Abstract
Effective stimulated emission cross-section spectra, sigma(em,eff)(gamma), stimulated emission cross-section spectra, sigma(em)(lambda), and excited-state absorption cross-section spectra, sigma(ex)(lambda) = sigma(em)(lambda) - sigma(em,eff)(lambda), are determined in the fluorescence region for the conjugated polymer poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) in tetrahydrofuran, xylene, toluene, and for the model molecule t,t'-(didecyloxy)-II-distyrylbenzene. For the conjugated polymer sigma(em,eff)(lambda) is found to be small compared to sigma(em)(lambda) indicating that sigma(ex)(lambda) and sigma(em)(lambda) are of similar strength and spectral shape, while for the model compound sigma(ex)(lambda) is small compared to sigma(em)(lambda). In the polymer the laser excitation to the broad S-1 absorption band leads to the formation of discrete lattice relaxed exciton conformations extending on the average over four monomer units which are responsible for upper-to-lower-state stimulated emission and lower-to-upper-state (excited-state) absorption. (C) 1997 Elsevier Science B.V.