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

在本项目中，我们的目标是全面了解分子团簇/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。重点放在无参数测定所研究的分子化合物的光谱性质上。对孤立分子和周期性分子晶体进行线性和非线性光谱、红外和拉曼光谱以及EELS特征建模。结构无序对光学性质的影响将在C1项目中根据第一性原理计算的基态几何图形的帮助下进行建模。我们在不同的精度水平上进行计算，从独立粒子近似到更精细的计算方案，包括准粒子效应和电子-空穴相互作用。在第一个项目阶段，我们扩展了我们的计算方案，以便计算时域内的非线性光学磁化率，这使得即使对于扩展系统，包括分子晶体，也能以足够的精度有效地计算光学响应。一方面，我们处理了众所周知的模型体系，如典型金刚烷形化合物[(RT)4R6] （R =有机基团；T = C, Si, Ge, Sn; E = O， S, Se, Te， NH, CH2, On•）。另一方面，我们研究了通式[(RT)4MxEz]的立方形化合物，M =过渡金属，以及它们在项目区域a中实现的化学变化，探讨了光学非线性的先决条件，如有序/无序、对称性和化学组成的基本问题。从C1项目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A和B区域的测量结果，并激发了具有定制光学特性的新化合物的合成。因此，我们的原子模型提供了两个关键的优势。第一个优点是光学响应是作为入射波长的函数来计算的。全光谱的知识将有助于建立激发波长与光学非线性之间的相关性，并确定是否可以采用除红外辐射以外的其他光源来驱动材料。我们的模型的第二个巨大优势是可以计算实验无法获得的亚稳结构的光学响应，例如亚稳团簇或非晶化合物的有序结构。这允许解开同时决定光学响应的不同效应，并解决有关单个方面的物理问题。

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.