Optical modes of t r a n s-polyacetylene

The Raman spectra of trans-polyacetylene excited with various laser wavelength (4579, 4880, 5145, 5682, and 6471 Å) show the lines at 1100 cm−1, 1500 cm−1 in (CH)x, and 860 cm−1 and 1400 cm−1 in (CD)x broadened. From a valence-force-field calculation of the optical modes, force constants in agreement with results from ab initio molecular orbital theories are obtained. It is shown that the broadening of the Raman lines may be interpreted as contributions to the scattered light from defect free polyene chains with various length. The experimentally observed spectra of the modes and the corresponding linewidths are explained by increasing the C=C stretch force constant from 7.7 mdyn/Å (infinite chain) up to 9.5 mdyn/Å (short chain). From the C=C double bond stretch force constant a bond order between 0.67 to 0.85 was found.