We present an x-ray absorption spectroscopy study from the carbon $K$, sulfur $K$, and sulfur $L_{2,3}$ edges of crystalline oligothiophenes of varying length, i.e., bithiophene (2T), quaterthiophene (4T), and sexithiophene (6T), performed from first principles by means of all-electron density-functional theory and many-body perturbation theory. A comprehensive assignment of all relevant spectral features is performed based on the electronic structure and the character of the target conduction states. The inclusion of electron-hole effects leads to significant redistribution of oscillator strengths and to strongly bound excitons with binding energies ranging from 1.5 eV to 4.5 eV. When going from 2T to 6T, exciton binding energies decrease by up to 1 eV, which we attribute to the reduction of the average Coulomb attraction with increasing oligomer length. These high values are significantly larger than their counterparts in the optical excitations of these systems and indicative of their localization on the respective molecules. For the same reason, local-field effects which typically dominate the optical absorption of organic crystals turn out to play only a negligible role at all edges. We identify two sets of carbon atoms, i.e., with or without sulfur bonding, which exhibit distinct features at the C $K$ edge. The sulfur atoms, on the other hand, yield similar contributions in the S, $K$, and $L_{2,3}$ edge spectra. Our results show excellent agreement with available experimental data.

中文翻译：

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基于多体微扰理论的低聚噻吩晶体的 X 射线吸收光谱

我们提出了碳的 X 射线吸收光谱研究$K$, 硫磺$K$和硫磺$L_{2,3}$不同长度的结晶低聚噻吩的边缘，即联噻吩（2T）、四噻吩（4T）和六噻吩（6T），通过全电子密度泛函理论和多体微扰理论从第一原理进行。根据电子结构和目标传导态的特征对所有相关光谱特征进行综合分配。电子空穴效应的加入导致振荡器强度的显着重新分布，并产生结合能范围为 1.5 eV 至 4.5 eV 的强结合激子。当从 2T 到 6T 时，激子结合能降低高达 1 eV，我们将其归因于平均库仑吸引力随着低聚物长度的增加而降低。这些高值明显大于这些系统的光学激发中的对应值，表明它们在各自分子上的定位。出于同样的原因，通常主导有机晶体光学吸收的局域场效应在所有边缘仅发挥可忽略不计的作用。我们确定了两组碳原子，即有或没有硫键，它们在 C 处表现出不同的特征$K$边缘。另一方面，硫原子在 S 中产生类似的贡献，$K$， 和$L_{2,3}$边缘光谱。我们的结果与现有的实验数据非常吻合。