Defect states in polyacetylene: A photoacoustic and diffuse reflectance study

We present a combined photoacoustic spectroscopy (PAS) and polarized diffuse reflectance (PDR) study of relatively thick freestanding cis and trans polyacetylene films in the visible-near IR regime. Both techniques are suitable for measuring low absorption coefficients in solids. The PAS signal for the cis isomer is saturated in the region of the strong interband transition. There is no absorption in the near IR except for a vibrational overtone band at 0.74 eV. Thermal isomerization leads to a very broad, featureless absorption band in the near IR consistent with a broad distribution of defect states in the bandgap of trans-polyacetylene. Polarized diffuse reflectance spectra also show a higher absorption in the near IR for the trans isomer. Upon doping with oxygen, an absorption peak grows with time with a maximum at 0.67 eV and a characteristic asymmetric line shape consistent with soliton absorption and similar to what has been observed in the absorption spectra of thin acceptor-doped (CH)x films. Our PDR results suggest an absorption coefficient of the order of 100 cm−1 for the defect absorption in undoped trans-polyacetylene.