Temperature, pressure, and oxygen quenching behavior of fluorescence spectra and lifetimes of gas-phase o-xylene and 1,2,4-trimethylbenzene

Abstract

Ortho-xylene (1,2-dimethylbenzene, XL) and 1,2,4-trimethylbenzene (TMB) are promising aromatic fluorescence tracer species for gas-phase imaging measurements of concentration, temperature, and oxygen partial pressure. In the present work, temperature-dependent gas-phase ultraviolet absorption spectra of XL and TMB were measured. In the investigated temperature range (296–725 K), the absorption bands red-shift with increasing temperature for both species and their absorption cross-sections increase. Time-resolved fluorescence spectra were recorded after picosecond laser excitation at 266 nm as a function of temperature (XL 296–1025 K, TMB 296–775 K), pressure (1–10 bar), and O₂ concentration using a streak camera coupled to a spectrometer. The fluorescence spectra of both species show a noticeable red-shift with increasing temperature and O₂ concentration. In N₂ as bath gas, the fluorescence lifetime of XL and TMB decreases by three orders of magnitude at the peak temperatures compared to room temperature. For both species, fluorescence quenching by N₂ (up to 10 bar) is temperature-dependent and is strongest at about 500 K. Quenching by O₂ shortens the fluorescence lifetime for both species significantly. This effect is much reduced at higher temperatures. The temperature dependence of the Stern–Volmer coefficients that describe the effect of O₂ quenching can be approximated by an exponential decay. Semi-empirical exponential fits to all investigated data (for XL and TMB) as well as published data for toluene were used to provide signal prediction models that are capable of predicting the signal intensities over a wide range of environmental conditions.