H2O、HDO和D2O冰的光解吸及其对分馏的影响
Photodesorption of H2O, HDO, and D2O ice and its impact on fractionation
摘要：
星际气体中的HDO/H2O比通常用于得出恒星形成区域和地球上水的起源的结论。在冷核和原行星盘的外部区域，气相水来自水冰的光解吸。我们提出的拟合公式，在天体化学模型中使用光解吸效率的所有水冰同位素使用经典的分子动力学模拟。我们调查，如果气相HDO/H2O的比例反映了存在于冰或分馏过程中是否可以发生光解吸。在10-100 K的温度下，在H2O无定形冰中，H2O，D2 O和HDO光解后，X（X=H，D）原子，OX自由基，X2 O和HDO分子的光脱附的顶部四个单层的概率。结果表明：（1）H原子在H_2O冰上的光脱附几率是D原子在D_2O冰上的光脱附几率的1.1倍，HDO光解时H和D的光脱附比例是D的2倍。随着时间的推移，这一过程将使冰富含氘原子;（2）D2 O和H2O光解时的OD/OH光解吸比平均为2倍，但HDO光解时的比率在所有温度下几乎恒定为1;（3）D原子比H原子更有效地踢出邻近的水分子。然而，光解吸的HDO和H_2O分子的比例等于冰中HDO/H_2O的比例，因此，HDO和H_2O光解吸时没有同位素分馏。然而，由于光脱附，冰中D原子的富集可以随着时间的推移导致冰中HDO/H2O比率的增强，并且当光脱附时，也会导致气体中HDO/H2O比率的增强。在何种程度上的邻/对位比的H2O可以被修改的光脱附过程进行了讨论。（节录）
英文摘要：
The HDO/H2O ratio in interstellar gas is often used to draw conclusions on the origin of water in star-forming regions and on Earth. In cold cores and in the outer regions of protoplanetary disks, gas-phase water comes from photodesorption of water ice. We present fitting formulae for implementation in astrochemical models using photodesorption efficiencies for all water ice isotopologues obtained using classical molecular dynamics simulations. We investigate if the gas-phase HDO/H2O ratio reflects that present in the ice or whether fractionation can occur during photodesorption. Probabilities for the top four monolayers are presented for photodesorption of X (X=H,D) atoms, OX radicals, and X2O and HDO molecules following photodissociation of H2O, D2O, and HDO in H2O amorphous ice at temperatures from 10-100 K. Isotope effects are found for all products: (1) H atom photodesorption probabilities from H2O ice are larger than those for D atom photodesorption from D2O ice by a factor of 1.1; the ratio of H and D photodesorbed upon HDO photodissociation is a factor of 2. This process will enrich the ice in deuterium atoms over time; (2) the OD/OH photodesorption ratio upon D2O and H2O photodissociation is on average a factor of 2, but the ratio upon HDO photodissociation is almost constant at unity for all temperatures; (3) D atoms are more effective in kicking out neighbouring water molecules than H atoms. However, the ratio of the photodesorbed HDO and H2O molecules is equal to the HDO/H2O ratio in the ice, therefore, there is no isotope fractionation upon HDO and H2O photodesorption. Nevertheless, the enrichment of the ice in D atoms due to photodesorption can over time lead to an enhanced HDO/H2O ratio in the ice, and, when photodesorbed, also in the gas. The extent to which the ortho/para ratio of H2O can be modified by the photodesorption process is also discussed. (Abridged)
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
