CORE EXCITONS AND VIBRONIC COUPLING IN DIAMOND AND GRAPHITE

The C 1s exciton in diamond is studied by soft x-ray emission excited with high resolution monochromatic photons. A strong phonon sideband o f 5 eV wide is observed in the exciton recombination spectrum. With th e phonon contribution, the exciton binding energy is estimated to be 1 .5 eV. The excited atom is suggested to undergo Jahn-Teller relaxation similar to that of the nitrogen impurities in diamond. A similar exci ton is observed at the sigma-band threshold in graphite. These result s provide new insights to the understanding of the core exciton and th e donor problem in diamond and other systems.