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.
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