Wavelength-dependent Raman activity of D2h symmetry polycyclic aromatic hydrocarbons in the D-band and acoustic phonon regions

The experimental Raman spectra of some polycyclic aromatic hydrocarbons (PAHs) belonging to the D2h symmetry point group, recorded with different exciting laser energies, are presented and compared with simulations obtained by quantum chemical methods in the framework of Albrecht's theory restricted to the A term. It is shown that resonance with the La state enhances the activity of the low frequencies in the D-band region, while high frequency components of the D-band are preferentially enhanced by resonance with the Ba state. Moreover, low frequency vibrations, in the region of acoustic phonons, are identified both by computations and experimentally, which are characteristic fingerprints of the size and shape of the D2h symmetry PAHs