Puschnig P, Boese A D, Willenbockel M, Meyer M, Lüftner D, Reinisch E M, Ules T, Koller G, Soubatch S, Ramsey M G, Tautz F S
Institute of Physics, University of Graz, NAWI-Graz, Universitätsplatz 5, 8010 Graz, Austria.
Institute of Chemistry, University of Graz, NAWI Graz, Heinrichstraße 28/IV, 8010 Graz, Austria.
J Phys Chem Lett. 2017 Jan 5;8(1):208-213. doi: 10.1021/acs.jpclett.6b02517. Epub 2016 Dec 19.
Orbitals are invaluable in providing a model of bonding in molecules or between molecules and surfaces. Most present-day methods in computational chemistry begin by calculating the molecular orbitals of the system. To what extent have these mathematical objects analogues in the real world? To shed light on this intriguing question, we employ a photoemission tomography study on monolayers of 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) grown on three Ag surfaces. The characteristic photoelectron angular distribution enables us to assign individual molecular orbitals to the emission features. When comparing the resulting energy positions to density functional calculations, we observe deviations in the energy ordering. By performing complete active space calculations (CASSCF), we can explain the experimentally observed orbital ordering, suggesting the importance of static electron correlation beyond a (semi)local approximation. On the other hand, our results also show reality and robustness of the orbital concept, thereby making molecular orbitals accessible to experimental observations.
轨道对于构建分子内或分子与表面间的键合模型非常重要。当今计算化学中的大多数方法都是从计算体系的分子轨道开始的。这些数学对象在现实世界中有多大程度的类似物呢？为了阐明这个有趣的问题，我们对生长在三种银表面上的3,4,9,10-苝四羧酸二酐（PTCDA）单层进行了光电子能谱断层扫描研究。特征光电子角分布使我们能够将各个分子轨道与发射特征对应起来。当将所得的能量位置与密度泛函计算结果进行比较时，我们观察到能量排序存在偏差。通过进行完全活性空间计算（CASSCF），我们可以解释实验观察到的轨道排序，这表明了超越（半）局域近似的静态电子相关的重要性。另一方面，我们的结果也表明了轨道概念的真实性和稳健性，从而使分子轨道能够被实验观测到。
