Graduate Thesis Or Dissertation
 

Chemical and physical influences of the atmosphere upon the spatial and temporal characteristics of atomic fluorescence in laser microprobe plumes

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  • The temporal and spatial characteristics of the plume produced by a laser microprobe, are dependent upon the composition and pressure of the atmosphere surrounding it. It has been shown that certain atmospheres can enhance the analytical signal by increasing the free atom lifetime. Improvements were made to the existing laser microprobe instrumentation. A new sample chamber was constructed with many improvements over the old. The sample capacity has been increased by 275%. Sample handling has been dramatically reduced. The vacuum integrity of the new chamber is very good, allowing excellent atmospheric control. A new vacuum-gas mixing system was constructed with many advanced features. Five separate gases can be added to the chamber at one time. Most of the vacuum system is constructed cut of stainless steel for corrosion resistance, with stainless steel flexible tubing connecting it to the microprobe chamber. Major improvements were made to the optics of the microprobe laser. The stability and ease of tuning has been significantly increased. Laser atomic fluorescence studies cf lithium and copper atoms in the plume were conducted. Laser atomic fluorescence was used to study the plume because high spatial resolution is possible allowing specific areas of the plume to be probed. A fraction of the plume as small as 0.0005 could be examined by this technique. Lifetime measurements of the fluorescence of lithium and copper in the plume were obtained by varying the time between the firing of the microprobe laser and the fluorescence excitation laser. Three-dimensional scans of the fluorescence signals were completed for a spatial map of the plume. These studies were conducted in both argon and oxygen atmospheres. Since both copper and lithium react with oxygen, information about the mixing of the atmosphere with the plume was obtained. It was determined that for both lithium and copper analytes, 7x10¹³ analyte atoms were being ejected into the plume per shot. The detection limit for lithium was determined to be 10⁸ atoms. Studies indicate that significant numbers of lithium atoms are ionized in the plume. The effects of pre- and post-filter absorption were also studied. In addition to helping to improve analytical results these time and spatially resolved observations provide information about the chemical reactions that are taking place.
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