Far-infrared spectrum of di-tetramethyltetraselenafulvalene hexafluoroarsenate [(TMTSF)2AsF6]

New results are presented on the far-infrared properties of the quasi-one-dimensional compound di-tetramethyltetraselenafulvalene hexafluoroarsenate [(TMTSF${)}_2$ ${\mathrm{AsF}}_6$] below the spin-density-wave (SDW) transition temperature. The infrared conductivity is reduced at frequencies below 60 ${\mathrm{cm}}^{\mathrm{-}1}$ but we do not observe a well-developed energy gap. A 5-${\mathrm{cm}}^{\mathrm{-}1}$-wide mode centered at zero frequency can be observed with a strength that is consistent with the freezing out of carriers below the SDW transition as observed by dc conductivity. There is no evidence of high metallic single-particle conductivity in our measurements above the transition temperature in the conducting state. A strong continuous absorption in the 50–500-${\mathrm{cm}}^{\mathrm{-}1}$ region can be seen and we associate it with a Holstein absorption with a ${\lambda }_{\mathrm{tr}}$ of 0.6, a value that is consistent with the temperature dependence of the dc conductivity.