Detection of Soliton Shape Modes in Polyacetylene

We observed soliton shape modes by photoinduced absorption in partially isomerized ${\left(\mathrm{CH}\right)}_x$ and ${\left(\mathrm{CD}\right)}_x$, coexisting with the soliton translational modes. The shape modes' ir activity, their dependence on the degree of isomerization, and the photoinduced spectra are explained in detail by a symmetry-breaking interaction which allows coupling of shape modes and translational modes. We also observed soliton shape-mode absorption in fully isomerized samples as phonon sidebands associated with optical transitions of neutral solitons above 1.5 eV.