Bulk rotational relaxation time and cross-section in gas mixtures

Abstract

In an earlier paper [M. Terzić, J. Jovanović-Kurepa, D.D. Markušev, J. Phys. B: At. Mol. Opt. Phys. 32 (1999) 1193] we have presented a method for evaluation of the bulk rotational relaxation times and cross-sections in gas mixtures from the measured saturation intensity, I s , vs. buffer gas pressure, p buff , at constant incident laser energy. According to this method, the bulk rotational parameters were determined from the slope, I s / Pbuff ("the slope method"). In this work, we report an alternative method to determine the same parameters, which does not require the knowledge of the functional dependence of I s vs. Pbuff . The method, which we propose, allows the evaluation of bulk rotational relaxation parameters from measured Is at a selected Pbuff . Hence, we termed this method "the point method". Both methods are tested with a calibrated photoacoustic technique for SF 6 -CH 4 gas mixtures at constant SF 6 gas pressure (0.47 mbar) and variable CH 4 gas pressure (0-140 mbar). The gas mixtures were irradiated by a pulsed 10P(16) CO 2 laser line in fluence range 0.1-0.5 J cm -2 . The accuracy of both methods is checked by quantitative comparison of the data with fitting laws. The result obtained shows that the proposed point method gives higher accuracy of the obtained parameters. The influence on the accuracy of the saturation intensity and of the cross-section of the absorbing molecule (SF 6 ) on rotational relaxation parameters are discussed also. © 2001 Elsevier Science B.V. All rigths reserved.