We develop a linked cluster method to calculate the spectral weights of
many-particle excitations at zero temperature. The dynamical structure factor
is expressed as a sum of exclusive structure factors, each representing
contributions from a given set of excited states. A linked cluster technique to
obtain high order series expansions for these quantities is discussed. We apply
these methods to the alternating Heisenberg chain around the dimerized limit
(位=0), where complete wavevector and frequency dependent spectral
weights for one and two-particle excitations (continuum and bound-states) are
obtained. For small to moderate values of the inter-dimer coupling parameter
位, these lead to extremely accurate calculations of the dynamical
structure factors. We also examine the variation of the relative spectral
weights of one and two-particle states with bond alternation all the way up to
the limit of the uniform chain (位=1). In agreement with Schmidt and
Uhrig, we find that the spectral weight is dominated by 2-triplet states even
at 位=1, which implies that a description in terms of triplet-pair
excitations remains a good quantitative description of the system even for the
uniform chain.Comment: 26 pages, 17 figure