Photocurrent excitation cross-correlation on a laddertype polymer

The photocurrent of methyl substituted ladder type poly(para-phenylene) (m- LPPP) was measured when excited by two 150fs-pulses at 390nm and 780nm resp ectively in a cross correlation configuration. The first pulse at 390nm cre ates singlet excitons (S-0-->S-1), that are re-exited to a higher lying sta te by the time delayed second pulse at 780nm (S-1-->S-n). Our measurements show that the charge generation rate is highly increased if the exciton is re-excited within the first picosecond, pointing out the role of exciton mi gration in the ionization process. For longer time delays, we see a charge generation rate, that follows the excited state population, suggesting enha nced dissociation from a higher lying state.