Peculiarities of the SEHRS and SERS Spectra of - Bipyridine Molecule

TL;DR

This paper analyzes the SEHRS and SERS spectra of 4,4'-bipyridine using Dipole-Quadrupole theory, revealing strong vibrational lines linked to molecular symmetry and quadrupole interactions in nano-sized rough metal regions.

Contribution

It applies Dipole-Quadrupole theory to interpret the spectra of 4,4'-bipyridine, identifying symmetry-related vibrational lines and the role of quadrupole interactions in nano-sized roughness.

Findings

Strong lines due to vibrations with D2 and D2h symmetry groups.

Lines caused by contributions from specific vibrational representations.

Degeneration of vibrations related to molecular geometry.

Abstract

The SEHRS and SERS spactra of 4,4 - Bipyridine are analyzed on the base of the Dipole-Quadrupole theory for two possible geometries of the molecule. It is demonstrated that there appear strong lines caused by vibrations transforming after a unit irreducible representation both for the geometry with D2 and D2h symmetry groups, which may probably describe the symmetry properties of the molecule. Appearance of these lines is associated with a strong quadrupole light-molecule interaction, which arises in nano size rregions of sharp roughness of the metal. In addition, there are the lines caused by contributions from both the vibrations transforming after the unit irredicible representations A or Ag and the representations B1 or B1u, respectively, which describe transformational properties of the Ez component of the dipole moment, which is perpendicular to the surface for both geometries. This result is associated with a specific geometry of the molecule, when the indicated vibrations can be nearly degenerated and can not be resolved by the SEHRS and SERS spectra analysis. This issue is in a full compliance with the results of the SEHRS and SERS Dipole-Quadrupole theory.