Electronic effects in SERS by liquid water

Abstract

By exploiting the attenuated total reflection (ATR)-Otto configuration, surface-enhanced Raman scattering of water at monocrystalline copper and silver electrodes in 0.1 M Na 2 SO 4 was observed at cathodic electrode potentials with hydrogen evolution at a laser wavelength of 647.1 nm. The proposed enhancement model is based on the condition that the upshift of the Fermi level of the electrode and the absorption of one laser photon allow an electron of the electrode at the Fermi level to reach the lower level of the energy band of delocalized electrons in liquid water. The electron interaction with the water molecules in the first hydration shell of cations in the Helmholtz plane is so-called impulse scattering, which is the short-time, non-resonant excursion to the anionic ground state of molecules known for a long time in low-energy electron scattering from free molecules.

SERS spectrum of imazalil. Experimental and quantum‐chemical vibrational analysis

“…Previous studies of imidazole by SERS have shown that this molecule can adsorb onto a metal surface in two different ways: through the unshared pair of electrons on the N (2) atom or through the π‐electrons system of the imidazole ring . Coordination through the N (2) atom leads the molecule to adopt a perpendicular orientation of the aromatic ring plane with respect to the metal surface, as was also observed for other heteroaromatic rings . In the case of imazalil, this type of coordination implies that the vinyl group may also interact with the metal through the π electrons of the C═C double bond.…”

“…However, interactions of the benzene ring with the metal are not predicted to justify a shift of this band. Alternatively, this intense band may be related to the presence of water molecules in interaction with the metal surface, according to reported SERS spectra of pure water that showed a band at 1,610 cm −1 .…”

Theoretical Vibrational Raman and Surface‐Enhanced Raman Scattering Spectra of Water Interacting with Silver Clusters

“…The experimental results of water on rough silver at a negative potential of À1.5 V, given by Chen and Otto, [41] predict an enhancement factor of 4.2 10 6 for the bending vibrational mode and a factor of 7.3 10 4 for the stretching vibrational mode. The same authors indicate that these values increase if the potential becomes more negative.…”

“…[40,41,56] To model these negative potentials, we performed calculations by using negatively charged clusters. [40,41,56] To model these negative potentials, we performed calculations by using negatively charged clusters.…”

“…Thus, the intensity of the bending mode is enhanced greatly if a hydrogen bond is formed between water and some anionic species [29][30][31] and if there are hydrated electrons produced from irradiating liquid water (even though they have a short lifetime of the order of microseconds). [38][39][40][41] At negative potentials, water molecules were found to be adsorbed with one hydrogen-down atom to form a hydrogen bond with the metal surface, unlike the configuration with the oxygen-end down that appears at positive potentials. [38][39][40][41] At negative potentials, water molecules were found to be adsorbed with one hydrogen-down atom to form a hydrogen bond with the metal surface, unlike the configuration with the oxygen-end down that appears at positive potentials.…”