Petrik Nikolay G, Kavetsky Alexander G, Kimmel Greg A
Fundamental Sciences Directorate, Pacific Northwest National Laboratory, Mail Stop K8-88, Richland, WA 99352, USA.
J Chem Phys. 2006 Sep 28;125(12):124702. doi: 10.1063/1.2345367.
The low-energy, electron-stimulated production of molecular oxygen from thin amorphous solid water (ASW) films adsorbed on Pt(111) is investigated. For ASW coverages less than approximately 60 ML, the O(2) electron-stimulated desorption (ESD) yield depends on coverage in a manner that is very similar to the H(2) ESD yield. In particular, both the O(2) and H(2) ESD yields have a pronounced maximum at approximately 20 ML due to reactions at the Pt/water interface. The O(2) yield is dose dependent and several precursors (OH, H(2)O(2), and HO(2)) are involved in the O(2) production. Layered films of H(2) (16)O and H(2) (18)O are used to profile the spatial distribution of the electron-stimulated reactions leading to oxygen within the water films. Independent of the ASW film thickness, the final reactions leading to O(2) occur at or near the ASW/vacuum interface. However, for ASW coverages less than approximately 40 ML, the results indicate that dissociation of water molecules at the ASW/Pt interface contributes to the O(2) production at the ASW/vacuum interface presumably via the generation of OH radicals near the Pt substrate. The OH (or possibly OH(-)) segregates to the vacuum interface where it contributes to the reactions at that interface. The electron-stimulated migration of precursors to the vacuum interface occurs via transport through the hydrogen bond network of the ASW without motion of the oxygen atoms. A simple kinetic model of the nonthermal reactions leading to O(2), which was previously used to account for reactions in thick ASW films, is modified to account for the electron-stimulated migration of precursors.
研究了吸附在Pt(111)上的非晶态薄固态水(ASW)薄膜在低能电子激发下产生分子氧的过程。对于ASW覆盖率小于约60 ML的情况，O(2)电子激发脱附(ESD)产率随覆盖率的变化方式与H(2) ESD产率非常相似。特别是，由于Pt/水界面处的反应，O(2)和H(2) ESD产率在约20 ML时都有一个明显的最大值。O(2)产率与剂量有关，并且几种前体(OH、H(2)O(2)和HO(2))参与了O(2)的产生。使用H(2) (16)O和H(2) (18)O的分层薄膜来描绘水膜中导致氧气产生的电子激发反应的空间分布。与ASW薄膜厚度无关，导致O(2)产生的最终反应发生在ASW/真空界面处或其附近。然而，对于ASW覆盖率小于约40 ML的情况，结果表明，ASW/Pt界面处水分子的解离可能通过在Pt衬底附近产生OH自由基，从而有助于ASW/真空界面处的O(2)产生。OH(或可能是OH(-))会偏析到真空界面，在那里它有助于该界面处的反应。前体向真空界面的电子激发迁移是通过ASW的氢键网络进行传输而发生的，其中氧原子没有移动。一个先前用于解释厚ASW薄膜中反应的导致O(2)产生的非热反应的简单动力学模型被修改，以解释前体的电子激发迁移。
