Dispersion of the interband pi electronic excitation of highly oriented pyrolytic graphite measured by high resolution electron energy loss spectroscopy

We measured the momentum dependency of the energy and width of the pi plasm on in graphite, using high-resolution electron energy-loss spectroscopy. Fo r low momentum transfer the energy dispersion is positive and for high mome ntum transfer it becomes constant, in contrast with previous transmission e lectron energy-loss measurements. The width dispersion is negative for low momentum transfer and positive for higher momentum transfer. In addition, w e report an additional loss structure that disperses in the range 1-4 eV. ( C) 2000 Elsevier Science B.V. All rights reserved.