水冰的光脱附：分子动力学研究
Photodesorption of water ice: a molecular dynamics study
摘要：
覆盖在星际颗粒上的水冰吸收紫外线会导致冰分子的解离和解吸。这些过程在分子云和原行星盘中的气体-颗粒化学中被认为是重要的，但是很少有定量的研究。我们计算了无定形水冰的光脱附效率，并阐明了脱附发生的机制。经典的分子动力学计算进行了紧凑的无定形冰表面在10 K被认为是星际冰的代表。被认为是在1300-1500埃范围内的光子吸收到第一激发电子态后，解离和解吸的H2O分子在前六个单层。跟踪H和OH光碎片的轨迹，直到它们逃逸或被困在冰中。在顶部三个单层中，每个吸收的UV光子的H2O解吸的概率为0.5-1%，然后在接下来的两个单层中降低到0.03%，并且可以忽略不计。在顶部两个单分子膜中的主要H2O去除机制是通过H和OH碎片的单独解吸。从冰中去除H2O分子，无论是作为H2O本身还是其产物，在顶部两个单层中每个吸收的UV光子的总概率为2-3%。在第三个单层的概率约为1%，更深的冰的光解吸的概率下降到微不足道的数字。每个入射光子去除H2O的概率估计为3.7 × 10 ^-4，完整H2O分子光解吸的概率为1.4 × 10 ^-4。当没有发生解吸时，H和OH产物可以在表面内部或表面顶部行进高达70和60埃，在此期间它们可以与其他物质反应。
英文摘要：
Absorption of ultraviolet radiation by water ice coating interstellar grains can lead to dissociation and desorption of the ice molecules. These processes are thought to be important in the gas-grain chemistry in molecular clouds and protoplanetary disks, but very few quantitative studies exist. We compute the photodesorption efficiencies of amorphous water ice and elucidate the mechanisms by which desorption occurs. Classical molecular dynamics calculations were performed for a compact amorphous ice surface at 10 K thought to be representative of interstellar ice. Dissociation and desorption of H2O molecules in the top six monolayers are considered following absorption into the first excited electronic state with photons in the 1300-1500 Angstrom range. The trajectories of the H and OH photofragments are followed until they escape or become trapped in the ice. The probability for H2O desorption per absorbed UV photon is 0.5-1% in the top three monolayers, then decreases to 0.03% in the next two monolayers, and is negligible deeper into the ice. The main H2O removal mechanism in the top two monolayers is through separate desorption of H and OH fragments. Removal of H2O molecules from the ice, either as H2O itself or its products, has a total probability of 2-3% per absorbed UV photon in the top two monolayers. In the third monolayer the probability is about 1% and deeper into the ice the probability of photodesorption falling to insignificant numbers. The probability of any removal of H2O per incident photon is estimated to be 3.7x10^-4, with the probability for photodesorption of intact H2O molecules being 1.4x10^-4 per incident photon. When no desorption occurs, the H and OH products can travel up to 70 and 60 Angstroms inside or on top of the surface during which they can react with other species.
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