Large-amplitude quantum fluctuations and subgap optical absorption intrans-polyacetylene

We show that in the frequency regime of the subgap tail, large-amplitude fluctuations of the (CH${)}_{\mathrm{x}}$ chain are involved in the optical absorption process. A recently developed semiclassical technique is applied to calculate the absolute absorption coefficient which is related to multidimensional tunneling of soliton pairs. We include the renormalization effects of weak electron-electron interactions and background transverse phonons. Good agreement with recent experimental data is found. Experimental predictions are made concerning the isotope dependence and the effect of the soliton shape mode on the absorption coefficient.