Electronic Structure of the Organic Conductorsκ-ET₂Cu(SCN)₂andκ-ET ₂Cu[N(CN)₂]Br Studied Using Soft X-ray Absorption and Soft X-ray Emission

The problem of resolving molecular components of the electronic structure of complex, organic solids with respect to their chemical and orbital character has been approached using core-level photon spectroscopies. Specifically, the bulk C 2poccupied and unoccupied partial densities of states (PDOS) of the organic superconductorsκ-ET₂Cu(SCN)₂andκ-ET ₂Cu[N(CN)₂]Br were measured using a combination of high-resolution soft X-ray absorption (SXA) and soft X-ray emission (SXE). The PDOS was also calculated using a tight binding model, and the measured spectra compared directly to that predicted by the calculations. The emission and absorption spectra from both materials were found to be quite similar, reflecting mostly contributions from the common conductive ET layers. The presence of two nonequivalent carbon sites of the ET molecule was identified in the SXA spectra. Contributions from theπandσstates were identified in the emission spectra. The occupied C 2pbandwidth was found to be approximately 17eV. An observed dependence of the SXE spectra on the excitation energy is partly accounted for by a simple model that considers the presence of the two nonequivalent ET carbon sites. We also find evidence in the SXE spectra for a high degree of localization in the lowest unoccupied states.