Investigation of photophysical processes as the basis for quantitative imaging of gas-phase mixing processes with tracer LIF
Final Report Abstract
The research project investigated the fluorescence properties of toluene and other aromatic tracers with improved fluorescence properties under different excitation and environmental conditions to improve quantitative imaging techniques. To investigate self-quenching of aromatic tracers, which occurs preferentially at high laser fluences, both laser fluence and tracer concentration were varied. The study confirmed that collisions between two excited tracer molecules are responsible for self-quenching. The temperature dependence was measured and no self-quenching was observed at slightly elevated temperatures. Subsequent measurements with o-xylene showed that self-quenching due to collisions of excited tracer molecules is not unique to toluene. By systematically varying excitation wavelengths and temperatures, an existing model for determining the population after laser excitation was further developed for anisole and toluene. A picosecond laser system was used to measure the effective fluorescence lifetimes at different pressures at excitation wavelengths between 246 and 274 nm. The determination of the pressure independent lifetimes allowed the identification of the excitation wavelength to excite the average thermal level. Ab initio calculations confirmed the accuracy of the energy transfer methods. The measurements also showed a relationship between the internal conversion and the oxygen-dependent redshift of the fluorescence spectrum. Both low-pressure and high-pressure high-temperature measurements were carried out. The low-pressure measurements provided the first evidence of a transition from photoinduced cooling to heating in toluene. High-pressure high-temperature measurements in a new measurement cell provided calibration data for high-pressure applications in a previously unexplored pressure-temperature range. These high-pressure experiments investigated the effective fluorescence lifetimes of substances such as toluene and 1,2,4-trimethylbenzene in the pressure range 1 to 50 bar. These data confirmed the results from the previous funding period up to pressures of 10 bar and showed a further non-linear decrease in fluorescence lifetime up to about 20 bar. It was also found that the influence of vibrational relaxation increases with temperature. Fluorescence spectra at pressures up to 50 bar showed stronger redshifts than at lower pressures. At temperatures of 873 K and pressures above 20 bar, degradation of toluene into pyrolysis products was observed. Based on commonly used step-ladder models and our modified calculation of the original excitation energy in S1, a model was created by global optimization that aims to accurately reproduce the fluorescence lifetime as a function of excitation wavelength and temperature. However, the implementation revealed problems with the simultaneous global fitting of all existing measurement data, indicating a limitation of the model. Meanwhile, the Tracer-Sim database created in the first phase has grown to approximately 1,300 data sets and has been enhanced with an external user interface, a collection of empirical models and a two-color detection evaluation module.
Publications
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Excitation wavelength dependence of the fluorescence lifetime of anisole. Physical Chemistry Chemical Physics, 21(27), 14562-14570.
Baranowski, Thomas; Dreier, Thomas; Schulz, Christof & Endres, Torsten
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Wavelength dependence of anisole, Gordon Conference Laser Diagnostics in Energy and Combustion Science (2019)
T. Baranowski; T. Endres; T. Dreier & C. Schulz
