Exploring the Foundations of Photoemission Tomography

确定价轨道特征的实验方法从飞秒激光光谱到超低温扫描探针技术都有。尽管这些方法吸引了广泛的兴趣，但也有几个限制，例如，只能研究受限条件下的相当简单的分子，例如防止分子扩散的超低温温度。相反，这里提出的实验方法，即角度分辨光电子能谱，也可以应用于技术相关的温度和大范围的分子/底物组合。为此，将在超高真空下沉积有机分子的样品用紫外光照射。然后根据光电发射电子的能量和角分布对其进行分析。这种方法提供了获得分子轨道三维图像的可能性，因此也被称为光电子断层成像。然而，对实验数据的解释并不直接。具体地说，必须对电子从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。它的优点是以一种特别简单的方式解释实验数据，允许确定分子几何结构，测量电子动量分布和重建轨道图像。这个项目的目的是探索在哪些实验条件下，这些简化的假设会导致可靠的结果。我们的团队由格拉茨大学和德国Jülich大学的表面科学家以及Physikalisch-Technische Bundesanstalt产生的计量特征UV同步辐射专家组成，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不同程度，该项目团队还包括来自格拉茨大学的量子力学从头计算领域的专家，该领域涉及分子和分子界面的电子结构。成像技术相关分子轨道的可能性必将拓宽我们对量子力学电子轨道概念的基本理解。它将允许详细研究物理和化学过程以及有机分子和无机表面之间的界面。可能的技术应用包括定制催化剂表面、传感器、用于能量收集(例如光伏)或能量存储的新型分子和纳米结构，或识别和表征未知分子物种。

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.