晶体冰和非晶冰中水分子光解离的分子动力学研究。

Andersson Stefan, Al-Halabi Ayman, Kroes Geert-Jan, van Dishoeck Ewine F

Leiden Observatory, The Netherlands.

J Chem Phys. 2006 Feb 14;124(6):64715. doi: 10.1063/1.2162901.

We present the results of classical dynamics calculations performed to study the photodissociation of water in crystalline and amorphous ice surfaces at a surface temperature of 10 K. A modified form of a recently developed potential model for the photodissociation of a water molecule in ice [S. Andersson et al., Chem. Phys. Lett. 408, 415 (2005)] is used. Dissociation in the top six monolayers is considered. Desorption of H(2)O has a low probability (less than 0.5% yield per absorbed photon) for both types of ice. The final outcome strongly depends on the original position of the photodissociated molecule. For molecules in the first bilayer of crystalline ice and the corresponding layers in amorphous ice, desorption of H atoms dominates. In the second bilayer H atom desorption, trapping of the H and OH fragments in the ice, and recombination of H and OH are of roughly equal importance. Deeper into the ice H atom desorption becomes less important and trapping and recombination dominate. Motion of the photofragments is somewhat more restricted in amorphous ice. The distribution of distances traveled by H atoms in the ice peaks at 6-7 Angstroms with a tail going to about 60 Angstroms for both types of ice. The mobility of OH radicals is low within the ice with most probable distances traveled of 2 and 1 Angstrom for crystalline and amorphous ices, respectively. OH is, however, quite mobile on top of the surface, where it has been found to travel more than 80 Angstroms. Simulated absorption spectra of crystalline ice, amorphous ice, and liquid water are found to be in very good agreement with the experiments. The outcomes of photodissociation in crystalline and amorphous ices are overall similar, but with some intriguing differences in detail. The probability of H atoms desorbing is 40% higher from amorphous than from crystalline ice and the kinetic-energy distribution of the H atoms is on average 30% hotter for amorphous ice. In contrast, the probability of desorption of OH radicals from crystalline ice is much higher than that from amorphous ice.

我们展示了为研究在10K表面温度下结晶冰和非晶冰表面水的光解离而进行的经典动力学计算结果。使用了一种最近开发的用于冰中水分子光解离的势模型的改进形式[S. Andersson等人，《化学物理快报》408, 415 (2005)]。考虑了最上面六个单层中的解离情况。对于这两种类型的冰，H₂O的解吸概率都很低（每吸收一个光子的产率低于0.5%）。最终结果强烈依赖于光解离分子的原始位置。对于结晶冰的第一双层和非晶冰中的相应层中的分子，H原子的解吸占主导。在第二双层中，H原子解吸、H和OH碎片在冰中的捕获以及H和OH的重组大致同等重要。在冰中更深的位置，H原子解吸变得不那么重要，捕获和重组占主导。光解离碎片的运动在非晶冰中受到的限制稍大一些。对于这两种类型的冰，H原子在冰中行进的距离分布在6 - 7埃处达到峰值，尾部延伸至约60埃。OH自由基在冰中的迁移率很低，对于结晶冰和非晶冰，最可能行进的距离分别为2埃和1埃。然而，OH在表面顶部相当活跃，在那里发现它行进的距离超过80埃。发现结晶冰、非晶冰和液态水的模拟吸收光谱与实验结果非常吻合。结晶冰和非晶冰中光解离的结果总体相似，但在细节上有一些有趣的差异。非晶冰中H原子解吸的概率比结晶冰高40%，并且非晶冰中H原子的动能分布平均高30%。相比之下，结晶冰中OH自由基的解吸概率比非晶冰高得多。