Rayleigh Scattering Measurements of Heating and Gas Perturbations Accompanying Femtosecond Laser Tagging

Planar Rayleigh scattering and Rayleigh scattering polarimetry are applied toward measurements of molecular dissociation, thermometry, and the visualization of fluid dynamics in the afterglow of a femtosecond laser plasma under conditions relevant to the femtosecond laser tagging diagnostic. The generation of shock waves and a hightemperature region are observed, with a corresponding temperature increase of 280 +/- 40 K (320 mu J, N-2) and 860 +/- 160 K(780 mu J, N-2) produced with a 175mmfocusing lens. The spatial profile of the temperature rise along the beam is found to scale linearly with the optical emission intensity. The transverse width of the emission region is also observed to correlate with increased temperature. Rayleigh polarimetry measurements show a high dissociation fraction of up to 0.46 +/- 0.04 (780 mu J, N-2). Modeling of three-body recombination into the N-2 (B-3 Pi(g), v') state using the measured dissociation fraction is able to reproduce the emission intensity time history between 1 and 50 mu s. Local heating is found to play a key role in the observed trends in the spatial profile, width, intensity, and temporal evolution of femtosecond laser tagging at laser intensities near or above 3.9 x 10(14) W/cm(2).