Measurement and extrapolation modeling of PAH laser-induced fluorescence spectra at elevated temperatures

Abstract

The laser-induced fluorescence technique is widely used in the measurement of polycyclic aromatic hydrocarbons (PAHs) in sooting flames. One of the main limitations is the absence of the PAH fluorescence spectra at elevated temperatures. In this study, fluorescence spectra and photophysical properties of five typical PAHs were experimentally studied in the temperature range of 673–1373 K in an optical cell. The experimental results indicated that the fluorescence spectra of PAHs were greatly sensitive to PAH structures and were likely to shift to the red and became broader as temperature increases. Further, we also observed that absorption cross sections of PAHs increased linearly as a function of temperature. Fluorescence quantum yields, which were calculated using integral fluorescence intensities and absorption cross sections, decreased monotonically with increasing temperature. However, the descent gradients of different PAHs were quite different, e.g., naphthalene, fluoranthene and fluorene were more sensitive to temperature, and their fluorescence production was much lower at elevated temperature compared with phenanthrene and pyrene. In order to investigate the fluorescence quantum yields at higher temperatures (up to 2000 K), which cannot be measured in the optical cell, a multistep decay model was established and optimized based on experimental results. At temperatures between 1373 and 2000 K, the extrapolation results indicated that fluorescence quantum yields of phenanthrene and pyrene would be two orders of magnitude higher than those of naphthalene, fluorene and fluoranthene. This contributed to explaining that the PAH fluorescence signals emitted from phenanthrene and pyrene were stronger than those emitted from naphthalene in flames, although the concentrations of phenanthrene and pyrene were much lower than that of naphthalene in flames.