Evaluation of The Laser Ablation of Transition Metals/Metal Compounds by Time-of-Flight and Optical Spectroscopy
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EVALUATION OF THE LASER ABLATION OF TRANSITION METALS/METAL COMPOUNDS OPTICAL SPECTROSCOPY
BY TIME-OF-FLIGHT
AND
Terry L. Thiem, Lyn R. Watson, James A. Gardner, Rainer A. Dressier, Richard H. Salter and Edmond Murad Spacecraft Interactions Branch, Phillips Laboratory, Hanscom AFB, MA
ABSTRACT A fast pulsed beam of neutral metal atoms is produced by laser vaporization of a solid metal or metal compound sample in a modified high-temperature mass spectrometer. Atomic beams of several eV kinetic energy are generated as measured using time-of-flight spectroscopy. The energy range can be controlled with the laser power, similar to studies conducted on thin metal films. The solid samples, however, overcome the problem of short sample lifetime associated with irradiating thin films. Samples have been irradiated for several hours without observing a change in beam intensity or energy, thus offering an interesting source for kinetic studies. Initial results of studies done on copper, nickel, zinc and related oxide, sulfide, bromide, and chloride salts will be discussed as to their applicability to serve as a fast atom source. Spectroscopic data from these compounds will also be presented. INTRODUCTION In this work, the authors have found three characteristic areas of particle generation when using intense pulsed laser radiation. Early work on the interaction of high-power laser 2 1 radiation with solids has been summarized by Ready and Demichelis . At low laser power 2 densities, 30 eV) atoms. The ions are mass selected by a 600, 12-inch radius magnetic mass filter and are detected by a microchannel plate (TOF-2003, Galileo, Inc). Time-of-flight measurements are started by a "TL pulse from the laser's Q-switch. Spectra are recorded and displayed on a multichannel scaler (SRS-430 Stanford Research Systems, Inc.) RESULTS A solid zinc sample was used to produce a pulsed atom beam but the intensity decreased quickly with time. One possible explanation is a thin oxide coating may have been present being preferentially vaporized due to a lower thermal conductivity. Once this layer was removed exposing zinc metal itself the heat spread quickly into the sample decreasing the amount of vaporization occurring. To test this theory, zinc oxide was vaporized using the same method. The zinc oxide provided a more intense Zn atom signal for a longer period of time than the zinc metal. Other zinc compounds were subsequently tested to determine which would give the most intense zinc atom signal. Time-of-flight spectra for the zinc compounds at laser power 3 x 108 W cm 2 accumulated over 300 laser shots can be seen in Figures 1 through 4. The time-of-flight spectra show both ZnO and ZnS providing up to four times the signal level of the Zn metal itself. The Zn atom's energy is also greater and both ZnO and ZnS displayed an energy tunability with laser power as displayed in Table I. ZnBr 2 irradiation resulted in intensity levels less than the zinc metal. This, combined with the difficulty of working with an extremely hydrophilic material
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