Abstract

The aim of this work is to present the data regarding the measurements of the vibrational-translational time constant (τ_V-T) for the vibrational relaxation of SF₆molecule by Ar as the buffer gas. The applied experimental apparatus¹was the time resolved photoacoustic technique simultaneously used with transmission spectroscopy technique, which are rather powerful tools for V-T relaxation investigations.^2-4 Such experimental device is than with high sensitivity and reveals the final deposition of the laser energy into translational modes of the bath gas. This is the case actually whenever is justified to apply the results obtained from the linearized acoustic wave equation.²Namely, by determination of ${\text{P}}_{\text{a}}^{-}/{\text{P}}_{\text{a}}^{+}$ ratio of the first detected photoacoustic signal (P_a) in some media, the correlation with time constant τ_V-Tcould be obtained. ${\text{P}}_{\text{a}}^{-}$ and ${\text{P}}_{\text{a}}^{+}$ are the amplitudes of rarefaction and condensation picks of the measured photoacoustic signal. The ratio ${\text{P}}_{\text{a}}^{-}/{\text{P}}_{\text{a}}^{+}$ actually depends on the quantity ϵ, which is defined²as the ratio of the time that the sound wave takes to cross the radius of the excited volume (τ_sonic) to the vibrational relaxation time (τ_V-T). It is proposed²that for every experimental apparatus the ${\text{P}}_{\text{a}}^{-}/{\text{P}}_{\text{a}}^{+}$ dependence of ϵ and should be found by using Green's function method to solve linearized acoustic wave equation, assuming the following: (1) the precise knowledge of the diameter of the irradiated volume in front of the photoacoustic detector; (2) the distance of the detector from the axis of the laser beam; (3) "tophat" geometry for the laser beam intensity; and (4) an exponentially decaying energy source. When ${\text{P}}_{\text{a}}^{-}/{\text{P}}_{\text{a}}^{+}$ correlation with ϵ is determined, the τ_V-Tshould be obtained straightforward from the experimental data for investigated media, if the all parameters of the setup are precisely known. On the other hand, it is possible, too, to calibrate the used apparatus by an empirical normalization procedure,²i.e., to determine the correlation ${\text{P}}_{\text{a}}^{-}/{\text{P}}_{\text{a}}^{+}$ vs τ_V-Tfrom the known relaxation rate constant from investigated media. Then, this calibration curve is valid for all absorbing species in the same (calibrated) bath gas.

© 1994 IEEE