Investigation of laser-induced iodine fluorescence for the measurement of density in compressible flows

An experimental technique is needed for the nonintrusive measurement of the molecular number density at a point in a compressible flowfield. Laser-induced fluorescence (LIF} is an attractive approach but due to the complication of collisional quenching does not produce a signal that is directly related to the number density. The objective of this work is to investigate the use of LIF for the quantitative measurement of density in compressible flows. Two approaches for minimizing the quenching effect are explored: saturation and frequency-detuned excitation. Iodine is chosen as the molecular system in which to evaluate the feasibility of these approaches due to its convenient visible absorption spectrum. It is shown theoretically that complete saturation would eliminate the quenching dependence of the fluorescence signal. Saturation experiments are performed which indicate that with available continuous-wave laser sources of Gaussian transverse intensity distribution only partial saturation can be achieved in iodine at the pressures of interest in gasdynamics. Therefore, it is concluded that saturation is not a viable approach to eliminating the quenching complication. Using a fluorescence lineshape theory it is shown that for sufficiently large detuning of a narrow bandwidth laser from a molecular transition the quenching can be cancelled by collisional broadening over a large range of pressures and temperatures. Experimental data are obtained in a room temperature static cell which allow determination of the molecular quenching, collisional broadening and i effective hyperfine-width constants. Data are obtained in a Mach 4.3 underexpanded jet of nitrogen, seeded with iodine, for various single-mode argon laser detunings from a strong iodine transition at 5145/_. Using the experimentally-determined