Molecular Orbital Tomography: Organic Interfaces and Surface Reaktions

分子的性质受最高占有率和最低未占有率分子轨道的影响。因此，对这些轨道的空间分布进行成像在科学上是非常有趣的。最近建立了角分辨光电子能谱(ARPES)来实验获取这种空间轨道信息。与高次谐波产生、电子动量谱或扫描隧道显微镜等其他技术相比，ARPES的这种分子轨道层析成像具有基本的优势，例如，数据采集速度更快。ARPES的理论描述一般是复杂的。然而，使用终态的平面波近似，可以将其简化为一种简单的形式，这使得与角度相关的ARPES强度分布成为相应分子轨道的傅里叶变换。这种方法可以相当直接地将分子轨道与ARPES测量联系起来，从而解释实验数据。在DFG自2014年以来资助的SCHO 1260/4-1项目中，我们在过去两年半中非常成功地开展了工作，所取得的部分成果已在六份出版物上发表。这些结果和专题的特殊及时性为这一延续提议提供了若干联系，其中打算进行以下研究：(I)方法学方面：平面波近似的应用限制：轨道层析成像的限制与平面波对最终状态的近似直接相关。有迹象表明，在出射电子的某些能量下，偏离了这个平面波的终态。在这个子项目中，光子能量和角度相关的数据将被记录在一个很大的动量和能量区，以区分和量化前向散射、后向散射或对邻近原子的衍射可能产生的贡献。轨道层析成像对非平面系统的适用性将在模型系统C60上进一步测试。由于这种分子的球对称性，在这种情况下，散射效应的影响是明显的。(Ii)轨道层析成像在特定系统中的应用：高能量分辨率的轨道成像：在振动激发的情况下，分子主干的运动导致电子波函数的几何分布的变化。为了在真实空间成像这些扭曲的轨道并重建相，我们计划对弱耦合、低对称的分子进行实验。分子-分子-界面的杂化及其对轨道的影响：通过轨道层析成像，将研究分子-分子-接触处可能出现的新杂化状态，这将对理解有机半导体材料中的掺杂起到至关重要的作用。

The properties of molecules are crucially influenced by the highest occupied and lowest unoccupied molecular orbitals. Imaging of the spatial distribution of these orbitals is thus scientifically highly interesting. Recently angle resolved photoelectron spectroscopy (ARPES) has been established to experimentally access this spatial orbital information. Against other techniques like higher harmonics generation, electron-momentum spectroscopy or scanning tunneling microscopy this molecular orbital tomography by ARPES has essential advantages such as, e.g., a faster data acquisition. The theoretical description of ARPES is generally complicated. However, using a plane-wave approximation for the final state it can be brought into a simple form, which renders the angle dependent ARPES intensity distribution as a Fourier transform of the respective molecular orbital. This approach allows rather directly connecting the molecular orbital with the ARPES measurement and thus interpreting the experimental data. Within the project SCHO 1260/4-1, funded by the DFG since 2014, we have been able to work very successfully in the last 2½ years and parts of the achieved results have been published already in six publications. These results and the particular timeliness of the topic provide several links for this continuation proposal, in which the following investigations are intended:(i) Methodological Aspects: Limitations for the Application of the Plane-Wave-Approximation:The limitations of -orbital tomography are immediately connected to the approximation of the final state by a plane wave. There are indications for deviations from this plane wave final state at certain energies of the outgoing electrons. In this subproject photon energy- and angle-dependent data will be recorded over a large momentum and energy region to distinguish and quantify possible contributions from forward scattering, back scattering, or diffraction on neighboring atoms. The applicability of orbital tomography to non-planar systems will furthermore be tested on the model system C60. Due to the spherical symmetry of this molecule a pronounced influence of scattering effects is expected in this case.(ii) Application of Orbital Tomography to Particular Systems:Imaging of orbitals with high energy resolution: In case of excitation of vibrations the motion of the molecular backbone leads to a change of the geometric distribution of the electronic wave function. To image these distorted orbitals in real space and reconstruction of the phase, experiments on weakly coupled, low-symmetric molecules are planned.Hybridization at molecule-molecule-interfaces and its influence on orbitals: By means of orbital tomography possible new hybrid states at molecule-molecule-contacts will be investigated, which will crucially contribute to the understanding of doping in organic semiconductor materials.