  Cluster expansion methods are developed for calculating the spectral weight
contributions of multiparticle excitations - continuum and bound states - to
high orders. A complete 11th order calculation is carried out for the
alternating Heisenberg chain. For $\lambda=0.27$, relevant to the material
$Cu(NO_3)_2.2.5D_2O$, we present detailed spectral weights for the two-triplet
continuum and all bound states. We also examine variation of the relative
weights of one and two-particle states with bond alternation from the dimerized
to the uniform chain limit.
