System of pulsed photoacoustics based on a TEA CO₂ laser applied to SF₆-N₂ mixtures

Abstract
A transversal single-mode TEA CO₂ laser, whose cavity design is a diffraction coupled resonator, tuned at the 10P(16) line, is used for photoacoustic detection of SF₆ traces in N₂. The acoustic cavity, resonant at a longitudinal mode, is designed on the basis of an electric analog model of a one-dimensional resonator. The acoustic signal from SF₆-N₂ and the signal from a laser energy detector enter simultaneously into a PC through its sound port. The photoacoustic signal, proportional to the average number of absorbed photons per molecule, is found to increase with the laser energy as a power of 0.5. The constant of the setup, previously measured from pulsed photoacoustics on NO₂-N₂ mixtures excited with the second harmonic of a Q-switched Nd:YAG laser, allows to quantitatively determine the SF₆ absorption cross section as a function of the CO₂ laser fluence. The calibration of this system is performed down to 150 ppbV of SF₆ in N₂ at atmospheric pressure for different values of laser energy.