Theory of phonon-assisted forbidden optical transitions in spin-gap systems

We consider the absorption of light with emission of one S_tot=1 magnetic excitation in systems with a spin gap induced by quantum fluctuations. We argue that an electric-dipole transition is allowed on the condition that a virtual phonon instantaneously breaks the inversion symmetry. We derive an effective operator for the transition and argue that the proposed theory explains the polarized experiments in CuGeO₃ and SrCu₂(BO₃)₂ .