Ultrafast charge separation and recombination dynamics in a nanometer thin film of polyimide observed by femtosecond transient absorption spectroscopy

Femtosecond transient absorption spectroscopy was used to study how polyimide (PI) interacts with light in a nanometer-thick PI thin film. The PI containing pyromellitic diimide (PMDI) shows absorption by PMDI anions around 710 nm. Its decay occurs on two time scales: one is on the order of 10 ps and the other is on the order of a nanosecond. Each decay time is independent of the excitation wavelength and energy density, indicating that a geminate recombination would be dominant in a PI film. Under excitation by 390 nm light, the faster decay process shows one-photon absorption, but the slower decay process shows a two photon-absorption process.