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

Abstract.

Picosecond laser-induced radiative emission from flames injected with aromatic substances has been measured spectrally and temporally resolved. The measurements were performed in various seeded regions and for different stoichiometric ratios of the surrounding gas. The wavelength of the excitation radiation was 266 nm.

Changes in the lifetime and the spectral composition of the emission were observed with changes in the equivalence ratio and the position in the flame. Considerable agreement with previously reported cell measurements was obtained for those regions close to the injection zone. Temperatures were determined from spectrally and temporally resolved measurements. The comparison with elastic scattering gave reasonable results at low seeding rates for naphthalene, and is hoped to be improved even further in future experiments by increasing the time resolution and the signal-to-noise ratio of the measurements. Downstream and towards the surrounding gas, the lifetimes increased and the spectral profiles shifted and broadened towards the red. This effect increased when the equivalence ratio for the surrounding gas decreased 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 substance to the formation of soot. An indicator for molecular or particle growth was the composition of the spectral emission in terms of UV, blue and green–yellow bands and the ratio between elastic-scattering signal and total emission signal. Spatially resolved measurements across the seeding region using a gated intensified CCD camera allowed a closer study of the molecular-growth region from the parent aromatic substance seeded to the soot formed. The fluorescence properties of dimers and their cyclodehydrogenated compounds and polymers containing aryl units are also discussed.