Ultrafast Electron Dynamics at Alkali/Ice Structures Adsorbed on a Metal Surface
Ultrafast Electron Dynamics at Alkali/Ice Structures Adsorbed on a Metal Surface
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吸附在金属表面的碱/冰结构的超快电子动力学
DOI：
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发表时间：
2011
期刊：
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通讯作者：
M. Meyer
中科院分区：
文献类型：
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作者：
M. Meyer
这项工作的目的是研究吸附在金属表面的水冰结构中的过剩电子与其他带电或中性物种(如碱性离子)或化学反应分子(如氯氟烃)之间的相互作用。冰中多余的电子可以直接或间接地通过氢键水分子与离子相互作用。在这两种情况下，碱的存在都会影响冰层中过剩电子的布居、局域化和寿命。考虑到过剩电子的高活性特性，这些性质具有很大的相关性，因为过剩电子可以通过解离电子附着(DEA)来介导化学反应。利用飞秒时间分辨双光子光电子能谱研究了碱吸附对冰结构中电子的溶剂化和迁移动力学的影响。在第一个系统中，碱原子被共吸附在吸附在铜(111)上的润湿的无定形冰膜的顶部。在60~100K温度范围内，碱吸附导致冰/真空界面上形成带正电的碱性离子。表面的碱性离子与周围水分子的偶极矩之间的相互作用导致水分子的重新取向。结果，形成了新的电子捕获位置，即在局部电势极小值。将多余的电子光注入到这些被碱离子覆盖的无定形冰层中，导致溶剂化的电子被捕获在碱离子/水的复合体中。纯无定形冰膜中的溶剂化电子位于冰层的主体中，而碱离子/水复合体中的溶剂化电子位于冰/真空界面。它们的寿命为几皮秒，并表现出快速的能量稳定。对于持续的溶剂化，即泵浦-探测时间延迟，电子转移是通过隧道穿过由冰层厚度决定的势垒来调节的。在第二个系统中，研究了直接在金属衬底上制备的碱/水小团簇的电子溶剂化。在这些实验中，可以控制这样一个团簇中水分子的平均数量，从而可以研究过量电子的布居和稳定动力学作为D2O覆盖率的函数。观察到了两个主要效应：(I)碱分子通过团簇中周围溶剂分子的重新取向而溶剂化；(Ii)在每个碱基上有临界数量的水分子以上，多余的电子可以定位在团簇上，在那里它们是能量稳定的。这一临界比取决于碱的类型，并与碱诱导的偶极矩成反比。最后，证明了吸附在Ru(001)晶面上的晶冰中捕获的电子可以非常有效地通过解离电子吸附来促进化学反应。当像CFCl3这样的电负性分子与结晶冰共吸附时，捕获的电子与CFCl3分子之间发生DEA过程，导致·CFCl2自由基和Cl−阴离子的形成。这些结果表明，光激发俘获电子可以在冰表面的非均相化学过程中发挥重要作用，因此可能与极地平流层化学有关。
The goal of this work is to study the interaction between excess electrons in water ice structures adsorbed on metal surfaces and other charged or neutral species, like alkali ions, or chemically reactive molecules, like chlorofluorocarbons (CFC), respectively. The excess electrons in the ice can interact with the ions directly or indirectly via the hydrogen bonded water molecules. In both cases the presence of the alkali influences the population, localization, and lifetime of electronic states of excess electrons in the ice adlayer. These properties are of great relevance when considering the highly reactive character of the excess electrons, which can mediate chemical reactions by dissociative electron attachment (DEA). The influence of alkali adsorption on electron solvation and transfer dynamics in ice structures is investigated for two types of adsorption configurations using femtosecond time-resolved two-photon photoelectron spectroscopy. In the first system alkali atoms are coadsorbed on top of a wetting amorphous ice film adsorbed on Cu(111). At temperatures between 60 and 100 K alkali adsorption leads to the formation of positively charged alkali ions at the ice/vacuum interface. The interaction between the alkali ions at the surface and the dipole moments of the surrounding water molecules results in a reorientation of the water molecules. As a consequence new electron trapping sites, i.e. at local potential minima, are formed. Photoinjection of excess electrons into these alkali-ion covered amorphous ice layers, results in the trapping of a solvated electron at an alkali-ion/water complex. In contrast to solvation in pure amorphous ice films, where the electrons are located in the bulk of the ice layer, solvated electrons at alkaliion/water complexes are located at the ice/vacuum interface. They exhibit lifetimes of several picoseconds and show a fast energetic stabilization. With ongoing solvation, i.e. pump-probe time delay, the electron transfer is mediated by tunneling through a potential barrier which is determined by the thickness of the ice layer. In the second system electron solvation at small alkali/water clusters directly prepared at the metal substrate is investigated. In these experiments the average number of water molecules in such a cluster can be controlled so that the population and stabilization dynamics of excess electrons can be investigated as a function of D2O coverage. Two main effects are observed: (i) the alkalis are solvated by a reorientation of the surrounding solvent molecules in the cluster; and (ii) above a critical number of water molecules per alkali excess electrons can localize at the clusters where they are energetically stabilized. This critical ratio depends on the type of alkali and is inversely proportional to the alkali-induced dipole moment. Finally, it is demonstrated that trapped electrons in crystalline ice adsorbed on Ru(001) can very efficiently mediate chemical reactions via dissociative electron attachment. When electronegative molecules like CFCl3 are coadsorbed with crystalline ice a DEA process between trapped electrons and CFCl3 molecules occurs, resulting in the formation of ·CFCl2 radicals and Cl− anions. These results suggest that photoexcited trapped electrons can play an important role in heterogeneous chemical processes on ice surfaces and could thus be relevant in the polar stratosphere chemistry.