An electron spin resonance selection rule for spin-gapped systems

The direct electron spin resonance (ESR) absorption between a singlet groun d state and the triplet excited states of spin gap systems is investigated. Such an absorption, which is forbidden by the conservation of the total sp in quantum number in isotropic Hamiltonians, is allowed by the Dzyaloshinsk ii-Moriya interaction. We show a selection rule in the presence of this int eraction, using the exact numerical diagonalization of the finite cluster o f the quasi-one-dimensional bond-alternating spin system. The selection rul e is also modified into a suit;able form in order to interpret recent exper imental results on CuGeO3 and NaV2O5.