Photoexcitations in polyacetylene

Abstract

The results of experimental studies of photoluminescence and photoconductivity in cis- and trans-${\left(\mathrm{CH}\right)}_x$ are presented. For cis-${\left(\mathrm{CH}\right)}_x$, we find recombination luminescence in the scattered light spectrum at 1.9 eV, near the interband absorption edge. The luminescence turns on sharply for excitation energies greater than 2.05 eV, implying a Stokes shift of 0.15 eV. Studies of the temperature dependence ($T>~7$ K) show no loss of luminescence intensity even at temperatures as high as 300 K. Isomerization of the same sample quenches the luminescence; we find no indication of luminescence near the interband absorption edge of trans-${\left(\mathrm{CH}\right)}_x$ even at temperatures as low as 7 K. These results are discussed in the context of parallel phototransport studies. The quenching of the luminenscence upon cis-trans isomerization is concurrent with the appearance of a large photoconductive response. The photoconductivity in trans-${\left(\mathrm{CH}\right)}_x$ has a threshold at 1.0 eV, well below the interband absorption edge at 1.5 eV, implying the presence of states deep inside the gap. The observation of luminescence in cis-${\left(\mathrm{CH}\right)}_x$, but not in trans-${\left(\mathrm{CH}\right)}_x$, and the observation of photoconductivity in trans-${\left(\mathrm{CH}\right)}_x$, but not in cis-${\left(\mathrm{CH}\right)}_x$ provide confirmation of the proposal that solitons are the photogenerated carriers. In trans-${\left(\mathrm{CH}\right)}_x$, the degenerate ground state leads to free soliton excitations, absence of band-edge luminescence, and photoconductivity. In cis-${\left(\mathrm{CH}\right)}_x$ the nondegenerate ground state leads to confinement of the photogenerated carriers, absence of photoconductivity, and to the observed recombination luminescence.