The Reverse Mirage Effect: Catching the Thermal Wave at the Solid/Liquid Interface

Abstract

Detection of species at the solid/liquid interface using infrared spectroscopy is severely limited by the opacity of most liquids to the infrared beam. In this work we use a variant of the photothermal beam deflection (“mirage effect”) method to avoid this problem. With this variant of the method (the “reverse mirage effect”), the IR beam passes through a transparent solid first, and then is absorbed by a liquid medium or by chromophoric species at the solid/liquid interface. The probe laser beam grazes the nonilluminated (back) surface of the solid and is deflected by the thermal gradient in the liquid. Results are presented that were obtained with the use of the reverse mirage technique with single-crystal silicon as the transparent solid and the use of pure acetonitrile as the absorbing sample and beam deflection medium. Studies of the position of the laser probe beam center with respect to the Si/CH₃CN interface reveal interesting qualities about photothermal detection within the absorbing medium. The resulting spectra are analyzed in terms of the Rosencwaig-Gersho model.