Temperature, pressure, and bath gas composition dependence of fluorescence spectra and fluorescence lifetimes of toluene and naphthalene

Time-resolved fluorescence spectra of gas-phase toluene and naphthalene were investigated upon picosecond laser excitation at 266 nm as a function of temperature (toluene 296-1,025 K, naphthalene 374-1,123 K), pressure (1-10 bar), and bath gas composition (varying concentrations of N-2, O-2, and CO2) with a temporal resolution of 50 ps. In the investigated temperature range, the fluorescence spectra of both toluene and naphthalene show a significant red-shift, whereas the fluorescence lifetime decreases with increasing temperature, more pronounced for toluene than for naphthalene. Increasing the total pressure of either N-2 or CO2 from atmospheric to 10 bar leads to an increase by about 20 % (naphthalene at 373 K) and a decrease by 60 % (toluene at 575 K) in fluorescence lifetimes, respectively. As expected, at atmospheric pressure collisions with O-2 shorten the fluorescence lifetime of both toluene and naphthalene significantly, e.g., by a factor of 30 and 90 when changing O-2 partial pressure at 373 K from 0 to 0.21 bar, respectively. The fluorescence model of Koban et al. (Appl Phys B 80: 777, 2005) for the dependence of the toluene quantum yield on temperature and O-2 partial pressure at atmospheric pressure describes toluene fluorescence lifetimes well within its range of validity. The model is modified to satisfactorily predict effective toluene fluorescence lifetimes in N-2 at pressures up to 10 bar. However, it still fails to predict the dependence at simultaneously elevated temperatures and pressures in air as bath gas. Similarly, an empirical model is presented for predicting (relative) fluorescence quantum yields and lifetimes of naphthalene. Although the fitting models have their shortcomings this publication presents a data set of great importance for practical LIF applications, e.g., in-cylinder mixture formation diagnostics in internal combustion engines.