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 D-2h 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 L-a 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 B-a 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 D-2h symmetry PAHs. (C) 2004 Elsevier B.V. All rights reserved.