Observation of Three-Magnon Light Scattering in ${\mathrm{CuGeO}}_3$

Temperature and magnetic field dependent polarized Brillouin spectra of ${\mathrm{CuGeO}}_3$ are reported. In addition to a bound singlet state at 30 ${\mathrm{cm}}^{-1\mathrm{}}$, a new feature has been observed at 18 ${\mathrm{cm}}^{-1\mathrm{}}$. This feature is interpreted in terms of a novel three-magnon light scattering process between excited triplet states.