Photoacoustic spectroscopy on trace gases with continuously tunable CO2 laser

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 (O-3), 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. (C) 1996 Optical Society of America