Remote Detection of Trace Species for Combustion and Atmospheric Magnetometry

Abstract

This thesis effort focuses on extending the development of radar REMPI, a laser-based

diagnostic, for concentration measurements. Radar REMPI combines the selective ionization

of resonance enhanced multi-photon ionization spectroscopy (REMPI) with the high detection

sensitivity afforded by coherent microwave scattering (radar). The incorporation of

microwaves not only permits sub-ns temporal resolution of the concentration measurement

but also supports long distance propagation required for remote sensing applications. Here,

the development of radar REMPI is facilitated by evaluating its performance with respect to

two different applications, namely, the measurement of combustion species and the use of

trace species for atmospheric magnetometry. While the underlying physics of both these

fields are relatively distinct, they share a common need for trace species detection in

environments with a multitude of buffer gases. A primary contribution of this thesis is the

observation that the amount and type of buffer gas has a significant impact on both the

amplitude and temporal dynamics of the radar REMPI signal. Measurements of radical

species in a combustion environment have been conducted by probing atomic oxygen (O) in a

well-calibrated flame produced by a Hencken burner. Absolute O concentrations are obtained

via calibration against an inert gas (i.e. xenon (Xe)) with a similar REMPI scheme. Buffer

gas effects are avoided by seeding known amounts of Xe into the flame and performing the

calibration in-situ. The results generally agree well with their expected equilibrium

concentrations, but display an overshoot in both the fuel-rich and fuel-lean regimes, a

phenomenon which is likely to be due to laser photolysis. A framework for utilizing the 129Xe

isotope as the magnetic medium for atmospheric magnetometry has been proposed and

partially demonstrated. The concept involves spin polarizing and measuring the long-lived

magnetic field precession of 129Xe, which has a native concentration of about 24 ppb in the

atmosphere. Suitable two-photon Xe transitions have been identified and detection

sensitivities of < 150 ppb in pure N2 and < 1.5 ppm in air have been successfully realized

using radar REMPI. A spin polarization scheme using two-photon pumping on a deltaJ=0

transition around 249 nm has also been demonstrated.