UV absorption and fluorescence properties of gas-phase p-difluorobenzene

Abstract

1,4-Difluorobenzene (p-DFB) is a promising aromatic tracer for determining concentration, temperature, and O₂ partial pressure in mixing gas flows based on laser-induced fluorescence (LIF). Signal quantification requires the knowledge of absorption and fluorescence properties as a function of environmental conditions. We report absorption and fluorescence spectra as well as fluorescence lifetimes of p-DFB in the temperature, pressure, and oxygen partial pressure range that is relevant for many applications including internal combustion engines. The UV absorption cross section, investigated between 296 and 675 K, has a peak value close to 266 nm and decreases with temperature, while still exceeding other single-ring aromatics. Time-resolved fluorescence spectra were recorded after picosecond laser excitation at 266 nm as a function of temperature (296–1180 K), pressure (1–10 bar), and O₂ partial pressure (0–210 mbar) using a streak camera (temporal resolution 50 ps) coupled to a spectrometer. The fluorescence spectra red-shift (~2 nm/100 K) and broaden (increase in full width at half maximum by 58% in the investigated temperature range) with temperature. In N₂ as bath gas (1 bar), the fluorescence lifetime τ _eff decreases with temperature by a factor of about 20 (from 7 ns at 298 K down to 0.32 ns at 1180 K), while at 8 bar the shortest lifetime at 975 K is 0.4 ns. A noticeable pressure dependence (i.e., reduced τ _eff) is only visible at 675 K and above. Quenching of p-DFB LIF by O₂ (for partial pressures up to 210 mbar) shortens the fluorescence lifetime significantly at room temperature (by a factor of 8), but much less at higher temperatures (by a factor of 1.8 at 970 K). For fixed O₂ partial pressures (52 mbar and above), τ _eff shows a plateau region with temperature which shifts toward higher temperatures at the higher O₂ partial pressures. O₂ quenching is less prominent for p-DFB compared to other aromatic compounds investigated so far. The temperature dependence of O₂ quenching can be approximately expressed by an exponential function. The influence of temperature, total pressure, and O₂ partial pressure on absorption cross sections and fluorescence quantum yields are given as empirical functions that allow for interpolation. For typical applications, p-DFB LIF provides up to three orders of magnitude stronger signal compared to toluene LIF.