First Principles Simulations of Soft X-ray Spectroscopy of Molecular Charge and Energy Transfer Processes

本项目的总体目标是开发和数值实现描述分子能量和电荷转移的软x射线光谱的理论方法。在用作光催化剂或有机光伏发电的新型太阳能材料中，涉及价态激发的能量和电荷转移是普遍存在的过程。在这里，未来的性能改进将受益于分子尺度上的深入机制理解。此外，利用非平凡量子效应作为工作原理，可以克服经典物理的性能限制。利用光学检测对这些过程进行共同表征，往往面临着光谱拥挤的挑战，这很难明确地分配。相比之下，x射线光谱学是元素特异性的，因此分子复合物的单个原子可以通过它们的吸收边来定位。在这个快速发展的领域，大型设施和桌面软x射线源越来越多，需要足够的理论发展。虽然用光泵和x射线探针技术研究含单一过渡金属中心的化合物已成为当今的标准，但对具有多个金属中心的体系的相应研究仍然很少。特别是第一原理理论在涉及重要的l边时面临其局限性，因为它们的光谱是由多构型和自旋轨道耦合效应形成的。该项目将采用一种新的策略来应对这种情况。利用x射线吸收的局部性质，用相互作用的碎片来描述多色络合物。这个想法是借鉴了Frenkel激子理论，这是建立在价态激发的背景下。应用于多色络合物的价态和核心激发态，将为描述光触发价态动力学的x射线光谱提供一个统一的框架。我们将考虑两种类型的申请。首先，在原理验证研究中，将考虑双金属化合物，从而扩大原始建议的范围。其次，将结合实验研究有机光伏模型系统中的能量传递和界面电荷分离。本文将特别强调激子波包的相干性和离域特征的揭示，这将给该领域正在进行的争论带来新的变化。

The overarching goal of this project is the development and numerical implementation of theoretical methods for the description of the soft X-ray spectroscopy of molecular energy and charge transfer. Energy and charge transfer involving valence excitations are ubiquitous processes in novel solar energy materials used as photocatalysts or in organic photovoltaics. Here, future performance improvements will benefit from an in-depth mechanistic understanding on the molecular scale. Moreover, performance limitations due to classical Physics might be overcome by making use of nontrivial quantum effects as a working principle. Common characterization of these processes employing optical detection often faces the challenge of congested spectra, which are difficult to assign unambiguously. In contrast, X-ray spectroscopy is element specific such that individual atoms of a molecular complex can be addressed by means of their absorption edges. Large scale facilities but also table top soft X-ray sources are becoming increasingly available in this rapidly developing field, demanding for adequate theoretical developments. While the investigation of compounds containing a single transition metal center with, for instance, optical pump and X-ray probe techniques is becoming standard nowadays, respective studies of systems with multiple metal centers are still rare. In particular first principles theory faces its limits when it comes to the important L-edges since their spectra are shaped by multiconfigurational and spin-orbit coupling effects. This project will follow a new strategy to cope with this situation. Making use of the local nature of X-ray absorption, multichromophoric complexes will be described in terms of interacting fragments. The idea is borrowed from Frenkel exciton theory, which is well-established in the context of valence excitations. Application to valence and core excited states of multichromophoric complexes will provide a unifying frame for the description of the X-ray spectroscopy of optically triggered valence state dynamics. Two types of applications will be considered. First, in a proof-of-principle study bimetallic compounds will be considered, thus extending the scope of the original proposal. Second, energy transfer and interfacial charge separation in model systems for organic photovoltaics will be investigated in close cooperation with experiment. Particular emphasis will be paid to unravel the signatures of exciton wave packet coherence and delocalization, which shall give a new twist to an ongoing debate in this field.