H2O对冰光化学的影响
The effect of H2O on ice photochemistry
摘要：
简单的冰的紫外线照射，提出有效地产生复杂的有机物种在恒星和行星的形成。通过一系列的实验室实验，我们调查的H2O浓度的影响，占主导地位的冰成分在空间上，更挥发性的物种，特别是CH 4，在冰混合物的光化学。在实验中，薄（~40 ML）冰混合物，保持在20-60 K，超高真空条件下照射的宽带紫外氢放电灯。使用反射-吸收红外光谱法测定了含水冰混合物中挥发性物质（CH 4和NH 3）的光破坏截面和新物质（C2 H6、C2 H4、CO、H2 CO、CH 3 OH、CH 3 CHO和CH 3CH 2 OH）的产生效率。在照射期间和随后的缓慢升温期间。增加H2O浓度的四个主要影响是：1）由于光解离后的逆反应减少，挥发性混合物成分的破坏效率增加高达一个数量级，2）向富含氧的产物例如CH 3OH和H2 CO的转变，3）在冰中捕获高达五倍以上的形成的自由基，以及4）与CH 3和HCO自由基相比，OH自由基的扩散势垒不成比例地增加。自由基扩散温度的依赖性是一致的计算H2O-自由基键强度。所有列出的影响是潜在的重要的生产复杂的有机物在H2O丰富的冰粒地幔周围的原恒星，因此应考虑到冰化学建模时。
英文摘要：
UV irradiation of simple ices is proposed to efficiently produce complex organic species during star- and planet-formation. Through a series of laboratory experiments, we investigate the effects of the H2O concentration, the dominant ice constituent in space, on the photochemistry of more volatile species, especially CH4, in ice mixtures. In the experiments, thin (~40 ML) ice mixtures, kept at 20-60 K, are irradiated under ultra-high vacuum conditions with a broad-band UV hydrogen discharge lamp. Photodestruction cross sections of volatile species (CH4 and NH3) and production efficiencies of new species (C2H6, C2H4, CO, H2CO, CH3OH, CH3CHO and CH3CH2OH) in water-containing ice mixtures are determined using reflection-absorption infrared spectroscopy during irradiation and during a subsequent slow warm-up. The four major effects of increasing the H2O concentration are 1) an increase of the destruction efficiency of the volatile mixture constituent by up to an order of magnitude due to a reduction of back reactions following photodissociation, 2) a shift to products rich in oxygen e.g. CH3OH and H2CO, 3) trapping of up to a factor of five more of the formed radicals in the ice and 4) a disproportional increase in the diffusion barrier for the OH radical compared to the CH3 and HCO radicals. The radical diffusion temperature dependencies are consistent with calculated H2O-radical bond strengths. All the listed effects are potentially important for the production of complex organics in H2O-rich icy grain mantles around protostars and should thus be taken into account when modeling ice chemistry.
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