Optical absorption of CuGeO3

The absorption coefficient of CuGe${\mathrm{O}}_3$ was measured in the energy range of 1-4 eV, in a temperature interval between 300 and 20 K, for light polarized along the $b$ and $c$ crystal axes. A weak absorption band at 1.7 eV is attributed to copper $d-d$ transitions partially allowed by electron-phonon interactions; this is followed by a region of transparency leading to a strong exponential edge above 3 eV, which is identified with the onset of charge-transfer transitions.