D2O 冰光解吸的分子动力学模拟。

Leiden Observatory, Leiden University, P. O. Box 9513, 2300 RA Leiden, The Netherlands.

J Chem Phys. 2011 Apr 28;134(16):164503. doi: 10.1063/1.3582910.

Molecular dynamics (MD) calculations have been performed to study the ultraviolet (UV) photodissociation of D(2)O in an amorphous D(2)O ice surface at 10, 20, 60, and 90 K, in order to investigate the influence of isotope effects on the photodesorption processes. As for H(2)O, the main processes after UV photodissociation are trapping and desorption of either fragments or D(2)O molecules. Trapping mainly takes place in the deeper monolayers of the ice, whereas desorption occurs in the uppermost layers. There are three desorption processes: D atom, OD radical, and D(2)O molecule photodesorption. D(2)O desorption takes places either by direct desorption of a recombined D(2)O molecule, or when an energetic D atom produced by photodissociation kicks a surrounding D(2)O molecule out of the surface by transferring part of its momentum. Desorption probabilities are calculated for photoexcitation of D(2)O in the top four monolayers and are compared quantitatively with those for H(2)O obtained from previous MD simulations of UV photodissociation of amorphous water ice at different ice temperatures [Arasa et al., J. Chem. Phys. 132, 184510 (2010)]. The main conclusions are the same, but the average D atom photodesorption probability is smaller than that of the H atom (by about a factor of 0.9) because D has lower kinetic energy than H, whereas the average OD radical photodesorption probability is larger than that of OH (by about a factor of 2.5-2.9 depending on ice temperature) because OD has higher translational energy than OH for every ice temperature studied. The average D(2)O photodesorption probability is larger than that of H(2)O (by about a factor of 1.4-2.3 depending on ice temperature), and this is entirely due to a larger contribution of the D(2)O kick-out mechanism. This is an isotope effect: the kick-out mechanism is more efficient for D(2)O ice, because the D atom formed after D(2)O photodissociation has a larger momentum than photogenerated H atoms from H(2)O, and D transfers momentum more easily to D(2)O than H to H(2)O. The total (OD + D(2)O) yield has been compared with experiments and the total (OH + H(2)O) yield from previous simulations. We find better agreement when we compare experimental yields with calculated yields for D(2)O ice than when we compare with calculated yields for H(2)O ice.

为了研究同位素效应对光解吸过程的影响，采用分子动力学（MD）方法研究了 10、20、60 和 90 K 下非晶态 D2O 冰表面上 D2O 的紫外（UV）光解。对于 H2O，UV 光解后的主要过程是碎片或 D2O 分子的捕获和脱附。捕获主要发生在冰的较深单层中，而脱附发生在最上层。有三种脱附过程：D 原子、OD 自由基和 D2O 分子光解吸。D2O 通过重新组合的 D2O 分子的直接解吸或通过将部分动量传递给周围的 D2O 分子而将光解产生的高能 D 原子从表面踢出去而发生解吸。计算了 D2O 在最上面四层中的光激发的脱附概率，并与先前在不同冰温下进行的非晶态水冰的 UV 光解的 MD 模拟中获得的 H2O 的脱附概率进行了定量比较[Arasa 等人，J. Chem. Phys. 132, 184510 (2010)]。主要结论是相同的，但平均 D 原子光解吸概率小于 H 原子的光解吸概率（约 0.9 倍），因为 D 的动能比 H 低，而平均 OD 自由基光解吸概率大于 OH 的光解吸概率（取决于冰温，约为 2.5-2.9 倍），因为对于研究的每个冰温，OD 都比 OH 具有更高的平动能量。平均 D2O 光解吸概率大于 H2O（取决于冰温，约为 1.4-2.3 倍），这完全是由于 D2O 踢出机制的贡献更大。这是一种同位素效应：对于 D2O 冰，踢出现象更为有效，因为 D2O 光解后形成的 D 原子比 H2O 中光生成的 H 原子具有更大的动量，并且 D 比 H 更容易将动量传递给 D2O 而不是 H 传递给 H2O。已经将总（OD+D2O）产率与实验进行了比较，并与先前模拟的总（OH+H2O）产率进行了比较。当我们将实验产率与 D2O 冰的计算产率进行比较时，我们发现与 H2O 冰的计算产率相比，我们的结果与实验产率更为吻合。