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Title: Diode laser absorption studies of gas phase species
Author: Thornton, Lee James
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2006
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Sensitive and selective absorption spectroscopy techniques are applied to the detection of the excited species present in a range of low pressure inductively coupled plasmas (ICPs). The state densities and temperatures of various species are investigated across the parameter space accessible (plasma power and pressure) to aid in the understanding of the kinetic processes occurring. The experimental methods are based upon various forms of absorption spectroscopy, incorporating wavelength modulation and/or an optical enhancement cavity. The probing radiation is generated either directly using a CW diode laser or indirectly through the use of frequency conversion techniques. The absolute number densities of all four levels (1s2, 1s3, 1s4 and 1s5) present in the first excited manifold of atomic argon and neon are determined as a function of plasma operating conditions. A kinetic model is constructed to simulate these populations using cross-sections taken from the literature together with further measurements on the electron density and temperature obtained with a Langmuir probe. The model elucidates the importance of populational redistribution within the 1s manifold via excitation to the 2pn levels, and highlights the mechanism of radiative decay (with radiative trapping taken into account) as the ultimate loss route for the 1s manifold. Measurements are made using cavity enhanced absorption spectroscopy (CEAS) on the 2p5 and 2p6 state densities in argon in order to draw additional conclusions about the nature of the discharge and to verify the kinetic model. The populations of the 1s3 and 1s4 states are probed in a neon plasma with helium, argon and nitrogen as a dopant gas, with the aim of manipulating the EEDFs. The addition of N2 and Ar to the neon discharge resulted in a reduction in the 1s3 and 1s4 populations, while the addition of He resulted in an increase. These observations are consistent with a decrease and an increase, respectively, in the electron temperatures. The populations of the vibrational levels v = 0, 1, 3, and 6 of the A(3Σu+) state of molecular nitrogen are determined as a function of plasma operating conditions in a N2 discharge using CEAS. A selection of vibrational bands within the B(3Πg)←A(3Σu+) system are probed, with calibration achieved using cavity ring-down spectroscopy. At 25 mTorr and 200 W power the populations of the v = 0, 1,3, and 6 levels are (1.31 ± 0.16) × 1011 cm-3, (8.44 ± 1.01) × 1010 cm-3, (2.83 ± 0.34) × 1010 cm-3 and (5.27 ± 0.63) × 109 cm-3, respectively, corresponding to a vibrational temperature of 3600 ± 150 K. In addition, the observation of the N2+(X2Σg+) molecular ion in v = 0 using both CEAS and CEAS in combination with wavelength modulation spectroscopy is presented (which is found to improve the sensitivity for this measurement by approximately an order of magnitude). At 10 mTorr and 400 W the total population in N2+(X2Σg+, v = 0) is (1.26 ± 0.15) × 109 molecules cm-3, consistent with data obtained using a Langmuir probe. The density of oxygen atoms present in their ground state (3P2) is investigated using the technique of CEAS, and at 500 W and 100 mTorr the concentration is estimated to be (2.2 ± 0.3) × 1014 cm-3. This corresponds to a dissociation efficiency, δ, of O2 of 0.06. Furthermore, a difference frequency generation (DFG) system is constructed to generate radiation at 1.9 μm in order to probe the (0,0) band of the O2(b1Σg+←a1Δg) quadrupolar system. A minimum detectable absorbance of 1.3 × 10-5 over a 10 cm cell is determined by calibrating the system on an ammonia absorption, placing a limit of 1.8 × 1016 cm-3 on the total v = 0 population of O2(a1Δg) in a microwave discharge operating with 5 Torr pure O2.
Supervisor: Hancock, Gus Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Absorption spectra ; Chemical lasers