Solitons or not in polyacetylene (and does it matter?)

The results of a series of experiments are described which demonstrate that solitons are the important excitations in trans-(CH)_x and that the properties of these non-linear excitations can be directly studied through measurements on trans-(CH)_x samples, either during photoexcitation of after doping. The importance of these concepts in the more general context of conducting polymers is addressed. Although the two-fold degenerate ground state of trans-(CH)_x is quite special, the relevant concepts can be generalized to confined soliton pairs (bipolarons) and applied to a wide variety of conjugated polymers in which the ground state degeneracy is not present.