Manifestation of the strong quadrupole light-molecule interaction in the SEHR spectra of phenazine and pyrazine

TL;DR

This paper investigates the giant enhancement of Surface Enhanced Hyper Raman Scattering (SEHRS) in phenazine and pyrazine, emphasizing the role of strong quadrupole light-molecule interactions and their impact on spectral features.

Contribution

It provides a theoretical and experimental analysis demonstrating the significance of quadrupole interactions in SEHRS spectra of specific molecules, confirming the dipole-quadrupole theory.

Findings

Observation of forbidden vibrational bands in SEHR spectra

Confirmation of the dipole-quadrupole interaction mechanism

Explanation of spectral features using symmetry considerations

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 lines, caused by totally symmetric vibrations, transforming after the unit irreducible representation in molecules with the groups, where the elements of symmetry change z on -z . These lines are forbidden in usual HRS spectra. Analysis of literature data on phenazine and pyrazine molecules demonstrates that their SEHR spectra can be explained by the SEHRS dipole-quadrupole theory. Analysis of the SEHR spectra of these molecules reveals appearance of strong forbidden bands, caused by vibrations transforming after the unit irreducible representation that strongly confirms this theory which allows to interpret the whole SEHR spectra in detail. The results corroborate this common mechanism of Surface Enhanced optical processes on molecules adsorbed on rough metal surfaces.