Development of Laser Spectroscopic Techniques for Trace Element Analysis and Combustion Diagnostics
Abstract: This thesis deals with the development of highly sensitive laser-based spectroscopic techniques for trace-element analysis and combustion diagnostics. The work consists of both theoretical as well as experimental applications of the interaction between laser light and atoms or molecules.The theoretical part is concerned with a development of the Density-Matrix formalism for description of one-and two-step excitations of atoms with degenerate states in high collisional media (e.g. GHz) by linearly polarised laser light of arbitrary bandwidth (e.g. GHz). This new treatment can take into account the influence of the laser light upon the atomic system, an effect that is neglected by the Rate-Equation formalism normally used for description of laser excitation of atoms in high collisional media. This implies that effects such as dynamic Stark broadenings, splits, and shifts as well as two-photon transitions can now be accurately taken into account. The treatment has furthermore allowed a reduction of the general system of Density-Matrix equations (n2 first-order coupled linear differential equations, where n is the number of states included in the system) to a time-dependent system of three first-order linear differential equations describing a partly steady-state situations. In addition, the full temporal solution for this type of system is also derived and a number of computer simulations describing some physical phenomena connected with two-step excitation of atoms is presented. The experimental work involves the application of three laser-based spectroscopic techniques, Laser-Enhanced Ionisation (LEI), Laser-Induced Fluorescence (LIF), and Degenerate Four-Wave Mixing (DFWM), to studies of fundamental atomic properties, ultra-sensitive trace-element analysis and non-intrusive combustion diagnostics. The LEI technique has been used for identification of high-lying states in Fe I and for the determination of atomic metastable state lifetimes in flames. LIF in Graphite Furnace (LIF-GF) has been developed and used for sensitive trace-element analysis. This includes a determination of ultra-low (pg/ml) concentrations of Tl in natural waters from the Göteborg area and a determination of the Al and Pb content in aerosol samples from the Arctic. An investigation of the possibility to apply the LIF-GF technique to the analysis of ultra-low concentrations of Ti in biological tissue is also included. In the area of combustion analysis, the possibilty to utilise the lifetimes of atomic metastable states as probes for local stoichiometric conditions in flames by either the LEI or the LIF technique has been investigated. In addition, some fundamental properties of the application of the non-linear laser-based spectroscopic technique, DFWM, to combustion analysis have been examined. This work consists of a development of 2-step DFWM - a technique that can reduce pre- and post-filtering effects that might influence measurements in optically thick media - and an investigation of the dependence of the DFWM signal strength on buffer gas pressure and temperature, properties of importance for the utilisation of the technique to diagnostics of combustion processes in engines.
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