Two-magnon Raman scattering in spin ladders with exact singlet-rung ground state

Using coordinate Bethe ansatze we construct two-magnon states for the family of spin-ladder models with exact singlet-rung vacuum suggested by A. K. Kolezhuk and H.-J. Mikeska. The explicit formula for the zero-temperature Raman scattering cross section is derived. The corresponding line shapes are strongly asymmetric and their singularities originate only from bound states. This form of a line shape is in good correspondence to the experimental data.