Manifestation of the strong quadrupole light-molecule interaction in the SEHR spectra of symmetrical molecules

TL;DR

This paper explores how strong quadrupole light-molecule interactions significantly enhance Surface Enhanced Hyper Raman Scattering (SEHRS) spectra of symmetrical molecules, supported by theoretical analysis and experimental data.

Contribution

It introduces a dipole-quadrupole SEHRS theory explaining forbidden bands in spectra and demonstrates its applicability to various symmetrical molecules.

Findings

SEHRS spectra can be explained by dipole-quadrupole interactions.

Forbidden vibrational bands are observed and explained.

The theory is supported by analysis of multiple molecular spectra.

Abstract

The paper demonstrates possibility of giant enhancement of Surface Enhanced Hyper Raman Scattering on the base of qualitative consideration of electromagnetic field near some models of rough metal surfaces and of some features of the dipole and quadrupole light-molecule interaction, such as it was made in the dipole-quadrupole SERS theory. Consideration of symmetrical molecules allows to obtain selection rules for their SEHR spectra and establish such regularity as appearance of the bands, caused by the totally symmetric vibrations, transforming after the unitary irreducible representation in molecules with C2h,D and higher symmetry groups, which are forbidden in usual HRS spectra. Analysis of literature data on trans-1,2-bis (4-pyridyle) ethylene and pyridine molecules demonstrates that their SEHR spectra can be explained by the SEHRS dipole-quadrupole theory, while analysis of the SEHR spectrum of pyrazine reveals appearance of the strong forbidden bands, caused by vibrations transforming after the unitary irreducible representation that strongly confirms this theory which permits to interpret the whole SEHR spectrum in detail. The results corroborate this common mechanism of Surface Enhanced optical processes on molecules adsorbed on rough metal surfaces.