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Title: Optical spectroscopy with continuous-wave lasers
Author: Thomas, D. W.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 1998
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The principles behind the production of laser beams and their important characteristics are outlined briefly in the first introductory chapter. In chapters 2 and 3, and theory is developed for the studies of the transmission and Raman scattering behaviour of aligned liquid crystal molecules with respect to laser beams of well-defined polarisations. The sample studied was a mixture of cyanobiphenyls, LCE7, and was aligned using the technique of Linear Photopolymerisation, LPP. Concentration-Modulated Absorption Spectroscopy, COMAS, a pump-probe technique is described in chapters 4 - 7. In chapter 4, the principles are described briefly for both pulsed and continuous-wave laser beams for 2-level kinetic schemes. In this thesis, the theory is developed for 3-level schemes, which allows the determination of the re-population lifetimes of species in the range ca. 10ms - 10ns, by measuring the COMAS "gain" as a function of the pump beam modulation frequency. The optical COMAS systems are described in chapter 5. The triplet lifetimes of the Xanthene dyes Rose Bengal and Eosin Y in ethanolic solutions have been measured in chapter 6 and are in general agreement with literature values. Using experimental data and the COMAS calculations it is possible to determine the absolute absorption cross-sections of the dye molecules at the probe laser wavelength. In chapter 7 the 3-level scheme has been modified to consider the re-population kinetics of Neon atoms in a hollow cathode discharge. The studies show that there exist two rates of repopulation for the lower level of the transition monitored, which give rise to two "gain" components with distinct rates of re-population ca. 2x105s-1 and 5x107s-1. It is shown that by taking into account the prominent role of saturation, it is possible to measure accurately species concentrations and absorption cross-sections.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available