Modelling and analysis of experimental photothermal beam deflection signals in gases

Photothermal beam deflection is a powerful tool for studying molecular dynamics in gases. We present a new theoretical model that extends the versatility of the technique substantially. The model yields excellent agreement with experimental temporal signal shapes obtained with a continuously tunable CO2 laser end C2H4 or SF6 gas buffered in Ar or He/Ar mixtures. In agreement with experimental data, the derived first-order rate coefficient k(e) or the molecular relaxation time tau(relax) are strongly affected by the change of the Ar content in the buffer gas, an experimental regime that could not be treated with previous models. Finally, we present a concept that permits to find out the shortest tau(relax) that can be determined from experimental signal shapes with a specified accuracy and signal-to-noise ratio, or vice versa, to quantify optimum pump beam radii and pump-probe beam distance in a certain experiment. (C) 1998 Elsevier Science B.V.