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.
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