[论文解读] Photodesorption of water ice: a molecular dynamics study

Stefan Andersson, E. F. van Dishoeck|ArXiv.org|Oct 10, 2008

Astrophysics and Star Formation Studies参考文献 61被引用 120

一句话总结

本研究采用经典分子动力学模拟，研究10 K下水冰在紫外光照射下的光致脱附行为，模拟星际环境条件。结果表明，光致脱附量子产率在最上层两单层中最高（每吸收一个光子产率为2–3%），主要通过H和OH碎片脱附实现，而完整H₂O分子的脱附产率约为每入射光子1.4×10⁻⁴，与天体物理模型一致。

ABSTRACT

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.

研究动机与目标

• 量化非晶态水冰在星际温度下经紫外辐射照射后的光致脱附效率。
• 识别紫外光吸收后主要的脱附机制——完整H₂O分子还是光致碎片。
• 评估光致脱附在星际冰化学中的作用，特别是在分子云和原行星盘中的作用。
• 为天体化学模型提供基于物理的光致脱附产率估算。

提出的方法

• 在10 K下对紧凑的非晶态冰表面进行经典分子动力学模拟。
• 将波长范围为1300–1500 Å的紫外光子吸收至最上六单层中H₂O分子的第一激发电子态。
• 追踪H和OH光致碎片的轨迹，直至其逃逸或被冰体捕获。
• 按每吸收一个光子计算脱附概率，区分完整H₂O与碎片脱附。
• 模拟考虑了能量传递、碎片重新结合以及碎片在表面的扩散行为。
• 对结果进行分析，估算每入射光子的总光致脱附产率，同时考虑吸收与逃逸过程。

实验结果

研究问题

• RQ1 在最上层几单层中，每吸收一个紫外光子，非晶态水冰的光致脱附产率是多少？
• RQ2 主要的脱附路径是完整H₂O分子还是H和OH碎片？
• RQ3 脱附概率如何随冰层深度变化，从表面到深层？
• RQ4 光致碎片（H、OH）在被俘获前能保持多大程度的移动性和反应活性？
• RQ5 模拟结果与天体化学模型中使用的观测和实验产率相比如何？

主要发现

• 在最上层三单层中，每吸收一个紫外光子，H₂O脱附的概率为0.5–1%，在接下来的两单层中降至0.03%，在更深层则可忽略不计。
• 在最上两单层中，任何H₂O去除（完整分子或碎片）的总概率为每吸收一个光子2–3%。
• 完整H₂O分子的光致脱附产率为每入射光子1.4×10⁻⁴，与天体化学模型中常用的数值一致。
• 每入射光子的任何H₂O去除总概率估计为3.7×10⁻⁴。
• H和OH碎片可在冰体内部或表面分别移动达70 Å和60 Å后才被俘获，从而可能与CO等物种发生反应。
• H原子脱附的概率约为OH或H₂O脱附的2–3个数量级更高，而OH脱附概率约为H₂O的两倍。