Spectrally-Resolving the Emission of Laser-Induced Fluorescence

Conventional laser-induced fluorescence (LIF) measurements obtain the Doppler-broadened velocity distribution function (VDF) by recording injected laser wavelengths while monitoring induced fluorescence. Signal is proportional to laser intensity to the n^th power, where n is the number of photons absorbed per particle (n = 1 for single photon, n = 2 for two-photon absorption, etc.). To maximize laser intensity and therefore fluorescent signal, pulse-widths may be shortened. Ultrashort pulse (< ns) lasers produce linewidths too broad in wavelength to resolve VDFs. Still, these ultrashort pulse lasers are useful for temporally resolving the time scales of fluorescent decays and for high cadence measurements of absolute densities in fusion-relevant plasmas, thrusters, and plasma processing. Extracting the VDF is possible if emission is spectrally resolved, rather than the laser absorption. Conventional LIF also integrates over the velocity dimensions orthogonal to the laser k vector. If a distribution function in two velocity dimensions is desired, one must resort to more difficult optical pumping techniques such as optical tagging. Spectrally-resolving LIF emission using a narrow-linewidth laser provides the VDF in the direction of emission collection, only for particles with the selected velocity in the laser propagation direction, providing a 2V measurement. Presented are preliminary results showing the feasibility of the technique and future studies intend to measure 6V VDFs.