Raman activation in disordered graphites of the A1′ symmetry forbidden k≠0 phonon:: The origin of the D line

The analysis of the tensors of polarizability derivatives, computed with quantum-chemical methods for several large polycyclic aromatic hydrocarbon molecules, allows to rationalize the origin of the Raman D line observed in disordered graphites as due to a relaxation of the phonon selection rules. It is shown that a minimal rearrangement of the atomic and electronic structure in a lattice different from that of a perfect graphite crystal causes a selective activation of a specific totally symmetric phonon whose frequency falls in the region of the D band and whose motion is dominated by the breathing vibration of aromatic rings. We numerically proof this conjecture on the basis of polarizability derivatives obtained with Density Functional Theory calculations. (C) 2001 American Institute of Physics.