Velocity Selection of Laser Ablated Metal Atoms by a Novel Non-Mechanical Technique
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MARIO E. FAJARDO AND MICHEL MACLER* Emerging Technologies Branch, Propulsion Directorate, Phillips Laboratory, OLAC PL/RKFE, 10 E. Saturn Blvd., Edwards Air Force Base, CA 93524-7680. *AFMC PL/NRC Post-doctoral Research Associate.
ABSTRACT We present the results of experiments on velocity selection of fast laser ablated Al, Ga, and In atoms by a novel, non-mechanical, technique. Pulses of atoms with broad velocity distributions are produced by laser ablation of a single component pure metal target in vacuum. After a delay of - 1 ýts, there exists a strong one-to-one correlation between atomic velocity and distance traveled from the ablated surface. Thus, a second pulsed laser, delayed by - 1 [ts and crossed at a right angle to the atomic beam, can be used to photoionize only those atoms with unwanted velocities, i.e.: atoms moving too fast or too slow to be hidden behind an opaque mask placed - 1 cm from the ablated surface. The photoions, and any ions surviving from the ablation event, are subsequently deflected from the beam by a static magnetic field. By a fortunate coincidence, Al, Ga, and In atoms all have very large single photon photoionization cross sections at 193 rim, the output wavelength of the ArF excimer laser; thus, well over 95 % of the unwanted atoms can be easily photoionized and rejected. We have demonstrated velocity selected Al, Ga, and In atom fluxes equivalent to (D - 1011 atoms/(cm 2 -eV-pulse) at a working distance of 10 cm. INTRODUCTION Pulsed laser ablation of solid targets is an increasingly popular technique for the deposition of a wide variety of thin film materials [1]. There are many fundamental and applied studies underway of e.g.: laser/surface interactions, plume plasma hydrodynamics, plume composition vis-d-vis ions/neutrals/clusters/particulates, internal and kinetic energy content of ablated species, etc., and of how all these factors ultimately affect the processes of thin film deposition and growth. Unfortunately, progress towards this ultimate goal is hampered by the bewildering complexity of the phenomena involved, a predicament that has been likened to studying "a tornado in a garbage can." We believe that any modification of "standard" laser ablation techniques which result in a simplification of this situation, and/or in improved parametric control over deposition conditions, will prove to be highly valuable to the thin film deposition community. As a case in point: we have been employing pulsed laser ablation of metal targets as a source of metal atoms for matrix isolation spectroscopy (MIS) studies for several years now [2-6]. We prepare our MIS samples by codepositing the products of a laser ablated plume along with a large excess of an inert matrix host gas onto a cryogenically 39 Mat. Res. Soc. Symp. Proc. Vol. 388 ©1995 Materials Research Society
cooled substrate. These experiments have lead us to hypothesize that the incident kinetic energy (KE) of the ablated metal atoms plays a key role in determining the atomic isolation efficiency of the matrix deposition p
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