Nonlinear laser wave mixing is a versatile spectroscopic method for trace elemental analysis at high spectral resolution. Sub-Doppler spectral resolution allows isotope and hyperfine structure measurements of some of the elements even when using a room-pressure analytical flame (i.e., sub-Doppler but Lorentzian broadened spectra). A non-planar wave-mixing optical setup offers some advantages as compared to the conventional planar wave-mixing setup including better signal collection efficiency and easier optical alignment. Using our absorption-based wave mixing, a detection limit of 0.05 ng/mL (i.e., 50 parts-per-trillion) is reported for Rb in an air/acetylene flame, while still maintaining sub-Doppler spectral resolution for the infrared 780.0-nm Rb transition line.
Doppler-free flame spectrometry based on resonant degenerate four-wave mixing (D4WM) is shown to be an effective analytical tool for the determination of barium at high spectral resolution. Optical phase conjugation is produced in an air/acetylene flame, resulting in an optical signal that is a coherent time-reversed replica of the probe beam. The degenerate four-wave mixing spectrum of barium for the 6 s21 S0-6 s6 p1 P1 transition at 18,060.19 cm−1 is collected with a linewidth of 0.08 cm−1 (FWHM) with the use of a convenient continuously flowing atmospheric-pressure flame atomizer. A preliminary detection limit of 5 μg/mL barium, while a Doppler-free linewidth is maintained at 0.08 cm−1, is reported.
Resonant degenerate four-wave mixing is presented as an unusually sensitive nonlinear laser method that yields Doppler4ree spectral resolution at trace-concentmtion levels even when using low laser power levels. Using a non-planar four-wave mixing optical setup, one can extract the signal beam from the input beams more easily, and hence, suppress the background noise more effectively and improve signal-to-noise ratios. Optical alignment is simple and convenient for this multi-photon setup. Sub-Doppler spectral resolution allows reliable measurement ofmany isotope and hyperfine lines using room-pressure flame atomizers, low-pressure discharge atomizers or room-pressure graphite furnace atomizers. While maintaining sub-Doppler spectral resolution, four-wave mixing still yields parts-per-trillion level detection sensitivity using these popular analytical atomizers. While flame atomizers offer convenient and fast sample introducüon, low-pressure discharge atomizers offer better spectral resolution (i.e., sub-Doppler plus sub-Lorentzian), and graphite furnace atomizers yield lower atomizer background noise. Since laser power requirements are low (e.g., mW for CW lasers and n.J for pulsed lasers), many compact lasers (e.g., solid-state lasers) could be used in this simple yet sensitive nonlinear laser method.
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