High-resolution nonlinear laser wave-mixing spectroscopy for gas-phase environmental and atmospheric studies.

摘要

Nonlinear wave mixing is presented as a sensitive and high-resolution technique for environmental and atmospheric studies. Coupling of the wave-mixing method to the graphite furnace, inductively coupled plasma, and hollow cathode discharge atomizers yields sub-parts-per-trillion (ppt) detection sensitivity levels with the spectral resolution needed to study hyperfine and isotope signatures.; Cesium is detected in liquid samples with an excellent concentration detection limit of 3.75 parts-per-quadrillion, i.e., ∼700 atoms inside the laser probe volume. To our knowledge, this is the best detection sensitivity obtained for cesium so far. In addition, wave mixing allows high spectral resolution that is suitable for measurement of radioactive cesium-137. Hyperfine scans collected from ICP and graphite furnace atomizers are least squares fitted to those simulated based on nonlinear optical coherence theory (NOCT).; A preliminary concentration detection limit of 318 ppb is determined for strontium in the graphite furnace using the weak intercombination line. The feasibility of in-situ analysis of radioactive strontium-90 is also discussed.; A backward-scattering wave-mixing setup is applied to non-resonant absorption measurements of non-metal species such as oxygen and chlorine. These excited-state transition lines are normally not available for conventional atomic spectroscopic methods since they lay several eV above the ground state. Taking advantage of unique features of a hollow-cathode discharge and a wave-mixing setup, one can excite these non-resonant transition lines with user-friendly red/IR diode lasers instead of bulky VUV or UV sources. Preliminary concentration and mass detection limits of 11 ppmV and 14 attomole, respectively, are determined for atomic oxygen. Wave mixing allows better sensitivity and a more direct measurement of atomic oxygen in the low ppm range.; Nonlinear laser wave mixing offers excellent detection sensitivity while yielding a spectral resolution that is suitable for isotope and hyperfine analyses. The technique promises many potential applications for atmospheric, forensic and environmental studies including detection of isotope and hyperfine signatures of oxygen, chlorine, and other non-metal species.