Single-photon pumping and two-photon probing spectroscopy for the determination of absorption cross-sections in an organic semiconductor.

We investigate the bleaching of two-photon absorption by single-photon excitation using femtosecond transient absorption measurements on the prototypical polyfluorene (F8), and thus introduce single-photon pumping and two-photon probing spectroscopy for the determination of absorption cross-sections in an organic semiconductor. Single-photon excitation at 3.1 eV rearranges the population distributions on the singlet excited state (1Bu) and on the ground state (1Ag), and probe pulses at 1.55 eV will thus be absorbed both by the singlet excited state through a single-photon process and by the partially depopulated ground state for two-photon transition from 1Ag to mAg. As a result, the two-photon absorption will be partially bleached, introducing a modulation to the total transient absorption. Probe intensity dependence of the transient absorption enables simultaneous determination of the two-photon absorption (mAg?1Ag) and exciton absorption (kAg?1Bu) cross-sections at 1.55 eV.