Temperature-dependent core-level x-ray photoemission spectroscopy of the organic conductors (MeCl-DCNQI)2Cu, (MeBr-DCNQI)2Cu, and (DI-DCNQI)2Cu

We study the quasi-one-dimensional (1D) organic conductors (MeCl-DCNQI)(2)Cu, (MeBr-DCNQI)(2)Cu, and (DI-DCNQI)(2)Cu using x-ray photoemission spectroscopy. We observe negligible change of the Cu 2p core-level spectra of (DI-DCNQI)2Cu and (MeBr-DCNQI)(2)Cu as a function of temperature, in the metallic phase. In contrast, Cu 2p spectra of (MeCl-DCNQI)(2)Cu, which has a charge-density-wave/metal-insulntor transition at 210 K exhibits clear changes across the transition. The spectral shape change occurring across the transition indicates charge transfer from Cu 3d states to the 1-D DCNQI p pi conduction band. A line-shape analysis shows that the average valence of Cu ions follows a systematic trend related to the tetrahedral angle cu between N-Cu-N, i.e., a more distorted tetrahedral angle corresponds to lesser charge transfer from the Cu ions to the p pi band. The results also provide direct evidence for coupling of the electronic states involved in charge-ordering with the charge-density-wave formation. [S0163-1829(98)09719-7].