I n s i t u infrared spectroelectrochemistry

Abstract

The vibrational spectrum of molecules at or near an electrode surface can be obtained using subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) and electromodulated infrared spectroscopy (EMIRS). The large electric field existing at the electrode/solution interface is sufficient to induce dipole moments in highly polarizable molecules. The magnitude of the induced dipole moment is proportional to the electric field strength. As a result of this interaction some totally symmetric normal vibrations are made infrared active, and other bands appear in violation of the surface selection rule. This effect is known as the electrochemical Stark effect. Calculation of the expected absorption coefficients for the totally symmetric modes of adsorbed species gives values which are close to those observed experimentally. Experimental evidence of Ag mode activation of adsorbed pyrene is presented. The electric field dependence of the integrated absorption coefficient is demonstrated for acrylonitrile adsorbed on a gold electrode.