Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA.
J Chem Phys. 2013 Aug 21;139(7):074702. doi: 10.1063/1.4818163.
The scanning tunneling microscope (STM) is a fascinating tool used to perform chemical processes at the single-molecule level, including bond formation, bond breaking, and even chemical reactions. Hahn and Ho [J. Chem. Phys. 123, 214702 (2005)] performed controlled rotations and dissociations of single O2 molecules chemisorbed on the Ag(110) surface at precise bias voltages using STM. These threshold voltages were dependent on the direction of the bias voltage and the initial orientation of the chemisorbed molecule. They also observed an interesting voltage-direction-dependent and orientation-dependent pathway selectivity suggestive of mode-selective chemistry at molecular junctions, such that in one case the molecule underwent direct dissociation, whereas in the other case it underwent rotation-mediated dissociation. We present a detailed, first-principles-based theoretical study to investigate the mechanism of the tunneling-induced O2 dynamics, including the origin of the observed threshold voltages, the pathway dependence, and the rate of O2 dissociation. Results show a direct correspondence between the observed threshold voltage for a process and the activation energy for that process. The pathway selectivity arises from a competition between the voltage-modified barrier heights for rotation and dissociation, and the coupling strength of the tunneling electrons to the rotational and vibrational modes of the adsorbed molecule. Finally, we explore the "dipole" and "resonance" mechanisms of inelastic electron tunneling to elucidate the energy transfer between the tunneling electrons and chemisorbed O2.
扫描隧道显微镜（STM）是一种令人着迷的工具，可用于在单分子水平上进行化学过程，包括键的形成、键的断裂，甚至化学反应。Hahn 和 Ho [J. Chem. Phys. 123, 214702 (2005)] 使用 STM 在精确的偏置电压下对化学吸附在 Ag(110)表面上的单个 O2 分子进行受控旋转和离解。这些阈值电压取决于偏置电压的方向和化学吸附分子的初始取向。他们还观察到一种有趣的电压方向依赖性和取向依赖性的途径选择性，表明分子结处存在模式选择性化学，例如在一种情况下，分子直接解离，而在另一种情况下，分子经历了旋转介导的解离。我们提出了一项详细的、基于第一性原理的理论研究，以研究隧穿诱导的 O2 动力学的机制，包括观察到的阈值电压的起源、途径依赖性和 O2 解离的速率。结果表明，观察到的过程的阈值电压与该过程的活化能之间存在直接对应关系。途径选择性源于旋转和解离的电压修饰势垒高度之间的竞争，以及隧穿电子与吸附分子的旋转和振动模式的耦合强度之间的竞争。最后，我们探讨了非弹性电子隧穿的“偶极子”和“共振”机制，以阐明隧穿电子和化学吸附 O2 之间的能量转移。
