  We measured the reflectivity of a single crystal of FeSi from the
far-infrared to the visible region (50-20000 wavenumber), varying the
temperature between 4 and 300 K. The optical conductivity function was obtained
via Kramers-Kronig analysis. We observed a dirty metal-like behavior at room
temperature and the opening of a gap of 70 meV at low temperature. The results
of a group theoretical analysis of the lattice vibrational modes are presented
and compared to the experimental data. Of the five optical phonons expected for
this material only four have been observed in the far-infrared region.
