New Aspects of Buffer Gas Effects in Resonant Spectrophones

Abstract

In studies of gas phase photoacoustic spectroscopy, the gas whose spectrum is being investigated is generally one of several present in the sample, and often only in trace amounts. The response of the spectrophone is determined by the characteristics of the sample gas as a whole, and depends on the various thermal and molecular relaxational properties of the gases present. In particular, for an acoustically resonant spectrophone, important characteristics will include the sound velocity, heat capacity, thermal conductivity, viscosity, and the energies and relaxation times of the molecular vibrations. The sound velocity determines the resonant frequencies of the cavity, while the other parameters govern the loss mechanisms that determine the quality factors of the resonances, and also cause small shifts in the resonant frequencies. In an earlier study,1 the resonant frequencies and quality factors of acoustical resonances were determined for various buffer gases at atmospheric pressure, and the results were compared to theoretical predictions2 based on classical surface viscous and thermal losses. Significant discrepancies were observed for all non-noble gases.3 It is the goal of this work to investigate the pressure dependent behavior of the spectrophone, and to incorporate molecular relaxation effects into the theoretical interpretation of that behavior. We have made a number of extensions and improvements in experimental and analytical technique. The spectrophone used was a stainless steel cylinder polished to a 1/3 micron surface finish to guarantee well defined boundary layers whose losses could be calculated accurately from theory. The spectrophone and vacuum system were bakeable to reduce impurity effects due to outgassing. Whereas in the earlier experiment a large concentration of the optically absorbing component (9000 ppm of CH4) was used, we kept the absorber concentration small (50 ppm of C2H4) so as not to disturb the properties of the buffer gas. Use of an electro-optic modulator system allowed access to modes at much higher frequencies, thus enabling the study of lighter gases. We have measured the resonant frequency and quality factor as a function of pressure from atmopsheric pressure down to 10 torr or lower for each buffer gas. Data analysis consisted largely of nonlinear least squares curve fitting of the resonant response curves, which gave more accurate determinations of the resonant frequency and Q than the half power point method used in the previous experiment.