Theory of phonon-assisted forbidden optical transitions in spin-gap systems -: art. no. 024404

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).