A comparison of surface-enhanced infrared and surface-enhanced Raman spectra of pyrazine adsorbed on polycrystalline gold electrodes

The adsorption of pyrazine at a polycrystalline Au film electrode has been investigated using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), chronocoulometry, and ac impedance. Combining the SEIRA data and the thermodynamic data (the surface charge density of the electrode and the relative Gibbs surface excess of pyrazine), pyrazine was found to adsorb on the surface with a vertical end-on configuration via one N atom. The SEIRA spectra were compared with published surface-enhanced Raman (SER) spectra of pyrazine in order to clarify the reason for the breakdown of the Raman selection rule in the SER spectra. The activation of originally Raman-forbidden modes in the SER spectra is well explained by a photo-driven charge-transfer mechanism. The charge-transfer is deduced to be from filled metal states near the Fermi level to the first and second excited states of pyrazine. It is emphasized that the comparative SEIRA and SER studies are important for a better understanding of the electrochemical interface.