Effective stimulated emission and excited-state absorption cross-section spectra of para-phenylene-ethynylene polymers

The effective stimulated emission cross-section spectra, sigma sub(em,eff)( lambda ), the stimulated emission cross-section spectra, sigma sub(em)( lambda ), and the excited-state absorption cross-section spectra, sigma sub(ex)( lambda ) identical with sigma sub(em)( lambda )- sigma sub(em,eff)( lambda ), of the conjugated polymers poly(2,5-dioctadecyloxy-paraphenylene-ethynylene-co-2,5-thienyl) (OPT) and poly(2,5-dioctadecyloxy-paraphenylene-ethynylene-co-2,5-pyridinyl) (OPP) in tetrahydrofuran are determined in the fluorescence region. sigma sub(em,eff)( lambda ) determined by a fluorescence amplification technique is found to be small compared to sigma sub(em)( lambda ) indicating that sigma sub(ex)( lambda ) is nearly as large as sigma sub(em)( lambda ). The laser excitation to the wide inhomogeneously broadened S sub(1)-absorption band leads to the formation of spectrally structured inhomogeneously broadened lattice relaxed exciton conformations extending on average over two monomer units which are thought to be responsible for upper-to-lower-state stimulated emission and lower-to-upper-state (excited-state) absorption.