Two-photon absorption laser induced fluorescence: rate and density-matrix regimes for plasma diagnostics

Abstract

Two-photon absorption laser induced fluorescence (TALIF) technique employing nanosecond lasers is often used to measure space- and time-resolved distributions of key atomic and molecular radicals in reactive environments such as plasmas and combustions. Although the technique was applied for about four decades, particularly in high pressure non-equilibrium plasmas accurate measurements of species densities remain challenging. With atomic oxygen as an example, central aspects of the technique including the role of photon statistics and line profiles on the two-photon absorption rates, selection rules, spatial and temporal resolutions, photolytic and quenching effects, and absolute calibration methods are discussed. Simulations using rate equations which include non-depletion regime, three-level and six-level models are compared and criteria for non-saturation regimes are given for low- and high-pressure plasmas. Solutions of the density-matrix model, which include coherent excitation and Stark detuning phenomena, and the rate equation model are compared. The validity criteria for non-depletion and photolytic-free regimes and rate models are given. The nanosecond TALIF quench-free regime at high laser intensities is investigated using the density-matrix model. The two-photon cross-sections for O, H and N atomic radicals and their ratio with Kr and Xe rare gases used for calibrations are revisited and recommendations are proposed. For TALIF applying ultrafast lasers, the appropriate model for the fluorescence probability is discussed.