ANALYSIS OF THE PHOTOTHERMAL DEFLECTION TECHNIQUE IN THE SURFACE REFLECTION SCHEME - THEORY AND EXPERIMENT

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