The investigation on low-temperature tunneling reactions and its application

光线基本上无法穿透稠密的星际云，而乌云内部的温度保持在10 K左右。在这样的暗云中，已经用毫米和亚毫米光谱学观察到了120多个星际分子。越来越清楚的是，在弥漫的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发挥了重要作用。在寒冷、稠密的云层中的固体尘埃颗粒上生长的星际冰也成为各种化学反应的基质。近年来，红外空间天文台（ISO）提供了大量关于星际冰和彗星冰的信息，但很难直接观测到致密云的固相化学。由于缺乏直接证据，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，但对气相离子-分子反应的研究还不够深入。 ...更多信息 虽然在类似喜剧尘埃表面上有过分子氢形成的实验研究，但各种分子的固相反应仍然很少.本文模拟了-10K下冷尘埃颗粒上星际分子的形成.甲醛和甲醇是最复杂的分子之一，它们的氢原子加成反应是：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，宇宙中普遍存在的CH_3OH的形成必须有另一种来源，例如，为了解决甲醇佯谬，在10 K下用50-300 eV的电子束辐照含有少量CH_4的H_2O冰膜。结果发现，在电子辐照的非常早期的阶段形成的甲醇作为一个主要的产品，具有非常高的产率。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。我们还详细地研究了H和D原子与固体甲醛和甲醇在-10K下的反应。在10 K下，H与固体H_2CO反应，主要产物为CO、CH_3OH和多聚甲醛，产率在1%以下，这些产物的产率随温度的升高而急剧下降，在-25K时几乎可以忽略不计。D与固体H_2CO在10 K下反应生成CO、HDCO、CH_2DOD和多聚甲醛。测得反应[D+HCO→CO+HD]与[D+HCO→HDCO]的分支比为约1：6。[H+H_2CO]反应生成CH_3OH的产率与[D+H_2CO]反应生成CH_2DOD的产率之比为7：1，即H和D与H_2CO的加成反应存在明显的同位素效应。在H和D与CH_3OH的反应中，没有检测到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