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We report on the valence orbital structure of poly(para-phenylenevinylene) (PPV)-like oligomers. We studied these molecules as isolated oligomers in the gas phase, as well as in thin films deposited on metal substrates. We use a simple model based on a previously reported Hamiltonian that accurately describes the development of the low lying electronic excitations as a function of the number of repeating units. In the study on the thin organic films we report on the energy level alignment at metal / organic and organic / organic interfaces, where the organic layer is either a PPV-like oligomer or C-60. The results are important for understanding organic photovoltaic devices.

We present a detailed investigation of the surface electronic structure of solid K;CM, and Rb3C60 in connection with their electronic transport properties. The best conducting samples with the highest superconducting transition temperature T, have the highest density of states at the Fermi level as measured by photoemission, but the valence of the CM ions at the surface deviates appreciable from the 3- bulk value, namely 2.5- or even 1.5- depending on the preparation procedure. Despite the large discrepancy in the electronic structure, we find, using ultra-high resolution photoemission, that these surfaces have a BCS-like superconducting gap A which is not reduced from the bulk value, i.e. close to the weak coupling 2Delta/k(B)T(i) = 3.53 value.

We study the screening of the Coulomb interaction in C-60 and carbon nanotubes. It is shown that for these systems the screening deviates strongly from the Clausius-Mossotti behavior. The short range interaction is strongly screened and the long range interaction is anti-screened, thereby strongly reducing the gradient of the Coulomb interaction. For the nanotubes a cross-over from quasi one dimensional to two dimensional screening is found with increasing radius of the nanotube. The unconventional screening explains the moderate effect of electron-electron correlations on the electronic structure of these molecules.

Large photovoltaic responses have been recently observed in devices based on conjugated polymer-C-60 blends. Their enhanced performance, which relies on the formation of a bicontinuous network of donor-acceptor heterojunctions, is very sensitive to the morphology of the blend. In this paper, we propose sexithiophene-C-60 blends, prepared by coevaporation, as a model system for the study of these devices. Preliminary results suggest a microphase-separated morphology with a domain size comparable to the exciton diffusion length. A photovoltaic device with quantum efficiency of 1% is demonstrated.

We present and discuss resonant photoemission (PES) spectra of Ti4O7 taken at 50 and 300 K. The PES spectra taken in the metallic phase at 300 K show Ti 3d bands at the Fermi level and O 2p bands at higher binding energies. The intensity of the Ti 3d bands increases for photon energies above 40 eV. This effect is attributed to a constructive interference with the Ti 3p --> 3d resonance which is confirmed by the corresponding constant-initial-state (CIS) spectra. The Ti 3d bands shift approximately 0.25 eV towards higher binding energies in the low temperature semiconducting phase at 50 K. This shift reflects the opening of the band gap and provides a rough estimate of the electronic contribution to the formation energy of the Ti3+-Ti3+ pairs in the low-temperature phase.

We present band-structure and cluster-model calCulatiOns Of LaCoO3 in the low-spin phase. The purpose of these calculations is to contrast and complement the results and conclusions of recent spectroscopic studies. The total density of states (DOS) is compared to the photoemission spectrum; the agreement is very good except for the many-body satellites which appear at higher binding energies. The unoccupied O p DOS reproduces fairly well the O 1s x-ray-absorption spectrum; the main discrepancy appears in the Co 3d region and is attributed to core-hole effects. The ground state predicted by the cluster-model calculation is highly covalent and contains mainly 62% of t2g6 (1A1) and 36% of t2g6e(g)(2E)L. The first (one-electron) removal state has more 3d6L than 3d5 character whereas the first addition state is almost completely dominated by the 3d7 state. This means that low-spin LaCoO3 is in the charge-transfer regime and the optical band pp is of the p-d type. The Co 3d contribution to the photoemission spectrum calculated with the cluster-model reproduces not only the leading peaks but also the many-body satellites. The main drawback in this case is the absence of the spectral weight coming from the O 2p bands.

We present soft-x-ray absorption spectra (XAS) of LaCoO3 taken at different temperatures (80-630 K). The shape of the multiplets in the Co 2p XAS spectra conveys information on the symmetry and spin of the ground state. The 0 Is XAS spectra are related to unoccupied metal bands through covalent mixing. The changes in the spectra taken at different temperatures provide information on the spin-state transition in this compound. At low temperature, 80 and 300 K, the material is in a highly covalent low-spin state. The main contribution to the ground state in this case is given by t2g6 (1A1) with an occupancy of 0.56. At higher temperature, 550 and 630 K, the results indicate a gradual transition to a mixed-spin state. The main contribution to the high-spin part of the mixture is given by t2g4e(g)2(5T2) with an occupancy of 0.71. There is no evidence of charge disproportionation in the temperature range 80-630 K. The O 1s XAS spectra reflect important changes in the unoccupied Co 3d bands across the spin-state transition.

