THz time domain spectroscopy of low-frequency vibrations in a quasi-one-dimensional system

The mixed-valence halide-bridged transition metal linear chain (MX) complexes are prototypical quasi-one-dimensional systems, with a charge density wave ground state and localized electronic excitations analogous to those of conjugated organic polymers. In this work, we present studies of the low-frequency infrared-active vibrational modes of the MX complex [Pt(en)$_{2}$][Pt(en)$_{2}$I$_{2}$](PF$_{6})_{4}$ (en = ethylenediamine, C$_{2}$H$_{8}$N$_{2})$ in the frequency range 0.3 - 3 THz using terahertz time-domain spectroscopic techniques. Distinct polarization-dependent complex refractive indices are observed in single-crystal samples of this highly anisotropic material. The measurements reveal a strong absorption at a frequency of 2.24 THz (75 cm$^{-1})$ polarized along the chain axis, which we assign to the infrared-active $\nu _{3}$ vibrational mode, involving relative motion of the mixed-valence ions in the charge density wave structure. This work is supported by NSF grant DMR-0706407.