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 respectively in a cross correlation configuration. The first pulse at 390nm creates singlet excitons (S 0 →S 1 ), that are re-exited to a higher lying state 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 migration in the ionization process. For longer time delays, we see a charge generation rate, that follows the excited state population, suggesting enhanced dissociation from a higher lying state.