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

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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.