Optical excitation of plasmons in a quasi-one-dimensional conductor

We have carefully determined the optical spectrum of ${\mathrm{K}}_2$Pt${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4$ ${\mathrm{Br}}_{0.3}$ · 3${\mathrm{H}}_2$ O near the reflectivity edge. Our data reveal, in addition to the expected monotonic background, a broad peak in ${\epsilon }_2\left(\omega \right)$ above a threshold coinciding with the zero of ${\epsilon }_1\left(\omega \right)$ near 16 000 ${\mathrm{cm}}^{-1\mathrm{}}$. We assign this band to indirect excitation of plasmons. Details of the excitation of plasmons are discussed with particular attention being paid to the problems presented by the one-dimensional character of the material. It is concluded that the electronic structure of this material cannot be adequately described by either a free-electron model or a one-band tight-binding model.