  The effect of quantum lattice fluctuations on the optical-absorption spectra
in the ground state of halogen-bridged mixed-valence transition-metal
linear-chain complexes is studied by using a one-dimensional extended
Peierls-Hubbard model. The nonadiabatic effects due to finite phonon frequency
$\omega_{\pi}>0$ are treated through an energy-dependent electron-phonon
scattering function $\delta(k^{\prime},k)$ introduced by means of an unitary
transformation. The calculated optical-absorption spectra do not have the
inverse-square-root singularity, but they have a peak above the gap edge and
there exists a significant tail below the peak, which are consistent with the
optical-absorption coefficient or the optical conductivity measurements of this
material.