We present an x-ray absorption study of the oxidation states of transition-metal-ions of LiMnO2 and its related materials, widely used as cathodes in Li-ion batteries. The comparison between the obtained spectrum and the configuration-interaction cluster-model calculations showed that the Mn3+ in LiMnO2 is a mixture of the high-spin and low-spin states. We found that Li deficiencies occur in the case of Cr substitution, whereas there are no Li deficiencies in the case of Ni substitution. We conclude that the substitution of charge-transfer-type Ni or Cu is effective for LiMnO2 battery materials., 3 pages, 3 figures

LiCoO2 can be viewed as the end member of the Li-doped LixCo1-xO2 system. It was suggested that this system exhibits a transition from high-spin Co ions in CoO to low-spin Co ions in LiCoO2. We present a systematic study of the electronic properties of LiCoO2 based on a density-functional calculation. From a good agreement between the theoretical results and the results of various spectroscopies (x-ray photoemission spectroscopy, bremsstrahlung isochromat spectroscopy, and x-ray absorption spectroscopy) we conclude that a one-particle band-structure approach is basically adequate for LiCoO2 while being still controversial for CoO) and we support the conclusion of low-spin Co ions in this compound.

The controlled-valence properties of La1-xSrxFeO3 and La1-xSrxMnO3 are studied by means of soft-x-ray absorption spectroscopy. A comparison between the transition-metal 2p spectra and atomic-multiplet calculations is used to determine the 3d count. The 0 Is spectrum is used to characterize changes in unoccupied states that contain oxygen p character. The results indicate that the holes induced by substitution for both series are located in states of mixed metal 3d -oxygen 2p character. The ground state of LaFeO3 is mainly 3d5 and becomes 3d5L (where L denotes a ligand hole) in the La1-xSrxFeO3 series for low Sr concentration. The main component of the ground state of LaMnO3 is 3d4 and becomes a mixture of 3d3 and 3d4L in the La1-xSrxMnO3 series. The trends in controlled-valence properties of similar oxides across the transition-metal series can be rationalized within the framework of the Zaanen-Sawatzky-Allen model.

We discuss the electronic state of Ni atoms in Ni metal and of Ni impurity in Cu and Au metals from the viewpoint of 3d configuration interaction (CI) using the Anderson impurity model including atomic multiplets. On the basis of the discussion, we give an interpretation for the Ni 2p-core X-ray photoemission spectrum (2p XPS) in Ni metal and Ni impurity systems and 3d photoemission spectra in Ni metal. The interpretation is, furthermore, shown to be consistent with the previous one for the 2p --> 3d circularly polarized X-ray absorption in ferromagnetic Ni using the CI approach.

The electronic structure of La2-xSrxNiO4 is studied by use of photoemission spectroscopy, bremsstrahlung-isochromat spectroscopy (BIS), and electron-energy-loss spectroscopy. Quantitative analyses are made on the valence-band and Ni 2p core-level photoemission spectra through configuration-interaction calculations on a NiO6 cluster model. On the basis of these analyses, it is concluded that La2NiO4 is a charge-transfer (CT) insulator and the magnitude of the band gap is about 4 eV, nearly the same as that of NiO. The BIS spectra show that unoccupied states induced by hole doping are spread over the CT gap, which is incompatible with a rigid-band picture for the hole doping. We discuss the origin of the different insulator-to-metal transition behavior between this system and La2-xSrxCuO4.

The electronic state of ferromagnetic Ni and an interpretation of magnetic circular dichroism (MCD) in Ni 2p --> 3d and 3p --> 3d X-ray absorption (XAS) are discussed from a viewpoint of 3d configuration interaction. Inclusion of the 3d8 configuration and an orbital-magnetic-moment contribution of 0.07-mu(B) to the total moment of 0.6-mu(B) in the ground state are shown to explain the observed MCD.

Experimental data and theoretical calculations are shown for Pr 3d and 4d x-ray photoemission spectra (XPS) of Pr2O3. The observed spectral structures are well explained by the theoretical results that are obtained with the impurity Anderson model by combining intra-atomic multiplet coupling with solid-state hybridization between the Pr 4f and O 2p states. For 3d XPS the main spectral structures are caused by the hybridization effect, while for 4d XPS the interplay between the multiplet coupling and the hybridization is essential.

