Interpretation of the spectra of surface-enhanced hyper-Raman scattering of symmetric molecules, based on dipole–quadrupole theory

Abstract

The spectra of enhanced hyper-Raman scattering (Hyper RS) by symmetrical molecules studied in the literature are analyzed on the basis of the dipole-quadrupole theory of surface-enhanced Hyper RS of light and the concept of the existence of a strong quadrupole interaction of light with the molecules. It is shown that the theory predicts the appearance of forbidden lines in the enhanced Hyper-RS spectra (in molecules with sufficiently high symmetry), caused by totally symmetric vibrations and transforming according to a single irreducible representation. A consideration of the experimental spectra of phenazine, pyrazine, and pyridine confirmed the existence of the indicated forbidden lines and made it possible to explain the features of the spectra of these molecules. A critique is presented of the theoretical analysis of the enhanced Hyper-RS spectra of trans-1,2-bis(4-pyridyl)ethylene and crystal violet carried out by other authors.

Effect of Electric Field Gradient on Sub-nanometer Spatial Resolution of Tip-enhanced Raman Spectroscopy

“…The first term of Eq. (1) accounts for the dipole Raman, and the higher order including A α,βγ and C αβ,γδ account for the electric field gradient multiple Raman corresponding to IR-active modes 23 24 25 . The selection rules for the four terms in Eq.…”

“…The selection rules for the four terms in Eq. (1) can be obtained by the same manner in the following form 23 , where Γ is the irreducible representation.…”

Plasmonic Gradient and Plexcitonic Effects in Single-Molecule Tip-Enhanced (Resonance) Raman Spectroscopy

“…Sub-nanometer resolution of molecular image and multiple-Raman scattering signals at the single molecule level can be simultaneously provided by STM-TERS. Physical mechanisms of such sub-nanoscale optical resolution and multiple-Raman scattering have been interpreted by the plasmon-enhancement and plasmonic gradient effect. ,,,,,,,,,,, Hamiltonian for the Raman spectra of a molecule placed in an inhomogeneous electromagnetic field is ,− H = H 0 + H 1 + H 2 + H 3 + ··· = 1 3 ∑ α β ( α α β E β + A α , β γ ∇ E β γ ) E α + 1 3 ∑ α β γ ( A γ , α β E γ + C α β , γ δ ∇ E γ δ ) normal∇ E α β + ··· …”

“…The first term of eq accounts for the dipole Raman, and the higher order including A α,βγ and C αβ,γδ account for the electric field gradient multiple Raman corresponding to IR-active modes. The selection rules for the four terms in eq can be obtained by the same manner in the following form ,− normalΓ ∈ normalΓ μ = normalΓ θ …”

Electromagnetic Field Gradient-Enhanced Raman Scattering in TERS Configurations

“…Raman and infrared spectra are excellent quantitative analysis technologies for identification of chemical composition and determination of samples. − Here, 3 N – 6 independent vibration modes of a nonlinear molecule composed of N atoms could be characterized by the two complementary spectrum technologies. In situ simultaneous measurement of Raman and infrared absorption vibration modes can not only achieve accurate positioning but also measure the absolute intensity ratio of the two vibration modes, which is of great significance for the analysis and application of the molecular vibration spectrum. , However, only (3 N – 6)/2 molecular vibration modes can be observed in a single measurement by using Raman or infrared absorption spectroscopy, because of the complementarity of transition rules, , which greatly limits the in situ precise measurement of molecules. Although plasmon-enhanced Raman spectroscopy (PERS) technology largely improves the scattering section of a molecule, − it is still a great challenge to simultaneously detect all molecular vibration modes at one time.…”