Performing infrared spectroscopy of chemical species at the electrochemical interface represents a difficult challenge in terms of sensitivity (1 monolayer approximately 10(15) species/cm2) 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 (approximately 1-10 mum/s). This approach allows one to reach high sensitivity (typical minimum detectable signal DELTAI/I approximately 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, adsorption-desorption effects, and Faradaic processes, for which useful information on the reaction mechanisms may be obtained.
