Abstract
Photoemission from polycrystalline diamond films irradiated with the harmonics of a Nd:YAG laser indicates that the amorphous or quasi-amorphous zones (patches) of sp2-C on the external surface of the diamond grains make a dominant contribution to the emission yield. This is deduced from: (a) the strong reduction of the yield as a consequence of the partial removal of amorphous carbon (a-C) patches by etching with acid and (b) the correlation between the yield and the a-C content determined by Raman spectroscopy. The junction between the a-C patches and the diamond determines a configuration of the energy bands, which allows one-photon emission with an energy of 4.7 eV (4th harmonic). We have measured samples with variable a-C content and found an efficient emission, with values of quantum efficiency on the order of 10-6. A phenomenological picture of the electron emission process leads us to locate it at the a-C–diamond–vacuum triple border. This indicates a high local efficiency and suggests that a diamond film with an appropriate distribution of a-C patches on the surface could be a very efficient material. Moreover it could work in moderate vacuum and have a high laser-radiation damage-threshold. Such a material would therefore be interesting as a robust photocathode .
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