Photothermal beam deflection using the reverse mirage geometry: Theory and experiment
This paper presents a mathematical model and supporting experimental data for the reverse mirage effect with absorption in the deflecting medium. The reverse mirage effect is defined as transverse photothermal beam deflection with the probe laser on the nonilluminated side of the interface. The model takes into account the Gaussian distribution of intensity in the probe beam, and is used to consider the phase of the photothermal beam deflection (PBD) response. The results of the mathematical treatment are shown to be in good accord with experiment. The theory shows, in agreement with Bićanić, that a plot of the natural log of the response magnitude versus probe distance is linear, and has a slope equal to the absorptivity. As an example of its application, this relationship is used to obtain absolute absorption coefficients in the mid-infrared for acetonitrile from step-scan Fourier transform ir PBD data. The theory is used to show that the largest absorptivity that can be measured by this method is approximately 650 cm-1. The phase is shown to exhibit monotonic behavior to a larger absorptivity, estimated to be greater than 3000 cm-1. The designs of the PBD accessory and sample cells are described in some detail, and a number of experimental considerations are discussed.
Manning, CJ; Palmer, RA; Chao, JL; Charbonnier, F
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