  Inelastic neutron scattering was used to measure the magnetic field
dependence of spin excitations in the antiferromagnetic S=1/2 chain CuCl_2
2(dimethylsulfoxide) (CDC) in the presence of uniform and staggered fields.
Dispersive bound states emerge from a zero-field two-spinon continuum with
different finite energy minima at wave numbers q=pi and q_i approx pi
(1-2<S_z>). The ratios of the field dependent excitation energies are in
excellent agreement with predictions for breather and soliton solutions to the
quantum sine-Gordon model, the proposed low-energy theory for S=1/2 chains in a
staggered field. The data are also consistent with the predicted soliton and
n=1,2 breather polarizations and scattering cross sections.
