Determination of Atomic Velocity Distributions Using Transient Absorption Measurements

ABSTRACTResults of experiments on Al, Li, and Na atomic beams produced by laser ablation using XeCI excimer laser pulses (I∼107 to l08 W/cm2) are presented. Plume emission spectra showed that further than ∼5 mm from the targets most of the excited ablated species are in neutral, atomic form. Transient absorption measurements permitted the detection of ground state atoms with different flight times, at fixed distances from the target. The resulting time of flight (TOF) distributions were converted into velocity and kinetic energy (KE) distributions.The measured fluxes of fast atoms always increased with ablation laser intensity. The velocity distributions could not be fitted to Maxwell-Boltzmann (MB), or MB plus stream velocity type distributions. For Al plumes, comparison of velocity distributions measured at various distances from the target surface indicates that Al atoms are produced within ∼100 nsec following the leading edge of the ablation laser pulse, implying that there is a one-to-one mapping of the initial velocities to the distances traveled by the atoms after ∼1 μsec. This result should encourage efforts to develop a method of velocity selection of laser ablated metal atoms based on the spatial separation of the various velocity components along the direction of travel.