冰的光脱附Ⅰ：CO、N2和CO2
Photodesorption of ices I: CO, N2 and CO2
摘要：
天体化学中一个长期存在的问题是如何在致密的星际和星周区域保持分子的气相。光脱附是一种非热脱附机制，它可以解释在云核和盘中面观测到的少量冷气体。本文的目的是确定紫外光脱附产额和约束的光脱附机制的三个天体化学相关的冰：CO，N2和CO2。此外，在混合和分层CO：N2冰共解吸的可能性进行了探索。在超高真空和15-60 K条件下，用氢气放电灯（7-10.5 eV）对冰的光脱附进行了实验研究。辐射过程中的冰解吸监测冰的反射吸收红外光谱和解吸分子的同时质谱。每个入射光子的UV光解吸产率和光解吸机制都是分子特异性的。CO光解吸而不从冰的表面层解离。在该波长范围内缺乏电子跃迁的N2具有比其低一个数量级以上的光解吸产率。当N2与CO冰混合或在CO冰顶部分层时，由于共解吸，该产率显着增加。CO2通过解离和随后的重组从顶部10层冰中光解吸。在低温（15-18 K）下，纯CO和纯N2的光脱附产额分别为2.7 × 10 ^-3和<2 × 10 ^-4分子光子-1。CO2的光脱附率为1.2x10^-3x（1-e^（-X/2.9））+ 1.1x10^-3x（1-e^（-X/4.6））分子光子-1，其中X是单层冰的厚度，表达式的两个部分代表CO2和CO的光脱附途径。
英文摘要：
A longstanding problem in astrochemistry is how molecules can be maintained in the gas phase in dense inter- and circumstellar regions. Photodesorption is a non-thermal desorption mechanism, which may explain the small amounts of observed cold gas in cloud cores and disk mid-planes. This paper aims to determine the UV photodesorption yields and to constrain the photodesorption mechanisms of three astrochemically relevant ices: CO, N2 and CO2. In addition, the possibility of co-desorption in mixed and layered CO:N2 ices is explored. The ice photodesorption is studied experimentally under ultra high vacuum conditions and at 15-60 K using a hydrogen discharge lamp (7-10.5 eV). The ice desorption during irradiation is monitored by reflection absorption infrared spectroscopy of the ice and simultaneous mass spectrometry of the desorbed molecules. Both the UV photodesorption yields per incident photon and the photodesorption mechanisms are molecule specific. CO photodesorbs without dissociation from the surface layer of the ice. N2, which lacks an electronic transition in this wavelength range, has a photodesorption yield that is more than an order of magnitude lower. This yield increases significantly due to co-desorption when N2 is mixed in with or layered on top of CO ice. CO2 photodesorbs through dissociation and subsequent recombination from the top 10 layers of the ice. At low temperatures (15-18 K) the derived photodesorption yields are 2.7x10^-3 and <2x10^-4 molecules photon-1 for pure CO and N2, respectively. The CO2 photodesorption yield is 1.2x10^-3x(1-e^(-X/2.9)) + 1.1x10^-3x(1-e^(-X/4.6)) molecules photon-1, where X is the ice thickness in monolayers and the two parts of the expression represent a CO2 and CO photodesorption pathway.
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