- Quantification of Laser-Induced Breakdown Spectroscopy at Low Energies
- Taschuk, Michael Thomas
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- Laser-induced breakdown spectroscopy (LIBS) is an elemental characterisation technique using spectrally resolved emission froma laser-induced plasma to determine the composition of a sample. Due to limited quantitifcation of emission levels, in the literature, it is difficult to compare results between different lab groups, and only qualitative comparisons can be made with theory. As a result, understanding of the underlying physical processes which govern LIBS has lagged the growth of applications. Most applications of the LIBS technique have employed laser pulse energies in the range of 10 - 100mJ, focal spot sizes of ∼ 100 μm, and an accumulation of 10 - 100 spectra for a single measurement. The high energies, large focal spots and number of shots acquired improves the sensitivity of LIBS. The primary focus of this thesis is the quantification of the LIBS technique, LIBS equipment and the extension of LIBS to much lower pulse energies. This new regime, referred to as μLIBS, utilises pulse energies below 100 μJ. In this thesis a theory of LIBS detector systems is developed, and used to define responsivity, noise-equivalent integrated spectral brightness and noise-equivalent spectral brightness in terms useful for the LIBS experimentalist. Four LIBS detection systems have been characterised. Laser ablation plume dynamics and absolute emission levels from a millijoule energy LIBS system were studied and compared with simple physical models for shock wave expansion and stagnation. A simple model for the emission is compared with the absolute emission levels of the LIBS plasma. The scaling of LIBS emission below 100 μJ pulse energies is investigated. The number of photons emitted is found to be a small fraction of the number of atoms ablated for the energy range between 100 nJ and 100 μJ. Using a thin film target, it is found that the ablated region which contributes to the LIBS emission is restricted to a layer much shallower than the ablation crater. Finally, two applications of the μLIBS technique are presented. Surface mapping of Al alloys with sub-microjoule laser pulses is demonstrated. Latent fingerprint detection and imaging is demonstrated using the μLIBS technique
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