Analysis of the infrared conductivity of the organic superconductor a-(BEDT-TTF)2(NH4)Hg(NCS)4

201 An optical study of the a phase superconducting compound, a-(ET)2(NH4)Hg(NCS)" " has been performed. The absolute reflectivity of a deuterated crystal has been measured as a function of temperature and polarization, from which the conductivity was obtained. The overall shapes of these spectra are discussed and compared with those obtained from compounds of different phases. In particular we were able to perform a Drude fit of the low-temperature conductivity for one polarization, rather than of the reflectivity dip. The plasma frequency and damping were extracted and these yielded a reasonable effective mass and mean free path. The isotopic vibrational frequency shifts were obtained by repeating the measurements on a protonated sample, which was, however, too small to provide absolute data. The vibrational features are assigned and discussed