Pressure dependence of the photoabsorption of polyacetylene

Abstract

The optical-absorption spectra of pure cis- and trans-polyacetylene at various hydrostatic pressures, in the region 0-13 kbar, are reported. We find a large red shift of the absorption edge indicating a reduction of the energy gap $E_g$ as a function of increasing pressure in both isomers. At 13 kbar, $E_g$ is reduced in trans-${\left(\mathrm{CH}\right)}_x$ by about 0.17 eV (i.e., ∼ 10%). In addition, a decrease in the peak height and a broadening of the leading edge of the interband absorption coefficient are observed at high pressures. These changes result from an increase in the interchain transfer integral $t_{\perp }$. Comparison of the data with the result of tight-binding calculations leads to estimates of the transverse bandwidth, $W_{\perp }\simeq 0.3$ eV, and a ratio of about 30 between $W_{\perp }$ and the total bandwidth. The resolved structure in the absorption spectrum of cis-${\left(\mathrm{CH}\right)}_x$ is observed to red-shift with the absorption edge, to decrease in magnitude, and to broaden at high pressures. These secondary peaks are interpreted as vibronic transitions associated with the interband absorption in the solid. Their behavior with pressure is accounted for by a model for vibronic transitions which takes into account the band structure.