THE INFRARED ACTIVITY OF POLYACETYLENE

In this paper, we study the dynamic and infrared activity of pure tran s- and cis-polyacetylene, doped systems, and systems with cis-trans de fects. First we study the behaviour of the Goldstone mode frequency of a simple linear chain. We find that this frequency depends strongly o n the value of the dimerization parameter u, in contrast with the expe rimental results, where this mode is particularly stable. Thus, taking u as a free parameter, we develop several realistic models for neutra l perfect cis and trans chains and doped systems. Our results are in v ery good agreement with experimental results. We show that for the dop ed chain the transverse component of conductivity is about 1% of the l ongitudinal component. The presence of the cis-trans defect allows us to give the origin of several peaks found experimentally and not inter preted until now, We show that in the presence of cis-trans defects th e photoinduced infrared activity may appear in systems with charged so litons and antisolitons on the same neutral chain. Computed results ar e also in very good agreement with the experimental data.