Modulated infrared spectroscopy at the electrochemical interface

Performing infrared spectroscopy of chemical species at the electrochemical interface represents a difficult challenge in terms of sensitivity (1 monolayer ∼10¹⁵ species/cm) and selectivity (presence of the electrolyte). These problems are efficiently addressed by using modulation coupled with lock-in detection of the optical signal. The electrode potential, which governs the interface behavior, is the most straightforward physical quantity that can be modulated. Such a modulation technique may be combined with Fourier transform spectroscopy by using an interferometer with a very slow scanning speed of the movable mirror (1 - 10 μ/s). This approach allows one to reach high sensitivity (typical minimum detectable signal δI/I ∼ 10 ^-6 in a single-reflection arrangement). In some special cases, other modulations may be of interest, for example, modulation of the light at a semiconducting photoelectrode. A common benefit of these modulation techniques is that the resulting response can be analyzed as a function of the modulation frequency or by consideration of the phase of the signal at a given frequency. As can be shown for several examples, this analysis allows one to distinguish between the various physical and electrochemical processes taking place at the interface: change of free-carrier concentration beneath the electrode surface or of ion populations in the ionic double layer, adsorptiondesorption effects, and Faradaic processes, for which useful information on the reaction mechanisms may be obtained.