Anisotropy and Stabilization of Photoinduced Charge Carriers in Highly Oriented, Partially and Fully Isomerized Polyacetylene

We have investigated polarized photoinduced absorption (PIA) in highly oriented polyacetylene during isomerization. The experiments have been performed at two different excitation (pump) energies and with pump and probe beams polarized parallel and perpendicular to the chain direction. The signals of the charged excitations show a maximum at a trans content of about (50 ÷ 60)%. The shape of the low-energy tail of the low-energy peak changes and the defect mass of the excitation decreases upon isomerization. The polarized studies presented herein allow us to question thermal modulation as the origin of the features at about 740 cm -1 in the PIA spectra reported up to now.