Pressure dependence of the photoabsorption of polyacety-lene

The optical absorption spectra of pure cis and trans-polyacetylene at various hydrostatic pressures, in the region 0–9 Kbar, are reported. We find a “red shift” of the absorption edge indicating a reduction of the energy gap as a function of pressure. At 9 Kbar, E is reduced by about 0.1 eV. In addition a decrease ing the slope of α (ω) is observed at high pressures. These changes result from an increase in the interchain transfer integrals, t1. Comparison of the data with the results of tight binding calculations lead to estimates of t1≃ 0.025 − 0.05 eV (ambient) and t1/t1≃ 10−2.