Diode laser absorption studies of gas phase species

• Main Author: Thornton, Lee James
• Other Authors: Hancock, Gus
• Published: University of Oxford 2006
• Subjects: 530.44; Absorption spectra : Chemical lasers
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.442951

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 2p_n 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(³Σ_u⁺) 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(³Π_g)←A(³Σ_u⁺) 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⁺(X²Σ_g⁺) 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(b¹Σ_g⁺←a¹Δ_g) 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(a¹Δ_g) in a microwave discharge operating with 5 Torr pure O₂.