A neutron scattering study of two-magnon states in the quantum magnet   copper nitrate
  We report measurements of the two-magnon states in a dimerized antiferromagnetic chain material, copper nitrate (Cu(NO3)2*2.5D2O). Using inelastic neutron scattering we have measured the one and two magnon excitation spectra in a large single crystal. The data are in excellent agreement with a perturbative expansion of the alternating Heisenberg Hamiltonian from the strongly dimerized limit. The expansion predicts a two-magnon bound state for q ~ (2n+1)pi*d which is consistent with the neutron scattering data. 
D.A. Tennant, C. Broholm, D.H. Reich, S.E. Nagler, G.E. Granroth, T.   Barnes, K. Damle, G. Xu, Y. Chen, and B.C. Sales