Oxygen 1s x-ray-absorption measurements of La(2-x)SrxNiO4+delta (0 less-than-or-equal-to x less-than-or-equal-to 1.15) are presented, together with O 1s polarization-dependent x-ray absorption on single-crystal La1.85Sr0.15NiO4. It is concluded that the charge-compensating holes have mainly oxygen 2p character. The Ni 3d9 final states reached because of the p-d hybridization in the ground state are strongly polarized in the (a,b) plane. The Sr-induced O 2p holes are found to be distributed equally over in-plane p(x),p(y) and out-of-plane p(z) orbitals. Possible explanations for this are presented. The intrinsic band gap of La2NiO4 is found to be at least 4 eV. The bremsstrahlung-isochromat-spectroscopy results show that the Sr-induced states are spread out over a large portion of the band gap and are not concentrated at the top of the valence band.

We present a calculation of magnetic x-ray dichroism at the L2,3 edges of Gd metal, which takes into account dipolar transitions to the spin-polarized conduction bands, as well as quadrupole transitions to the highly localized 4f states. The striking agreement with the data demonstrates the validity of the single-particle approximation in the whole range of energies, above the Fermi level, probed by the experiments. The quadrupolar dischroism is important in determining the pre-edge structure of the measured spectra. Results for the L2,3 edges of Ho are also briefly discussed.

The electronic state of ferromagnetic Ni and the expected magnetic circular dichroism (MCD) in Ni 2p --> 3d x-ray-absorption spectroscopy [2p (XAS)] are discussed from a viewpoint of 3d configuration interaction on the basis of the Anderson impurity model. The model includes multiplet effects arising from 3d-3d and 3d-2p electron interactions and a 3d interatomic exchange approximated by a molecular field acting on the 3d spin. By using commonly accepted values of the interaction strengths, it is shown that the ground state of a Ni atom in ferromagnetic Ni is expressed by a superposition of 3d10, 3d9, and 3d8 configurations with relative weights of 15%-20%, 60%-70%, and 15%-20%, respectively, and with an orbital-magnetic-moment contribution of approximately 0.07-mu-B to the total moment of 0.6-mu-B. With these results, we can explain the recently observed MCD in Ni 2p XAS.

The fine structure of the Ca 2p soft-x-ray-absorption edge is studied for a variety of bulk compounds (Ca metal, CaSi2, CaO, and CaF2), for surfaces and interfaces [CaF2(111), BaF2 on CaF2(111), Ca and CaF2 on Si(111)], and for defects (F centers in CaF2). The observed multiplet structure is explained by atomic calculations in a crystal field [cubic O(h) for the bulk and threefold C3-nu for the (111) surfaces and interfaces]. While the bulk spectra are isotropic, the surface and interface spectra exhibit a pronounced polarization dependence, which is borne out by the calculations. This effect can be used to become surface and/or interface selective via polarization-modulation experiments, even for buried interfaces. A change in valence from Ca2+ to Ca1+ causes a downwards energy shift and extra multiplet lines according to the calculation. The energy shift is observed for F centers at the CaF2 surface and for the CaF2/Si(111) interface.

The electronic structure of a nominally trivalent Cu compound, NaCuO2, is discussed through the detailed analysis of the multiplet structures of the Cu 2p-XAS (X-ray absorption spectroscopy) and XPS (X-ray photoemission spectroscopy) spectra. It is shown that the ground state is a local singlet state 1A1 as in low spin Cu3+ compounds but the charge is to a large extent on the oxygen in O 2p orbitals.

We present atomic calculations for the recently discovered magnetic x-ray dichroism (MXD) displayed by the 3d x-ray-absorption spectra of rare-earth compounds. The spectral shapes expected at T=0 K for linear polarization parallel and normal to the local magnetic field is given, together with the temperature and field dependencies of the intensities of the spectral lines. It is concluded that strong MXD effects can be expected for all rare-earth ions except those having a ground state with either J=0 or L=0. Calculations of magnetic x-ray dichroism in the 3d absorption spectra of rare-earth compounds. - ResearchGate. Available from: http://www.researchgate.net/publication/13344010_Calculations_of_magnetic_x-ray_dichroism_in_the_3d_absorption_spectra_of_rare-earth_compounds [accessed Jun 2, 2015].