Optical and photoemission experiments reveal unexpected spectral signatures
of one-dimensional band insulators. In the model compound (NbSe4)(3)I the
optical conductivity decays as a power law sigma(1)(omega) similar to omega
(-4.25) above a sharp gap edge. Photoemission observes both the valence and
a shadow band, produced w by a commensurate superstructure. We identify an
optical and photoemission band gap consistent with other measurements but
much smaller than the energy scale defined by the dispersion of the band pe
ak in the photoemission spectra.