Spectroscopic investigation of atomic and molecular species formed in a laser microprobe plasma using a wavelenth calibrated tunable dye laser Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/dz010s94h

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  • The temporal and spatial characteristics of a laser microprobe plume were investigated using the spectroscopic methods of emission, absorption and fluorescence. The plume was produced by rapid vaporization of a small quantity (1 μg) of a solid sample using a focussed pulsed dye laser (110 MW/cm²). For absorption and fluorescence experiments, a wavelength calibrated narrow band tunable dye laser was used as the excitation source. All microprobe experiments were performed with a metal sample (Al or Zr) placed in a sample chamber with the pressure and composition of the sampling atmosphere controlled. Sample chamber pressures of 25 torr to 300 torr were used. The chemical composition of the sample chamber atmosphere was varied from pure argon to pure oxygen. Most spectroscopic observations were made on binary molecules formed by rapid reaction of the vaporized sample material with oxygen gas. For sample chamber atmospheres rich in oxygen, the rate of reaction was found to be significantly greater near the periphery of the plume than that observed for the central region. The results of these experiments indicate that the plume material forces the sampling atmosphere out of the region into which it expands. Transfer of the sampling atmosphere back into the region of the plume was observed to be radial in nature. Complete mixing of the plume material with the sampling atmosphere takes more than 100 μs. Significant quantities of sample oxide (A10 and ZrO) were also observed in sampling atmospheres containing only trace levels of oxygen. These oxides were found to be at their peak concentration near the center of the plume, indicating the sample itself may be a significant source of oxygen. A unique method for accurate wavelength calibration of an etalon tuned dye laser, such as the one used in the laser microprobe experiments, is also reported. This method is based on an equation which relates the optical mount setting of an intracavity etalon to the lasing wavelength of a CMX-4 tunable dye laser. The parameters of this equation were curve fitted to data obtained by using laser induced impedance change (LIIC) signals from hollow cathode lamps to accurately determine the lasing wavelengths corresponding to optical mount settings. The equation is capable of predicting either the proper optical mount setting for any lasing wavelength or the lasing wavelength corresponding to any optical mount setting. The maximum observed error in the computed lasing wavelength for a given optical mount setting is 0.04 Å.
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