Sensitive multi-photon nonlinear laser spectroscopic methods for isotope analysis in atmospheric and environmental applications

Multi-photon nonlinear laser wave-mixing spectroscopy is presented as a sensitive optical detection method for environmentally hazardous gases with isotopelevel spectral resolution. When compared to other detection techniques, degenerate four-wave mixig offers unique advantages including high spatial resolution, excellent sensitivity and small laser probe volumes that are suitable for diagnostic studies. Unlike those in conventional optical methods, the wave-mixing signal is a coherent laser-like beam, and as such, it can be spatially and optically modulated to enhance signal-to-noise ratios. This signal also has a quadratic dependence on analyte concentration, making it a powerful tool for detecting small changes in analyte properties. In addition, the nonlinear signal has a cubic dependence on laser power, which provdes excellent detection sensitivity as well as the ability to use low-power laser sources such as portable solid-state laser diodes.;Four-wave mixing is demonstrated as a powerful tool for in-situ diagnostics. This novel optical setup allows simultaneous collection of signals from two distinct wave-mixing geomtries in a common analytical atomizer (radio-frequency inductively coupled plasma torch). Experimental data fit well to those calculated based on an iterative perturbation model, resulting in reliable determination of temperature and electron density levels for the analytical zone of the ICP atomizer. Three dimensional mapping yields temperature levels ranging from 3,500 to 14,000 K +/- 150 K and electron density levels from 6.1 (+/- 0.3) x 1015 cm-3 to 10.1 (+/- 0.3) x 1015 cm-3 with 5 % uncertainty.;Nonlinear wave mixing is also used for sensitive detection of atomic oxygen and chlorine with isotope or hyperfine spectral resolution. Preliminary mass detection limits of 3.48 picograms and 2.2 picograms are reported for oxygen and chlorine, respectively. Isotope and hyperfine splittings for chlorine-35 and chlorine-37 are obtained from a sample of the environmental contaminant, Freon 113.;Mid-infrared laser absorption techniques are applied to molecular detection of environmental pollutants using an 8 mm quantum cascade laser. Nitrous oxide and CFC Freon 113 are detected in their native forms at room temperature and room pressure. A preliminary concentration detection limit of 3 pptv (parts per trillion by volume) is determined for gas-phase acetone analytes.