  We investigate the optical conductivity of photogenerated solitons in
quasi-one-dimensional halogen-bridged binuclear transition-metal MMX complexes
with particular emphasis on a comparison among the three distinct groups:
A_4_[Pt_2_(P_2_O_5_H_2_)_4_X]nH_2_O (X=Cl,Br,I; A=Na,K,NH_4_,...),
Pt_2_(RCS_2_)_4_I (R=C_n_H_2n+1_) and Ni_2_(CH_3_CS_2_)_4_I, which exhibit a
mixed-valent ground state with the X sublattice dimerized, that with the M_2_
sublattice dimerized and a Mott-Hubbard magnetic ground state, respectively.
Soliton-induced absorption spectra for A_4_[Pt_2_(P_2_O_5_H_2_)_4_X]nH_2_O
should split into two bands, while that for Pt_2_(RCS_2_)_4_I and
Ni_2_(CH_3_CS_2_)_4_I should consist of a single band. The excitonic effect is
significant in Ni_2_(CH_3_CS_2_)_4_I.
