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We present an x-ray absorption spectroscopy study from the carbon $K$, sulfur$K$, and sulfur $L_{2,3}$ edges of crystalline oligothiophenes of varyinglength, i.e. bithiophene (2T), quaterthiophene (4T), and sexithiophene (6T),performed from first principles by means of all-electron density-functionaltheory and many-body perturbation theory. A comprehensive assignment of allrelevant spectral features is performed based on the electronic structure andthe character of the target conduction states. The inclusion of electron-holeeffects leads to significant redistribution of oscillator strengths and tostrongly bound excitons with binding energies ranging from 1.5 to 4.5 eV. Whengoing from 2T to 6T, exciton binding energies decrease by up to 1 eV, which weattribute to the reduction of the average Coulomb attraction with increasingoligomer length. These high values are significantly larger than theircounterparts in the optical excitations of these systems and indicative oftheir localization on the respective molecules. For the same reason,local-field effects which typically dominate the optical absorption of organiccrystals, turn out to play only a negligible role at all edges. We identify twosets of carbon atoms, i.e. with or without sulfur bonding, which exhibitdistinct features at the C $K$-edge. The sulfur atoms, on the other hand, yieldsimilar contributions in the S, $K$, and $L_{2,3}$ edge spectra. Our resultsshow excellent agreement with available experimental data.

X-ray absorption spectroscopy of oligothiophene crystals from ab initio many-body theory