Nonlinear laser interactions with saltwater aerosols

Intensity ratios of the resonant Na(λ = 589 nm) to H_α (λ = 656 nm) emission lines have been measured for 10 GW/cm² KrF laser radiation (λ = 248 nm, t_pulse = 17 ns) interacting with 50 μm diameter saltwater droplets as a function of position in the breakdown region parallel to the incident laser beam. Results indicate the H_α emission intensity is highly non-uniform through the plasma plume and is highest in the plume ejected back toward the laser beam. In contrast, the Na emission line intensity shows a much weaker spatial dependence as a function of position within the plasma plume. Calibration data are also reported for the ratio of Na to H_α emission intensities as a function of Na concentration (0-2000 ppm) for ≈ 20 μm diameter monodisperse droplets located at a fixed observation point in the plasma plume. Future research will extend the ratioing technique to remotely measure droplet salt concentrations in polydisperse saltwater sprays.