  The optical conductivity sigma (omega) of undoped YBa2Cu3O6 is studied in
detail in the mid-infrared range. Substitutions on all but the Ba site are used
to identify the prominent absorption processes at 2800 and 3800 cm^-1.
Experimental evidence for bimagnon-plus-phonon absorption is collected. A more
critical analysis of the lineshape and the spectral weight reveals the limits
of this approach. Although phonon-2-magnon multiple scattering seems to
reproduce the lineshape, the necessary coupling is unrealistically large. The
strong increase of high frequency spectral weight with increasing temperature
makes the failure of spin-wave theory even more evident.
