Dynamics of the 1D Heisenberg model and optical absorption of spinons in   cuprate antiferromagnetic chains
  We use numerical and analytical results to construct a simple ansatz for the energy dynamical correlation function of the 1D antiferromagnetic Heisenberg model. This is applied to compute the phonon assisted absorption spectra of magnetic excitations (spinons) in quasi-one dimensional spin 1/2 insulators and show to reproduce very well recent infrared measurements in Sr$_2$CuO$_3$. 
J. Lorenzana (Bariloche) and R. Eder (Groningen)