Digital Signal Processing for Step-Scan Phase and Electrochemical Potential Double-Modulation Fourier Transform Infrared Spectrometry

Step-scan double-modulation (phase and electrochemical potential) Fourier transform infrared (FT-IR) spectrometry provides both spectroscopic and dynamic information about faradaic reactions. Recently introduced digital signal processing (DSP) can be used, instead of two lock-in amplifiers, for the optical signal demodulation at two modulation frequencies. In order to establish the merits of double-modulation FT-IR spectrometry with DSP, spectro-electrochemical experiments are performed in the attenuated total reflection configuration and with the commonly used ferri/ferrocyanide redox couple. Because of a large potential drop associated with the uncompensated resistance, a satisfactory signal-to-noise ratio for the alternating current (ac) optical measurements is obtained only with the employment of positive feedback compensation. In this arrangement, the amplitude of electrochemical modulation is sufficiently large to convert a significant fraction of the reduced form to the oxidized form and back to the reduced form. Large amplitude ac voltammetry demonstrates that the phase of faradaic admittance at the formal potential is approximately 45° at 2.00 Hz. In addition, these experiments allow for calculation of the interfacial ac potential. This variable is needed for the normalization of the in-phase and the quadrature spectra in order to overcome the problem associated with the iR_u drop. Because of the integral relationship between the faradaic current and the electromodulation reflectance coefficient, the phases of electromodulation reflectance coefficient with respect to the interfacial ac potential are expected to be −45° and 135° for the reduced and oxidized forms, respectively. However, dynamic information from double-modulation FT-IR spectrometry is available only if demodulation at the electrochemical potential modulation frequency is performed with respect to a defined phase. Because of an undefined demodulation phase implemented in the current version of DSP software, step-scan double-modulation FT-IR spectrometry with DSP is suitable only to provide spectroscopic information. In order to overcome this limitation, the demodulation of the ac optical signal at the electrochemical modulation frequency must be synchronized in phase with the ac potential modulation applied to the electrochemical cell.