Picosecond laser-induced fluorescence from gas-phase polycyclic aromatic hydrocarbons at elevated temperatures. I. Cell measurements

The aim of the work was a first step to investigate the possibility to use temporal selection as a complement to spectrally resolved measurements in c ombustion diagnostics to discriminate between different aromatic species. P icosecond laser-induced fluorescence from gas-phase naphthalene, fluorene, anthracene and pyrene has been measured at elevated temperatures in a flow cell under atmospheric-pressure conditions at temperatures between 400 and 1200K and at oxygen mole fractions between 0 and 10%. Picosecond-laser radi ation at a wavelength of 256 nm was used for excitation. The fluorescence e mission was recorded temporally resolved with a photomultiplier tube or a s treak camera and spectrally resolved with a spectrograph and a gated optica l multichannel analyzer. The results show that most of the lifetimes of the fluorescence emission de crease between one and two orders of magnitude going from the lower to the higher temperature. The spectral profiles change considerably with temperat ure and slightly or hardly at all with the oxygen content. At low temperatu res fluorescence is considerably quenched by oxygen, and it follows closely a Stern-Volmer description, whereas at higher temperatures another depende nce starts to appear. At high temperatures oxygen mixtures are reactive and absorption or fluorescence emission is seldom revealed. Results imply that the lifetimes and the spectral profiles are both influenced by the density of states. Extrapolation of the results to flame temperatures, 1200-1800 K , indicates that naphthalene and pyrene are the species to present the long est-lifetime components in regions of low oxygen concentration at 266-nm ex citation. Most of the substances are expected to present lifetime component s significantly below 1 ns at flame temperatures.