Photothermal Spectroscopy in Attenuated Total Internal Reflection Geometry by Polarization Rotation and Deflection

Photothermal spectroscopy has proven to be an incisive analytical tool, with several offshoots and applications. It is desirable to extend this technique to interfacial spectroscopy to further broaden its scope. Toward that goal, we demonstrate photothermal infrared (IR) spectroscopy where a resonant IR beam heats an interface, while a visible beam probes the interface in the attenuated total internal reflection (ATR) geometry. The photothermal spectra of a test molecule show similar concentration dependence and solvatochromic shifts when compared to FTIR-ATR measurements. We also show that two mechanisms underpin the ATR photothermal signal. The first mechanism is polarization change of the probe beam due to the thermal refractive index change of the interface. This is confirmed by time-dependent changes in the polarization state of the probe beam due to heating. The second mechanism is deflection of the probe beam upon heating by the IR pump. Furthermore, we showed that the photothermal effect persists when a thin film of gold is added to the interface. Our work will open potential applications of this technique in interfacial sciences and electrochemistry and will guide new approaches to reach monolayer sensitivity.