Optimization of in situ infrared spectro-electrochemical accessory with the aid of reflectance and electric field and calculations

Abstract

Optical design of the in situ infrared reflection-absorption spectroscopy (IRRAS) accessory is discussed, with a particular attention to its optimization on the base of theoretical calculations. The sensitivity of this versatile technique applicable to most mono- and polycrystalline electrodes can be greatly enhanced by carefully adjusting the angle of incidence of infrared (IR) beam and the position of the electrode. Using Fresnel equations, we show that the reflectivity and the strength of the electric field of a stratified medium are interconnected, so that the magnitude of the electric field strength can be monitored by the reflectivity of the light into the first layer, and ultimately into the IR detector. The result of calculating these effects for the three-layer system, zinc selenide crystal / aqueous solution / Pt electrode, clearly shows that the intensity of the interferogram in the spectrometer can be used to precisely adjust the angle of incidence of the IRRAS accessory: the interferogram strength is increasing by pressing the Pt electrode on the ZnSe crystal at or near the optimal incident angle and decreasing when the angle of incidence is only a few degrees away from optimal. By combining the results of the calculation of the electric field with an experiment, an optimized system for identifying and tracking various species adsorbed on the surface of the electrode or within a few micrometers from the surface in the solution due to the change in the electrochemical potential can be achieved.