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A novel photoacoustic (PA) system that uses a continuously tunable high-pressure CO2 laser as radiation source is presented. A minimum detectable absorption coefficient of 10−6 cm−1 that is limited mainly by the desorption of absorbing species from the cell walls and by residual electromagnetic perturbation of the microphone electronics has currently been achieved. Although a linear dependence of the PA signal on the gas concentration has been observed over 4 orders of magnitude, the dependence on energy exhibits a nonlinear behavior owing to saturation effects in excellent agreement with a theoretical model. The calibration of the laser wavelength is performed by PA measurements on low-pressure CO2 gas, resulting in an absolute accuracy of ±10−2 cm−1. PA spectra are presented for carbon dioxide (CO2), ammonia (NH3), ozone (O3), ethylene (C2H4), methanol (CH3OH), ethanol (C2H5OH), and toluene (C7H8) in large parts of the laser emission range. The expected improvement in detection selectivity compared with that of studies with line-tunable CO2 lasers is demonstrated with the aid of multicomponent trace-gas mixtures prepared with a gas-mixing unit. Good agreement is obtained between the known concentrations and the concentrations calculated on the basis of a fit with calibration spectra. Finally, the perspectives of the system concerning air analyses are discussed.
© 1996 Optical Society of America
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