而黑云内部的温度保持在10K左右。通过毫米和亚毫米光谱学在这样的乌云中观察到120多个星际分子。越来越清楚的是，在弥漫性星际云中占主导地位的气相离子-分子反应在稠密的云中不那么重要。相反，宇宙尘埃颗粒上的化学反应起着重要作用。在寒冷稠密的云层中生长在固体尘埃颗粒上的星际冰也成为各种化学反应的基质。最近，红外空间天文台（ISO）提供了许多关于星际和彗星冰的信息。不幸的是，对致密云的固相化学的直接观测很难实现。在缺乏直接证据的情况下，我们不得不求助于实验室研究。关于气相离子-分子反应已经收集了大量的数据，但是对各种分子的固相反应的研究仍然很少，尽管已经有了在喜剧尘埃类似物表面形成分子氢的实验研究。在这项工作中，模拟了星际分子在-10K低温尘埃颗粒上的形成。甲醛和甲醇是最复杂的分子，已提出了连续的h原子加成反应：CO→HCO→H_2CO→CH_3O→CH_3OH。在H与固体CO膜的反应中，我们发现H_2CO的产率很低，没有检测到CH_3OH作为反应产物。这有力地表明，宇宙中无处不在的CH_3OH的形成一定有其他的来源，例如宇宙射线、紫外线等。为了解决甲醇悖论，用50-300 eV的电子束在10K下辐照含有少量CH_4的H_2O冰膜。研究发现，甲醇在电子辐照的早期就形成了，并以极高的收率为主要产物。这一结果清楚地表明，宇宙射线对星际分子的形成起着非常重要的作用。在-10K条件下，研究了H、D原子与固体甲醛和甲醇的反应。H与固体H_2CO在10K下反应，主要产物为CO、CH_3OH和多聚甲醛（聚甲醛），产率在1%左右。这些产物的产率随着温度的升高而急剧下降，在-25K时可以忽略不计。D与固体H_2CO在10K下反应生成CO、HDCO、CH_2DOD和多聚甲醛。测定了反应[D+HCO→CO+HD]与[D+HCO→HDCO]的支化比约为1:6。反应[H+H_2CO]的CH_3OH产率与反应[D+H_2CO]的CH_2DOD产率之比为7:1，即H、D加成反应对H_2CO有明显的同位素效应。H、D与CH_3OH反应时，未检出H_2CO作为反应产物。少

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 … More 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