Raman activation in disordered graphites of the A(1)(') symmetry forbiddenk not equal 0 phonon: The origin of the D line

The analysis of the tensors of polarizability derivatives, computed with qu antum-chemical methods for several large polycyclic aromatic hydrocarbon mo lecules, allows to rationalize the origin of the Raman D line observed in d isordered graphites as due to a relaxation of the phonon selection rules. I t is shown that a minimal rearrangement of the atomic and electronic struct ure in a lattice different from that of a perfect graphite crystal causes a selective activation of a specific totally symmetric phonon whose frequenc y falls in the region of the D band and whose motion is dominated by the br eathing vibration of aromatic rings. We numerically proof this conjecture o n the basis of polarizability derivatives obtained with Density Functional Theory calculations. (C) 2001 American Institute of Physics.