Femtosecond photo-current excitation cross-correlation on a ladder type polymer

Transient photo-current cross-correlation experiments were carried out on m ethyl substituted ladder type poly(para-phenylene) (m-LPPP). In this experi ment, the first pulse at 390 nm creates singlet excitons (S-0 --> S-1), tha t are re-exited to a higher lying state by the time delayed second pulse at 780 nm (S-1 --> S-n). The change in (390 nm)-induced photo-current caused by the 780 nm pulses is measured as a function of blue-red time delay. Our measurements show that the charge generation rate is highly increased if th e exciton is re-excited within the first picosecond, pointing out the role of exciton migration in the ionization process. For longer time delays, we see a charge generation rate, that follows the excited state population, su ggesting enhanced dissociation from a higher lying state. (C) 2001 Publishe d by Elsevier Science B.V.