Determination of Atomic Velocity Distributions Using Transient Absorption Measurements
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DETERMINATION OF ATOMIC VELOCITY DISTRIBUTIONS USING TRANSIENT ABSORPTION MEASUREMENTS
MICHEL MACLER* AND MARIO E. FAJARDO Emerging Technologies Branch, Propulsion Directorate, Phillips Laboratory, OLAC PI/RKFE, 9 Antares Road, Edwards Air Force Base, CA 93524-7680. *AFMC P1/NRC Post-doctoral Research Associate.
ABSTRACT Results of experiments on Al, Li, and Na atomic beams produced by laser ablation using XeCI excimer laser pulses (I - 107 to l08 W/cm 2) 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 pLsec. 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.
INTRODUCTION Recent successful efforts at isolating light metal atoms in cryogenic solid hydrogen have utilized a combination of pulsed laser ablation of the metal, and traditional matrix isolation spectroscopy (MIS) techniques [1,2]. These experiments suggest that the incident KE of the metal atoms plays a key role in determining the atomic isolation efficiency of the matrix deposition process, and in the formation of novel metal atom trapping site structures. Unfortunately, little work has been done to measure the KE distributions of laser ablated species prior to matrix isolation. Thus, the immediate goals of this research effort are: (1) to characterize the chemical identity and KE distributions of laser ablated species under our specific experimental conditions, and (2) to examine the possibility of modifying the nascent metal atom KE distributions through a modification of the traditional laser ablation process.
EXPERIMENTAL Fig. 1 depicts the experimental setup, configured for transient absorption measurements. The metal ablation targets are mounted on a rotatable rod within a vacuum chamber pumped directly by a small turbomolecular pump to - 10-6 Torr. The ablated plumes are formed by focusing a XeCI excimer laser beam on the targets at an incident angle of 45° from the surface normal. The incident laser pulse energies were varied between 3 and Mat. Res. Soc. Symp. Proc. Vol. 285. 01993 Materials Research Soc
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