EFFECTIVE STIMULATED-EMISSION AND EXCITED-STATE ABSORPTION CROSS-SECTION SPECTRA OF PARA-PHENYLENE-ETHYNYLENE POLYMERS

The effective stimulated emission cross-section spectra, sigma(em,eff) (lambda), the stimulated emission cross-section spectra, sigma(em)(lam bda), and the excited-state absorption cross-section spectra, sigma(ex )(lambda) = sigma(em)(lambda) - sigma(em,eff)(lambda), of the conjugat ed polymers adecyloxy-paraphenylene-ethynylene-co-2,5-thienyl) (OPT) a nd ecyloxy-paraphenylene-ethynylene-co-2,5-pyridinyl) (OPP) in tetrahy drofuran are determined in the fluorescence region. sigma(em,eff)(lamb da) determined by a fluorescence amplification technique is found to b e small compared to sigma(em,eff)(lambda) indicating that sigma(ex)(la mbda) is nearly as large as sigma(em)(lambda). The laser excitation to the wide inhomogeneously broadened S-1 -absorption band leads to the formation of spectrally structured inhomogeneously broadened lattice r elaxed exciton conformations extending on average over two monomer uni ts which are thought to be responsible for upper-to-lower-state stimul ated emission and lower-to-upper-state (excited-state) absorption.