In this paper we extend the theory of photothermal deflection spectroscopy for an isotropic film-on-substrate system to include the thermal contact resistance between the two materials and absorption of energy in the film and substrate. The model is formulated as a three-domain system (gas, film and substrate) with coupling conditions at the various interfaces, including a thermal contact resistance. Closed form expressions are obtained for the temperatures in each domain.

The analysis for probe beam deflection is confirmed by comparison to well-known limits of infinite film thickness, zero film thickness, zero contact resistance, and a thin absorbing layer at the surface of the film. The formulations are tested against NIST standard reference materials (SRM) using numerically generated beam deflection data to extract thermal diffusivity of a bulk material, and of two SRMs pressed together to extract thermal contact resistance. The results show the feasibility of using to determine the thermal contact resistance of a layered sample from beam deflection data.

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