Abstract

Photothermal studies of molecular dynamics in gases and gas mixtures have been performed by means of the photothermal beam deflection technique. The experimental setup employs a continuously tunable high pressure CO2 laser with a narrow linewidth of 0.017 cm^-1 and energy fluences up to 100 mJ cm^-2 for exciting the molecules to specific energy levels, and a collinear HeNe laser as probe beam [1]. The deflection of the probe beam is monitored with either a position sensor or the knife edge technique with a temporal resolution of ca. 0.1 μs. Measurements have been performed for different gases, gas pressures, pump beam diameters and distances between pump and probe beams. New experimental results are discussed for the excitation of the ν₃ mode of SF₆ as well as the ν₇ mode of ethylene (C₂H₄) and the structurally similar vinyl chloride (C₂H₃Cl) and vinyl bromide (C₂H₃Br). These compounds were diluted in nonabsorbing buffer gases such as Kr, Ar, He, N₂ and in mixtures of them. The measured signals are analyzed numerically with a model that considers a new “three-species” approach (excited and nonexcited absorbing molecules and nonabsorbing buffer molecules [1]. This model is generally applicable and valid for all time regimes. From such analyses nonradiative molecular deactivation rates k_e as well as thermal conductivity and sound speed data of the gas mixtures are obtained simultaneously. As example the photothermal beam deflection signals are plotted for C₂H₄ and C₂H₃Br in Figs. A and B. The derived rates k_e are generally in good agreement with literature data which demonstrates the powerful potential of this straightforward technique. However, in many cases, e.g. for C₂H₃Cl, C₂H₃Br, mixtures of gases or of buffer gases, our results are the first data to become available. Figure C shows an example for the deactivation rate k_e of C₂H₄ buffered in a varying mixture between Ar and Kr.

© 1998 IEEE