Graduate Thesis Or Dissertation
 

Effects of atmospheric pressure on line widths and spatial distributions of the transient atomic absorption signals of minor constituents in metal samples atomized by a dye laser microprobe

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  • Investigations of plasma production by a pulsed dye laser microprobe were made. Comparisons between a Q- switched and a pulsed dye laser system were made on the intensity of the continuum produced by atmospheric breakdown. The dye laser system was foundtominimize the production of the continuum which in turn allowed a greater fraction of the energy to be transmitted to the sample surface at atmospheric pressure. The magnitude of the atmospheric pressure over the sample surface had a significant effect on both the duration and magnitude of the peak absorption of Mn in a reference steel sample. Spatially resolved atomic absorption measurements on the plume indicated that the plume was distributed over a much larger volume at low pressure (1 torr) than at atmospheric pressure. The plume appeared to be more uniform in composition at low pressure. Absorption line width measurements of the low pressure plume indicated that the temperature of the plasma had cooled dramatically from an estimated 10, 000 -20, 000° K during its formation to 400 -1000° K a few microseconds after the termination of the dye laser pulse. Its measured absorption width coupled with the reported values of the emission line widths of high intensity pulsed hollow cathode lamps indicated that a source with a narrower spectral profile was desirable for atomic absorption measurements in the plume. The duration of the pulsed hollow cathode intensity can be dramatically reduced with the use of a high voltage power supply (800 V) coupled with a high voltage switching transistor. The reduced duration should result in improvement of the line profile and eliminate much of the self-reversal. The use of a short duration hollow cathode pulse allowed an alternate method of atomic absorption measurements to be carried out. A photoanodic charge integration apparatus was developed to greatly facilitate the atomic absorption measurements.
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