SPECTROSCOPIC MODULATION TECHNIQUES TO OBSERVE INFRARED SPECTRA FOR SPECIES IN THE ELECTRODE SOLUTION INTERFACIAL REGION

Two spectroscopic modulation techniques have been adapted for studies of infrared spectra of species in the electrode solution interfacial region. Electrochemically modulated infrared reflectance spectroscopy, EMIRS, extends into the infrared the modulated specular reflectance technique previously used in the uv-visible region for identification of species produced during redox reactions in electrochemical cells. Infrared reflection absorption spectroscopy, IRRAS, previously utilized for studies of gas adsorbates on metals has also been successfully adapted for use with metal substrates under potentiostatic control and adsorbates from aqueous solution. EMIRS Utilizes a square wave modulation of the electrode potential in order to discriminate infrared absorptions by oriented species on or near the electrode surface from absorptions of bulk solution species. Using a spectroelectrochemical cell with a working electrode mirror positioned a few microns from an infrared window, sufficient infrared through-put is achieved while maintaining potentiostatic control far modulation frequencies of 10--500 cps, Sensitivity levels are such that spectra of submonolayer quantities of adsorbates with strong infrared bands are observable. Since EMIRS spectra are difference Spectra between species present at the two selected modulation potentials, even strongly absorbing species whose surface concentrations, orientations, and spectra are potential independent are weakly dependent may not be observed. The complementary IRRAS method avoids this limitation. IRRAS utilizes polarization modulation of the incident infrared radiation to make visible for surface species only vibrational modes with non-zero surface normal dipole derivatives. Although IRRAS has the advantage of retaining the electrode potential as a variable experimental parameter, the sensitivity levels achieved to date are less than those obtained using EMIRS. Examples of spectra for CO, $HS{O_{4}}^{-}, C_{3}H_{3}N$, and $H_{2}O$ uwing Pt, Rh, Ag, and Au electrodes will illustrate: (1) a strongly adsorbed molecule visible by EMIRS due to the potential dependence of its vibrational frequency, (2) a species whose surface concentration changes with variation in the electrode potential, (3) a species whose infrared intensity depends upon the electrode potential, and (4) a species whose structure is changed by changing the electrode potential.