  We consider the easy-plane anisotropic spin-1/2 Heisenberg chain in combined
uniform longitudinal and transverse staggered magnetic fields. The low-energy
limit of his model is described by the sine-Gordon quantum field theory. Using
methods of integrable quantum field theory we determine the various components
of the dynamical structure factor. To do so, we derive explicit expressions for
all matrix elements of the low-energy projections of the spin operators
involving at most two particles. We discuss applications of our results to
experiments on one-dimensional quantum magnets.
