Specific features of photoelectron emission from palladium clusters on graphite

Subthreshold photoelectron emission was observed to emerge from palladium nanoclusters formed on pyrographite surface under irradiation by photons in the energy range 3.1–6.5 eV. The average size of the palladium nanoclusters on the pyrographite surface was 50–80 nm, and the average height, 2–4 nm. Besides conventional photoemission from states below the Fermi level, photoelectron emission was observed at the energies of photons irradiating the surface 0.9 eV below the work function of the Pd surface. It is assumed that this emission is stimulated by direct electron transitions from Pd states below the Fermi level to the unfilled electron surface states formed in the Coulomb potential of image forces (image states) and, subsequently, into vacuum. This phenomenon is assumed to originate from the contact spot field generated above the surface which is nonuniform in work function. This assumption is supported by the calculations presented in the paper.