TEMPERATURE-DEPENDENT CORE-LEVEL X-RAY PHOTOEMISSION SPECTROSCOPY OF THE ORGANIC CONDUCTORS (MECL-DCNQI)(2)CU, (MEBR-DCNQI)(2)CU, AND (DI-DCNQI)(2)CU

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 photoemissi on 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 temper ature, in the metallic phase. In contrast, Cu 2p spectra of (MeCl-DCNQ I)(2)Cu, which has a charge-density-wave/metal-insulntor transition at 210 K exhibits clear changes across the transition. The spectral shap e change occurring across the transition indicates charge transfer fro m Cu 3d states to the 1-D DCNQI p pi conduction band. A line-shape ana lysis shows that the average valence of Cu ions follows a systematic t rend 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 evidenc e for coupling of the electronic states involved in charge-ordering wi th the charge-density-wave formation. [S0163-1829(98)09719-7].