Resonance Raman scattering of polyacetylene doped with bromine, iodine and AsF₅ and compensated by ammonia

We have studied the resonance Raman scattering of polyacetylene doped with bromine, iodine and AsF₅, and compensated by ammonia. The measurements were performed on both cis and trans samples as grown and doped with various doping concentrations both compensated and uncompensated. Three laser lines were used: λ_L = 5145, 4880 and 4579 A ̊. To reduce the measuring time, we developed a novel optical set-up that uses a vibrating mirror. The laser power that can be focused on the sample without overheating it is about 30 times larger than in a conventional set-up using a cylindrical lens, yet all the scattered light is collected and focused into a sharp line at the entrance slit. Thus this arrangement reduces the required measuring time by about a factor of 30. The Raman bands of trans polyacetylene are analysed in terms of a distribution of uninterrupted segments of conjugated double bonds. The positions of the defects that limit the length of these segments are assumed to be independent of each other. The resonance scattering cross-section is estimated from the optical absorption function using the experimental relation between the phonon energy and the electronic gap energy measured in polyenes. We find that a bimodal distribution is needed to fit the experimental results. Very good fits to the higher energy parts of the Raman bands are obtained with only one parameter for all three wavelengths used in these experiments. The results of the measurements on the doped samples show that the doping process is not uniform: iodine preferentially dopes the amorphous parts of trans-polyacetylene and at low concentrations AsF₅ seems to dope the trans parts of cis-rich samples first. All three dopants reduce the length of the uninterrupted segments. Compensating the samples with ammonia increases the scattering intensity and removes additional scattering induced by the dopants. However, it does not restore the long uninterrupted segments. Samples doped with bromine are unstable with time in air at room temperature. The line at 160 cm^-1 associated with Br₃^- and the conductivity both decay with the same time constant. Only bromine and AsF₅ induce a cis-trans transformation. Iodine-doped cis-rich samples do not change with doping, but compensation induces the transformation to the trans form.