Date of Award
Doctor of Philosophy
Edward S. Yeung
Laser microprobe analysis (LMA) offers good spatial and depth resolution for solid sampling of virtually any material. Coupled with numerous optical spectroscopic and mass spectrometric detection methods, LMA is a powerful analytical tool. Yet, fundamental understanding of the interaction between the laser and the sample surface leading to the formation of the high temperature plasma (plume) is far from complete;To better understand the process of plume formation, an imaging method based on acousto-optic laser beam deflection has been coupled with light scattering methods and absorption methods to record temporal and spatial maps of the particle and molecule distributions in the plume with good resolution;Because particles can make up a major fraction of the vaporized material under certain operating conditions, they can reflect a large loss of atomic signal for elemental analysis, even when using auxiliary excitation to further vaporize the particles. Characterization of the particle size distributions in plumes should provide insight into the vaporization process and information necessary for studies of efficient particle transfer. Light scattering methods for particle size analysis based on the Mie Theory are used to determine the size of particles in single laser-generated plumes. The methods used, polarization ratio method and dissymmetry ratio method, provide good estimates of particle size with good spatial and temporal resolution for this highly transient system. Large particles, on the order of 0.02-0.2 [mu]m in radius, were observed arising directly from the sample surface and from condensation;Due to its ability to induce a rapid temperature increase at surfaces, LMA is used to promote desorption over decomposition for large thermally labile bio-organic compounds. Extremely large molecules can then be studied in the gas phase and detected with sensitive mass spectrometric methods. The desorption process is poorly understood. Molecular absorption is used to record the desorption of organic dyes from thin films. Classical absorption at the vaporization wavelength by the sample and power density of the vaporization laser is found to affect the extent of chromophore destruction. The major fraction of fragmentation appears to occur early (<100 [mu]s) in the desorption process.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Sandra Marshall Kimbrell
Kimbrell, Sandra Marshall, "Diagnostic probes for particle and molecule distributions in laser-generated plumes " (1990). Retrospective Theses and Dissertations. 9519.