Analysis of the photothermal deflection technique in the surface reflection scheme: Theory and experiment

The photothermal deflection technique has been usually applied, for the thermal diffusivity measurements, in the transverse skimming scheme. To overcome some limitations of the skimming, a surface reflection scheme (i.e., bouncing scheme) has been introduced in which the probe beam is reflected from the sample surface. In this configuration the probe beam deflection is obtained as a result of two different mechanisms: the thermal gradient in the gas near to the heated sample (mirage) and the sample surface deformation due to the thermal expansion (displacement). The superposition of these two effects must be taken into account when deriving the thermal diffusivity. In this article the mirage and the displacement have been studied from a theoretical and experimental point of view, and a new method for the measurement of thermal diffusivity in the bouncing scheme is presented. A special setup is described to obtain separately the mirage and the displacement signals from which the thermal diffusivity and the thermal expansion coefficient can be derived. The experimental values for different samples obtained by applying our method are in agreement with the literature values. (C) 1998 American Institute of Physics. [S0021-8979(98)00102-9].