The search for metastables and molecular ions in discharges

Abstract

Improvements to and use of an existing Raman Induced Kerr Effect (RIKE) spectrometer [Bhatia et al., J. Opt. Soc. Am. B, 14(2):263–270, February, 1997.] are described. Primary improvements were the use of wedged windows on the sample chamber, a new method of monitoring birefringence, and the addition of a photomultiplier tube (PMT) and double monochromator for monitoring Coherent Anti-Stokes Raman Spectroscopy (CARS) signals. This spectrometer is controlled through a Computer Automation and Control (CAMAC) crate. The construction and operation of a Linear Discharge Cell (LDC), a High Voltage Constant Current Sink for consistent operation of the LDC, and a Transverse Electric Atmospheric (TEA) discharge chamber are described in detail, as is synchronization of the pulsed discharge in the TEA with the pulsed output of the YAG laser using Hewlett-Packard Versatile Link fiber optic components. The atmospheric gases oxygen, carbon dioxide, and nitrogen were investigated with CARS in both discharge and non-discharge conditions. The influence of nuclear spin on the spectra and line strengths observed for all three gases is discussed. The origins of oxygen’s triplet ground state are discussed as well as simultaneous transitions in the visible of two colliding, excited oxygen molecules whose individual energies are in the infrared. The oxygen metastable singlet delta was observed, though with insufficient signal-to-noise ratio to extract molecular constant information. Also discussed for carbon dioxide are the profusion of state naming conventions, Fermi splitting, the calculation of the temperature of the discharge, quantum interference in the change of relative intensity of the two peaks in the ν1/2ν2 Fermi dyad from non-discharge to discharge conditions, and upper level hotband lines that appear when the discharge is turned on. Quantum interference in carbon dioxide was consistently observed in the LDC but not in the TEA discharge, most likely because the amount of power dissipated in the TEA was on the order of 1% of that dissipated in the LDC and the gas temperature was much lower. The molecular radical N3 was sought without success, though spectrometer characteristics set an upper bound on its concentration in the discharge.