The electronic structure of Li 2 RuO 3 was investigated using x-ray emission and absorption spectroscopy and by theoretical calculations employing two approaches: the local density approximation (LDA) and a combination of LDA with the cluster extension of dynamical mean-field theory (LDA+DMFT). The evolution of the spectral properties with the strength of electronic correlations is analyzed. We show that for moderate values of on-site Coulomb repulsion U and intra-atomic Hund's rule exchange J H , Li 2 RuO 3 is in an orbital-selective strongly correlated state in the sense that a part of the t 2g manifold (i.e., xz/yz) behaves as local atomic orbitals susceptible to Hubbard correlations, while the remaining (xy) orbitals must be described as bond-centered molecular orbitals. Both theoretical approaches succeed in explaining the x-ray data, and a comparison of the theoretical and experimental spectra provides a reasonable estimate of the possible correlation strength (U ) and Hund's coupling (J H ) in Li 2 RuO 3 .

The electronic structure of [6,6]-phenyl C61 butyric acid methyl ester (PCBM), poly(3-hexylthiophene) (P3HT), and P3HT/PCBM blends is studied using soft X-ray emission and absorption spectroscopy and density functional theory calculations. We find that annealing reduces the HOMO-LUMO gap of P3HT and P3HT/PCBM blends, whereas annealing has little effect on the HOMO-LUMO gap of PCBM. We propose a model connecting torsional disorder in a P3HT polymer to the HOMO-LUMO gap, which suggests that annealing helps to decrease the torsional disorder in the P3HT polymers. Our model is used to predict the characteristic length scales of the flat P3TH polymer segments in P3HT and P3HT/PCBM blends before and after annealing. Our approach may prove useful in characterizing organic photovoltaic devices in situ or even in operando.

We study anthracene and phenanthrene doped with potassium using X-ray absorption spectroscopy and electronic structure calculations. In addition, a comparison of molecular orbital calculations and solid state density functional theory calculations are presented. We find that potassium-doping partially populates the LUMO level of anthrancene and phenanthrene and that both the measured and calculated electronic structures of the doped systems are quite different from that of the pristine molecular systems. This suggests that the extra charge carriers in the doped system are responsible for the increased conductivity and greater intermolecular interaction. Finally, our calculations suggest that both K 3 :phenanthrene and K 3 :anthracene have reduced or nonexistent band gaps as compared to their pure counterparts, further supporting the conclusion that doping is responsible for increased conductivity.