Coexistence of Interchanged and Normal Orbital Levels in a Molecular Conductor Consisting of a Metal-Dithiolene Complex

We report the molecular vibrational spectra obtained from a powdered sample of the two-dimensional molecular conductor beta'-Me4Sb[Pt(dmit)(2)](2). Two monovalent dimers of this complex were found to change to one divalent and one neutral dimer below the metal-insulator transition temperature of 160 K. The magnitudes of the intra-dimer interactions were analyzed using the frequencies of vibrational modes and the excitation energy dependence of molecular vibrations. The results indicated that the orbital level crossing occurring near the Fermi energy above 160 K ceased in the divalent dimer but was preserved in the neutral dimer. This phenomenon suggests that the formation of degenerate orbitals was avoided by dimer distortion.