ELECTRONIC-STRUCTURE OF ALPHA(T)(BEDT-TTF)(2)I-3 - A PHOTOEMISSION-STUDY

Photoemission experiments have been performed on in situ cleaved cryst als of the organic superconductor alpha(t)-(BEDT-TTF)(2)I-3. Nine dist inct features are observed in the valence-band regime with binding ene rgies between 1.4 and 11.2 eV. None of the valence-band structures sho ws dispersion along the Gamma-Z (k(z)) direction, i.e., perpendicular to the conducting BEDT-TTF layers. The spectral intensity is zero at, and to within about 0.5 eV below the Fermi level. This is ascribed to the presence of a fairly large band gap in this direction, in agreemen t with band-structure calculations. The photoemission spectrum of the valence band shows remarkable changes when the photon energy is change d. These changes are qualitatively understood as a cross-section depen dence. The observed photon energy dependence suggests that both p orbi tals and the lower-lying s orbitals of the atoms constituting the BEDT -TTF molecule contribute significantly to the molecular orbitals formi ng the valence band. Core-level spectroscopy on the I 4d and S 2p leve ls revealed no surface or chemically shifted components. The large wid th of these core levels is ascribed primarily to phonon broadening, al though a contribution from disorder cannot be completely ruled out.