<p> A detailed experimental and theoretical study has been performed on several different photochemical systems. Lead-salt tunable diode lasers operating in the infrared region have been used as diagnostic tools to probe the molecules in these gaseous systems. Knowledge of these systems is expected to be useful in evaluating future schemes of laser isotope separation.</p> <p> In the course of this work a computerized digital technique has been developed that allows molecular parameters such as linestrength and linewidth to be obtained by measuring the infrared absorption on vibrational-rotational transitions of the molecule. Molecular concentrations can then be determined enabling one to gain valuable insight into the chemical processes occurring in the system.</p> <p> The digital technique was first tested on CO2 gas in a multi-pass White cell to verify the validity of the measurements. Subsequently, measurements were performed on mixtures of NH3/N2, NH3/Ar, HTO/H2O, and HTO/air. Those NH3 measurements that could be compared to previous measurements were found to be very accurate. For the HTO system, no previous measurements on linestrength and linewidth for the transitions examined have been performed.</p> <p> Described next is the application of the tunable diode laser diagnostic system to the investigation of infrared multiphoton dissociation of deuterated chloroform immersed in a chloroform bath. The sensitivity of the technique allowed for the measurement of the few parts per million of DCl formed by the photolysis of natural abundance CDCl3 in CHCl3.</p><p> In addition, the feasibility of transient detection with tunable diode lasers was examined. High fluence CO2 laser pulses were used to dissociate C3F6 or C2F3Cl and create CF2 radicals. Current-modulation of the tunable diode laser made it possible to monitor the transient CF2 radicals as they were formed, and as they subsequently decayed. The sensitivity of the transient detection technique was found to be limited by detector noise.</p> / Thesis / Master of Science (MSc)

The work described in this thesis involves the measurements of the optical gain and amplified spontaneous emission (A.S.E) spectrum of Pb1-xSnxTe epilayers and the establishment of conditions under which optically pumped Pb1-xSnxTe laser using a CO2 laser as pump source can be produced. Pb1-xSnxTe epilayers have been grown by a hot wall epitaxy (HWE) technique on BaF2 single crystal substrates and the optical gain which can be produced in these layers has been measured by pumping the films transversely with a N2 laser. A model for optical gain and stimulated emission as a function of pump intensity has been developed which has permitted for the first time in these materials, a direct comparison between the magnitude of the gain pumping rate, and the optical gain generated. The measured optical gain is in very good agreement with the model predicted gain. Good fits to the measured stimulated emission spectra were also obtained from the model prediction. It is shown that the gain for a given pump wavelength has a drastic dependence on the material doping density. According to the model, CO2 laser optically pumped Pb1-xSnxTe laser can be readily achieved, provided that epilayer doping densities can be reduced to values of 1017 cm-3 or less. Nevertheless, doping densities even in nominally undoped layers are generally at least an order of magnitude too high. In some initial attempts to achieve lower doping densities, using a thermal annealing technique, doping densities as low as 2 x 10 17 cm-3 have been obtained and significant pump absorption was achieved at CO2 laser wavelength, as predicted by the model. / Thesis / Master of Engineering (ME)

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