Picosecond laser-induced fluorescence from gas-phase polycyclic aromatic hydrocarbons at elevated temperatures. II. Flame-seeding measurements

Picosecond laser-induced radiative emission from flames injected with aroma tic substances has been measured spectrally and temporally resolved. The me asurements were performed in various seeded regions and for different stoic hiometric ratios of the surrounding gas. The wavelength of the excitation r adiation was 266 nm. Changes in the lifetime and the spectral composition of the emission were o bserved with changes in the equivalence ratio and the position in the flame . Considerable agreement with previously reported cell measurements was obt ained for those regions close to the injection zone. Temperatures were dete rmined from spectrally and temporally resolved measurements. Tl-ie comparis on with elastic scattering gave reasonable results at low seeding rates fur naphthalene, and is hoped to be improved even further in future experiment s by increasing the time resolution and the signal-to-noise ratio of the me asurements. Downstream and towards the surrounding gas, the lifetimes incre ased and the spectral profiles shifted and broadened towards the red. This effect increased when the equivalence ratio fur the surrounding gas decreas ed and the oxygen concentration increased. The study was also directed towards characterizing features in the emission that could be indicative of a transition from the seeded aromatic substanc e to the formation of soot. An indicator for molecular or particle growth w as the composition of the spectral emission in terms of UV, blue and green- yellow bands and the ratio between elastic-scattering signal and total emis sion signal. Spatially resolved measurements across the seeding region usin g a gated intensified CCD camera allowed a closer study of the molecular-gr owth region from the parent aromatic substance seeded to the soot formed. T he fluorescence properties of dimers and their cy clodehydrogenated compoun ds and polymers containing aryl units are also discussed.