Molecular dipole model for the internal optic modes of surface adsorbed molecules: Classical image theory

Abstract

A molecular dipole model is developed to describe dipolar coupling in surface adsorbed molecules using classical image theory. Only the internal optic modes of the adsorbed molecules are considered. Expanding the dipoles associated with both real and image molecules in terms of the same molecular normal coordinate results in a frequency shift twice that previously expected for dipolar coupling in adsorbed molecules. Application of the model to carbon monoxide adsorbed on platinum indicates that the observed frequency versus coverage behavior can be attributed to dipolar coupling. The irreducible corepresentations of Shubnikov type II, or grey, groups are used to describe the symmetry and spectroscopic activity of the molecule-plus-image system. Infrared activity is associated only with molecular vibrations having nonzero dipole moment derivative components perpendicular to the surface. Raman activity is associated only with those molecular vibrations having nonzero Raman scattering tensor elements αxx, αyy, αzz, or αxy, where z is perpendicular to the surface.

The Structures and Reactions of Linear and Cyclic C6Hydrocarbons Adsorbed on the Pt(111) Crystal Surface Studied by Sum Frequency Generation Vibrational Spectroscopy: Pressure, Temperature, and H2Coadsorption Effects

“…The SFG spectrum from the adsorbates on the metal surface should obey the IR surface selection rule that only the IR transition dipole vector component perpendicular to the metal surface accounts for the IR transition cross sections. Moreover, the classical theory of the Raman surface selection rule predicts that the Raman band intensity should scale as α( z 2 ) > α( xy ), α( yz ) ≫ (α x 2 ), α( y 2 ), α( xy ), where α is the Raman scattering tensor element; x , y , and z refer to the laboratory coordinates; and z is the axis along the surface normal. , This means that both the IR and Raman transition cross sections are sensitive to the molecular orientation with respect to the surface normal and so, therefore, is the SFG intensity. Moreover, the nonresonant SFG signal from a Pt surface is much smaller than that from other metal surfaces such as Au and Ag.…”

Adatom Hypothesis as a Predominant Mechanism of Surface Enhanced Raman Scattering: A Review of Experimental Argumentation

“…Frequencies of the above vibrations in silver sols also increase with the ratio between concentrations of Cl --and Br -ions [273], the increase being due to increasing relative coverage of sol particles by Cl -ions and qualitatively matching, to the authors' minds, theory of interaction of dipoles ion-image [274,275]. The authors of [273][274][275], however, absolutely ignore the role played by variations in the potential of sol particles, which are sure to bring about variations in partial charge transfer inside adsorbate particles. As follows from electrochemical works, adsorption of halide ions on silver involves considerable charge transfer, and it would be only natural to link at least a portion of the observed shift of the vibrational frequency of Ag-(Cl -) ads with precisely this effect.…”