The C 1s exciton in diamond is studied by soft x-ray emission excited with high resolution monochromatic photons. A strong phonon sideband of 5 eV wide is observed in the exciton recombination spectrum. With the phonon contribution, the exciton binding energy is estimated to be 1.5 eV. The excited atoms is suggested to undergo Jahn-Teller relaxation similar to that of the nitrogen impurities in diamond. A similar exciton is observed at the ${\mathrm{\sigma }}^{\mathrm{*}}$-band threshold in graphite. These results provide new insights to the understanding of the core exciton and the donor problem in diamond and other systems.

• Received 25 June 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.3725