This analytic technique is based on the illumination of a surface by infrared, visible or ultraviolet light; the generation of a small temperature gradient due to the heat released when the light is absorbed at the surface, and the deflection of a probe beam by the refractive-index gradient that accompanies the local temperature gradient. Here is a schematic diagram that illustrates the deflection principle:
Modern light-beam-position detectors are sensitive to nanoradian deflections of a probe beam, making this technique extremely powerful. It's most important feature, however, is its ability to detect surface species in situ, that is within a bounding liquid. Here is a photograph of an apparatus recently constructed at the Lawrence Berkeley National Laboratory:
It uses an infrared light source (cylinder at the top of the photo), a He-Ne laser as the probe-beam source (lower left), a sensitive detector (lower right), and associated optics and enclosures. Literature references to this work are listed below:
Z. Deng, J.D. Spear, J.D. Rudnicki, F.R. McLarnon and E.J. Cairns, "Infrared Photothermal Deflection Spectroscopy: A New Probe for the Investigation of Electrochemical Interfaces," J. Electrochem. Soc., 143, 1514-21 (1996).
J.D. Rudnicki, F.R. McLarnon and E.J. Cairns, "In Situ Characterization of Electrode Processes by Photothermal Deflection Spectroscopy," in Techniques for Characterization of Electrodes and Electrochemical Processes, R. Varma and J.R. Selman, eds., John Wiley and Sons, NY, and The Electrochemical Society, Pennington, NJ, pp. 127-66 (1991).
J.D. Rudnicki, G.M. Brisard, H.A. Gasteiger, R.E. Russo, F.R. McLarnon and E.J. Cairns, "Effect of the Supporting Electrolyte and Beam Diameter on Probe Beam Deflection Experiments," J. Electroanal. Chem., 362, 55-69 (1993).
O. Haas, J. Rudnicki, F.R. McLarnon and E.J. Cairns, "Mechanistic Investigations of Redox Polymer-Coated Electrodes Using Probe-Beam Deflection and Cyclic Voltammetry," J. Chem. Soc. Faraday Trans., 87, 939-45 (1991).