PHOTOACOUSTIC-SPECTROSCOPY ON TRACE GASES WITH CONTINUOUSLY TUNABLE CO2-LASER

A novel photoacoustic (PA) system that uses a continuously tunable hig h-pressure CO2 laser as radiation source is presented. A minimum detec table absorption coefficient of 10(-6) cm(-1) that is limited mainly b y the desorption of absorbing species from the cell walls and by resid ual electromagnetic perturbation of the microphone electronics has cur rently been achieved. Although a linear dependence of the PA signal on the gas concentration has been observed over 4 orders of magnitude, t he dependence on energy exhibits a nonlinear behavior owing to saturat ion effects in excellent agreement with a theoretical model. The calib ration 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), o zone (O-3), ethylene (C2H4), methanol (CH3OH), ethanol (C2H5OH), and t oluene (C7H8) in large parts of the laser emission range. The expected improvement in detection selectivity compared with that of studies wi th line-tunable CO2 lasers is demonstrated with the aid of multicompon ent trace-gas mixtures prepared with a gas-mixing unit. Good agreement is obtained between the known concentrations and the concentrations c alculated 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