  The magnetic excitation spectrum of copper pyrimidine dinitrate, a material
containing S=1/2 antiferromagnetic chains with alternating g-tensor and the
Dzyaloshinskii-Moriya interaction, and exhibiting a field-induced spin gap, is
probed using submillimeter wave electron spin resonance spectroscopy. Ten
excitation modes are resolved in the low-temperature spectrum, and their
frequency-field diagram is systematically studied in magnetic fields up to 25
T. The experimental data are sufficiently detailed to make a very accurate
comparison with predictions based on the quantum sine-Gordon field theory.
Signatures of three breather branches and a soliton, as well as those of
several multi-particle excitation modes are identified.
