We present the site-charge distribution in the charge-ordered and metallic states of ␣-͑BEDT-TTF͒ 2 I 3 , based on the assignment of the C v C stretching modes 2 , 3 , and 27 of ␣-͑BEDT-TTF͒ 2 I 3 with the aid of 13 C-and deuterium-substituted compounds. The nonuniform site charges in both the metallic and insulating phases were consistent with those determined by a recent x-ray diffraction study. Comparing the line shapes of the chargesensitive vibrational modes of ␣-͑BEDT-TTF͒ 2 I 3 with those of isostructural ␣-͑BEDT-TTF͒ 2 NH 4 Hg͑SCN͒ 4 , we propose a thermally activated fluctuation of charge order in the metallic phase of ␣-͑BEDT-TTF͒ 2 I 3. This fluctuation was considerably suppressed above 0.65 GPa. The optical conductivity in the metallic phase shows no Drude response.

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A dip-shaped anomaly appearing in the infrared spectrum of charge-transfer organic complexes has been investigated. The anomaly appears at approximately the same frequency (∼2700 cm −1 ), irrespective of light polarization as well as a composition of the complex, when the compounds undergo charge ordering. Isotope-shift measurements for θ-(BEDT-TTF) 2 RbZn(SCN) 4 [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] indicates a relationship between the overtone of a C=C stretching mode of the BEDT-TTF molecule and this anomalous signal. Calculations of electron-molecular vibration coupling based on a diatomic molecular dimer model reveals that the overtone is activated by an anharmonicity developed in the adiabatic potential in a charge-separated system. It is presented that numerical calculation based on the simple cluster model reproduces essential features of the experimentally obtained conductivity spectrum.

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