Dear Sir. Concerning your issue about the photothermal deflection spectroscopy (PDS) as a surface sensitive measurement. Photothermal spectroscopy is a group of high sensitivity spectroscopy techniques used to measure optical absorption and thermal characteristics of a sample. The basis of photothermal spectroscopy is the change in thermal state of the sample resulting from the absorption of radiation. Light absorbed and not lost by emission results in heating. The heat raises temperature thereby influencing the thermodynamic properties of the sample or of a suitable material adjacent to it. Measurement of the temperature, pressure, or density changes that occur due to optical absorption are ultimately the basis for the photothermal spectroscopic measurements. The absorptance is calculated as the difference between total light impinging on the sample and the transmitted (plus reflected, plus scattered) light, with the usual problems of accuracy when one deals with small differences between large numbers I think the following below links may help you in your analysis: