Abstract

The absorption of pulsed IR radiation by a gas or gas mixture creates photoacoustic (PA) and photolhermal (PT) effects. We have previously applied PA spectroscopy to successfully analyse gas mixtures with a tunable CO₂ laser. Our home-built CO₂ laser operated at ca. 10 bar total pressure allows continuous tuning across a total range of 76 cm^-1 within the 9R, 9P, 10R and 10P branches between 9.2 and 10.8 μm with a linewidth of only 0.017 cm^-1 [1]. Here we focus on the time dependence of the generation of the PA and PT signals and study the transfer of the excitation energy within the gas molecules in detail. For this purpose the photothermal beam deflection or thermal lensing technique [2,3] has been employed. The laser beam is slightly focused with a long focal length mirror to a diameter of 3.5 mm and directed through a cell containing the gas mixture under study. The beam of a HeNe laser as probe beam is directed through the cell collinearly at a variable distance d to the CO₂ laser pump beam. The probe beam deflection is measured with a position sensor with a spatial resolution of 10 μm and temporal resolution of presently 0.5 μs. Various gases and gas mixtures have been studied. As an example typical results obtained for SF₆/Ar mixtures are illustrated in Figs. 1 to 3.

© 1996 IEEE