Phase transition in the organic conductor (TTM-TTP)I${_{3}}$ studied by infrared and Raman spectroscopies

Polarised infrared reflectance (600–10000 cm$^{-1})$ as well as Raman scattering spectra of the organic conductor (TTM-TTP)I$_{3}$ were investigated as a function of temperature, below and above the metal–insulator phase transition at T=160 K. The IR electronic dispersion was analysed in terms of a Lorentz model and optical transport parameters were determined. The phase transition at 160 K has nearly no influence on IR spectra. The Raman scattering was studied for different excitations ($\lambda $ =514.5, 632.6 and 785 nm). Within the C=C stretching region three Raman bands at 1436, 1453 and 1486 cm$^{-1}$ were found and assigned to the three TTM-TTP modes of a$_{\rm g}$ symmetry. Due to the phase transition the band at 1486 cm$^{-1}$ splits into two lines at about 1487 and 1500 cm$^{-1}$. We relate this splitting to an asymmetric deformation of TTM-TTP. Key words. TTM-TTP salt - organic metal - IR and Raman spectra - phase transition.