The SEHRS spectrum of 4,4 Bipyridine is analyzed on the base of the Dipole Quadrupole theory. It is demonstrated that there appear strong lines caused by vibrations transforming after a unit irreducible representation of the D2 symmetry group. which
is most probably describes the symmetry properties of the molecule. These lines are nearly forbidden for the molecule, adsorbed on rough metal surface. Appearance of these lines is associated with a strong quadrupole light molecule interaction, which exists in this system. In addition, there are lines, caused by contributions from both the vibrations transforming after the unit irreducible representation A and the representation B1, which describes transformational properties of the dz component of the dipole moment, which is perpendicular to the surface. This result is associated with the specific geometry of the molecule, when the indicated vibrations can be nearly degenerated and cannot be resolved by the SEHRS spectra analysis. Analysis of the SEHRS spectra for the possible geometry of the molecule with the D2h symmetry group leads to similar results. This issue is in a full coincidence with the results of the SEHRS Dipole Quadrupole theory.
The Dipole-Quadrupole theory of Surface Enhanced Hyper Raman Scattering (SEHRS), created by the authors is expounded in details. Peculiarities of the behavior of electromagnetic field on rough metal surfaces are considered. It is demonstrated that th
ere is an enhancement of the dipole and quadrupole light-molecule interaction near the places with a large curvature. The expression for the SEHRS cross-section of symmetrical molecules, which consists of several contributions is obtained. Selection rules for the scattering contributions are obtained and a qualitative classification of the contributions after an enhancement degree is performed. Analysis of experimental spectra of pyrazine and phenazine, and also some another molecules is performed too. It is demonstrated a full coincidence of experimental regularities in these spectra with the theory suggested.
It is demonstrated that there is no chemical enhancement in Surface Enhanced spectroscopy, but the enhancement is of a pure electrodynamical nature.
The review is devoted to explanation of SERS in terms of the dipole and quadrupole light-molecule interactions arising in surface fields strongly varying in space in the region of the strongly irregular surface roughness. The main SERS characteristic
s, the theory of electromagnetic fields near some model kinds of rough surfaces and some other systems, the theory of SERS Raman tensor for arbitrary and symmetrical molecules, selection rules and analysis of the SER spectra, some anomalies in the SER spectra of symmetrical molecules for some specific conditions, electrodynamic forbiddance of the quadrupole scattering mechanism for the methane molecule and molecules with cubic symmetry groups are considered. The huge enhancement and blinking of the SERS signal arising in the phenomenon of Single Molecule detection by the SERS method are explained. The above theory is compared with some another SERS mechanisms, and the phenomena accompanying SERS are accounted for. It is demonstrated that the theory is in a good agreement with the experiment and explains quite a number of characteristics related to the SERS phenomenon.
