Exploring the Foundations of Photoemission Tomography

确定价轨道特性的实验方法从飞秒激光光谱学到超冷温度下的扫描探针技术。虽然这些方法已经引起了广泛的兴趣，但存在一些限制，例如，只能在限制条件下研究相当简单的分子，例如超冷温度以防止分子扩散。相反，这里提出的实验方法，即角度分辨光电子能谱，也可以应用在技术相关的温度和大范围的分子/基板组合。为此，用UV光照射其上已经在超高真空下沉积有机分子的样品。然后分析光发射电子的能量和角分布。该方法提供了获得三维分子轨道图像的可能性，因此也称为光电发射断层扫描。然而，对实验数据的解释并不直接。具体来说，必须对电子从其初始束缚态转移到的量子力学终态做出某些假设。最简单的解释是在这里使用自由电子态，即平面波。它提供了一个特别简单的方式来解释实验数据的优势，这使得可以确定分子的几何形状，测量电子动量分布和重建轨道图像，该项目的目的是探索在哪些实验条件下，这些简化的假设导致可靠的结果。我们的团队由来自格拉兹大学和于利希研究中心的表面科学家以及德国联邦物理技术研究所的计量学特征紫外同步辐射生成专家组成，他们将进行一系列实验，以追踪平面波近似的有效范围。为了解释实验结果并从理论上预测最终状态与这种自由电子状态的差异程度，项目团队还包括来自格拉兹大学的分子和分子界面电子结构量子力学从头计算领域的专家。对技术相关分子轨道成像的可能性肯定会拓宽我们对量子力学电子轨道概念的基本理解。它将允许物理和化学过程以及有机分子和无机表面之间的界面的详细调查。可能的技术应用包括催化表面、传感器、新分子和纳米结构的定制，以用于能量收集（如光催化）或能量储存，或未知分子种类的识别和表征。

Experimental methods to determine the characteristics of valence orbitals are ranging from femto-second laser spectroscopy to scanning probe techniques at ultra-cold temperatures. Although these approaches have attracted broad interest, there are several limitations, for instance, only rather simple molecules under restricted conditions, such as ultra-cold temperatures to prevent molecular diffusion, can be investigated. In contrast, the experimental approach proposed here, i.e. angle-resolved photoemission spectroscopy, can also be applied at technological relevant temperatures and for a large range of molecule/substrate combinations. To this end, a sample onto which the organic molecules have been deposited under ultra high vacuum is illuminated with UV light. The photoemitted electrons are then analyzed in terms of their energy and angular distribution. The method offers the possibility to obtain images of molecular orbitals in three dimensions and is, hence, also termed photoemission tomography. However, the interpretation of the experimental data is not straight forward. Specifically, certain assumptions have to be made about the quantum mechanical final state into which the electron is transferred from its initial bound state. The most simple ansatz is to use a free electron state here, i.e. a plane wave. It offers the advantage of interpreting the experimental data in a particularly simple manner, which allows to determine molecular geometries, to measure electron momentum distributions and to reconstruct orbital images.The aim of this project is to explore under which experimental conditions these simplifying assumptions lead to reliable results. Our team, consisting of surface scientists from the University of Graz and the Forschungszentrum Jülich and experts in the generation of metrologically characterized UV synchrotron radiation from the Physikalisch-Technische Bundesanstalt, will conduct a series of experiments to trace out the range of validity of the plane wave approximation. In order to interpret the experimental results and to theoretically predict to which extent the final state differs from such a free-electron state, the project team also comprises experts from the University of Graz in the field of quantum mechanical ab-initio calculations for the electronic structure of molecules and molecular interfaces. The possibility to image orbitals of technological relevant molecules will certainly widen our fundamental understanding of the concept of quantum mechanical electron orbitals. It will allow for the detailed investigation of physical and chemical processes and the interface between organic molecules and inorganic surfaces. Possible technological applications include the tailoring of catalytic surfaces, sensors, novel molecules and nano-structures to be used for energy harvesting (e.g. photovoltaics) or energy storage, or the identification and characterization of yet unknown molecular species.