Spectroscopic Signatures of Molecular Clusters and Molecular Cluster Aggregates Calculated from First Principles

在本项目中，我们的目标是深入了解分子团簇/团簇聚集体的原子结构、电子构型和化学变化之间的相互作用及其光学响应。重点放在所研究的分子化合物的光谱性质的无参数测定上。对于孤立的分子和周期性的分子晶体，应模拟线性和非线性光学光谱、红外光谱和拉曼光谱以及EELS特征。结构无序对光学性质的影响将借助于根据第一原理计算的基态几何来模拟。我们在不同的精度水平下进行计算，从独立粒子近似到包括准粒子效应和电子-空穴相互作用在内的更精细的计算方案。在第一个项目阶段，我们扩展了我们的计算方案，以便在时间域内计算非线性光学极化率，这使得即使对于扩展的系统，包括分子晶体，也能够以足够的精度有效地计算光学响应。一方面，我们讨论了一些典型的金刚烷型化合物[(RT)4R6](其中R=有机基团；T=C，Si，Ge，Sn；E=O，S，Se，Te，NH，CH2，On·)等著名的模型体系。另一方面，我们讨论了通式为[(RT)4MxEz]的立方烷型化合物，其中M=过渡金属，以及它们的化学变化，如在项目A中实现的光学非线性的基本问题，如有序/无序，对称性和化学组成等。从项目C1中确定的结构数据开始，我们的项目提供了所研究系统的光学响应，从而帮助解释了A区和B区的测量结果，并启发了具有定制光学性质的新化合物的合成。因此，我们的原子论模型提供了两个至关重要的优势。第一个优点是光学响应是作为入射波长的函数来计算的。对全光谱的了解将有助于建立激发波长和光学非线性之间的关系，并确定是否可以使用红外辐射以外的其他光源来驱动材料。我们模型的第二大优点是有可能计算实验上无法获得的亚稳态结构的光学响应，例如亚稳态团簇或非晶态化合物的有序结构。这允许解开同时作用于确定光学响应的不同影响，并解决与单个方面有关的物理问题。

In the present project, we aim at gaining a thorough understanding of the interplay between atomic structure, electronic configuration and chemical variations of molecular clusters/cluster aggregates and their optical response. The focus is set on the parameter-free determination of the spectroscopic properties of the investigated molecular compounds. Linear and nonlinear optical spectra, IR and Raman spectra and EELS signatures shall be modeled for isolated molecules and periodic molecular crystals. The effect of structural disorder on the optical properties will be modeled with the help of the ground state geometries calculated from first principles within the project C1. We perform calculations at different levels of accuracy, ranging from the independent particle approximation to more refined calculation schemes including quasiparticle effects and the electron-hole interaction. In the first project phase, we have extended our calculation schemes in order to calculate the nonlinear optical susceptibilities within the time-domain, which allow for an efficient calculation of the optical response with adequate accuracy even for extended systems, including molecular crystals. On the one hand, we deal with well-known model systems such as the prototypical adamantane shaped compounds [(RT)4R6] (with R = organic group; T = C, Si, Ge, Sn; E = O, S, Se, Te, NH, CH2, ON•). On the other hand, we address cubane shaped compounds of general formula [(RT)4MxEz], with M = transition metal, and their chemical variations as realized in project area A. Fundamental questions regarding the prerequisites for the optical nonlinearities such as order/disorder, symmetry and chemical composition are explored. Starting with structural data determined in project C1, our project provides the optical response of the investigated systems, thus helping the interpretation of the measurements in areas A and B and inspiring the synthetization of new compounds with tailored optical properties. Thereby our atomistic models offer two crucial advantages. The first advantage is that the optical response is calculated as a function of the incident wavelength. The knowledge of the full optical spectrum will help to establish the correlation between excitation wavelength and optical nonlinearities, and ascertain whether other light sources than IR radiation can be employed to drive the material. A second great advantage of our models is the possibility to calculate the optical response of metastable structures that are not experimentally accessible, such as metastable clusters or ordered structures of amorphous compounds. This allows to disentangle the different effects that simultaneously act to determine the optical response, and address the physical questions concerning the single aspects.