SINGULARITIES IN OPTICAL-SPECTRA OF QUANTUM SPIN CHAINS

The midinfrared optical absorption spectrum is reported of the one-dim ensional Mott insulator Sr2CuO3. The spectrum has an asymmetric cuspli ke structure at 0.48 eV and is analyzed in terms of the phonon-assiste d absorption in the one-dimensional quantum spin chain model. A good a greement is obtained between experiment and theory, and the exchange e nergy J is estimated to be J approximate to 0.26 eV, which is consiste nt with the magnetic susceptibility measurement.