Performing infrared spectroscopy of chemical species at the electroche
mical interface represents a difficult challenge in terms of sensitivi
ty (1 monolayer approximately 10(15) species/cm2) and selectivity (pre
sence 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 th
e most straightforward physical quantity that can be modulated. Such a
modulation technique may be combined with Fourier transform spectrosc
opy 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 ap
proximately 10(-6) in a single-reflection arrangement). In some specia
l 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 anal
yzed 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 seve
ral examples, this analysis allows one to distinguish between the vari
ous physical and electrochemical processes taking place at the interfa
ce: change of free-carrier concentration beneath the electrode surface
or of ion populations in the ionic double layer, adsorption-desorptio
n effects, and Faradaic processes, for which useful information on the
reaction mechanisms may be obtained.