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A model has been developed for photothermal refraction using flowing liquid samples. In photothermal refraction, a cylindrical thermal lens is formed within the sample because of the temperature rise produced by the absorbance of a pump laser beam. This cylindrical thermal lens is intersected at right angles with a second probe laser. For static samples, the maximum signal results when the pump and probe beams are coplanar. Defocusing of the probe beam by the cylindrical thermal lens is detected as a change in the far-field probe beam center intensity. Flow acts to distort the temperature distribution by transporting heat down stream. For flowing samples, the optimum signal is found when the probe beam is located about one pump beam spot size down stream from the pump beam axis.
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Weimer et al. (1985) studied this question.
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