Institute for Physical Chemistry, Georg-August University of Göttingen, 37077 Göttingen, Germany.
Department of Dynamics at Surfaces, Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany.
Proc Natl Acad Sci U S A. 2018 Jan 23;115(4):680-684. doi: 10.1073/pnas.1710587115. Epub 2018 Jan 8.
The Born-Oppenheimer approximation (BOA) provides the foundation for virtually all computational studies of chemical binding and reactivity, and it is the justification for the widely used "balls and springs" picture of molecules. The BOA assumes that nuclei effectively stand still on the timescale of electronic motion, due to their large masses relative to electrons. This implies electrons never change their energy quantum state. When molecules react, atoms must move, meaning that electrons may become excited in violation of the BOA. Such electronic excitation is clearly seen for: () Schottky diodes where H adsorption at Ag surfaces produces electrical "chemicurrent;" () Au-based metal-insulator-metal (MIM) devices, where chemicurrents arise from H-H surface recombination; and () Inelastic energy transfer, where H collisions with Au surfaces show H-atom translation excites the metal's electrons. As part of this work, we report isotopically selective hydrogen/deuterium (H/D) translational inelasticity measurements in collisions with Ag and Au. Together, these experiments provide an opportunity to test new theories that simultaneously describe both nuclear and electronic motion, a standing challenge to the field. Here, we show results of a recently developed first-principles theory that quantitatively explains both inelastic scattering experiments that probe nuclear motion and chemicurrent experiments that probe electronic excitation. The theory explains the magnitude of chemicurrents on Ag Schottky diodes and resolves an apparent paradox--chemicurrents exhibit a much larger isotope effect than does H/D inelastic scattering. It also explains why, unlike Ag-based Schottky diodes, Au-based MIM devices are insensitive to H adsorption.
玻恩-奥本海默近似（BOA）为几乎所有化学结合和反应性的计算研究提供了基础，它也是广泛使用的“球和弹簧”分子图像的依据。BOA 假设原子核在电子运动的时间尺度上有效地保持静止，这是由于它们的质量相对于电子很大。这意味着电子永远不会改变它们的能量量子态。当分子反应时，原子必须移动，这意味着电子可能会在违反 BOA 的情况下被激发。这种电子激发在以下方面显而易见：（）银表面上 H 的吸附产生电“化学电流”的肖特基二极管；（）基于 Au 的金属-绝缘体-金属（MIM）器件，其中化学电流来自 H-H 表面复合；（）非弹性能量转移，其中 H 与 Au 表面的碰撞表明 H-原子的平移激发了金属的电子。作为这项工作的一部分，我们报告了与 Ag 和 Au 碰撞的同位素选择性氢/氘（H/D）平移非弹性测量结果。这些实验共同为同时描述核和电子运动的新理论提供了机会，这是该领域的一个挑战。在这里，我们展示了最近开发的第一性原理理论的结果，该理论定量解释了探测核运动的非弹性散射实验和探测电子激发的化学电流实验。该理论解释了 Ag 肖特基二极管上化学电流的大小，并解决了一个明显的悖论——化学电流的同位素效应比 H/D 非弹性散射大得多。它还解释了为什么与基于 Ag 的肖特基二极管不同，基于 Au 的 MIM 器件对 H 吸附不敏感。
