The fluorescence lifetime is an important characteristic of fluorescence spectroscopy.The measurement of lifetime can be used in a number of ways to obtain dynamic and structural information about molecules.But for the lifetimes in the magnitude of nanoseconds a high technique is required to measure them.Although there have been a large number of reports in the literature on the fluorescence spectra of polynuclear aromatic hydrocarbons(PAH)

but the lifetime data of these molecules are not so abundant and in many cases the published values are in poor agreement with each other.For example

for fluoranthene

D.J.Ehrlich et al obtained 24 ns for 220℃ vapor and 22 ns for the cyclohexane solution by exciting the molecule at 351 nm

while L.J.Jan-dris et al obtained 38 ns for 60-95℃ vapor by exciting at 337 nm.This discrepancy is considered to be abnormal and repeated investigations are recommended.Using the presently most precise time-correlated single photon counting technique

we have measured eight PAHs which are important in fluorescence spectroscopy and in biochemistry.All solutions were bubbled with high purity nitrogen before the measurement.The excitation wavelength was 337 nm for all compounds and additionally 351 nm was also used for fluoranthene to verify the discrepancy between the values of both groups of authors.Our experiment gave out 39 ns for excitation at 337 nm and 36 ns for excitation at 351 nm

showing that there should be no essential dilference between the two cases.The lifetimes measured for other PAHs are either in agreement with literature value or are reasonable.