Superefficient diffusion of cesium atoms into rhenium covered by a 2D graphite film

Superefficient diffusion (SED) of cesium atoms into (1010)Re covered by a 2D graphite film has been demonstrated. Indeed, if a flux of Cs atoms of density v = 1 × 1013 cm−2 s−1 is incident on uncoated rhenium at T = 1800 K, the efficiency of their diffusion into the bulk is extremely low, ∼ 10−10. If, however, Cs atoms are deposited at 1800 K on a rhenium surface covered by a film of graphite, the efficiency of their diffusion into the bulk rises dramatically to values comparable to unity, ∼ 0.1-0.5. The mechanism of the SED involves efficient migration of Cs atoms from the surface under the graphite islands in the adlayer on the metal, and is based essentially on their extremely long lifetime under the islands, from where they diffuse into the bulk of the metal. Rhenium ribbons with the near-surface region saturated by cesium through the SED mechanism may serve as a novel efficient source of Cs+ ions (or Cs atoms) capable of emitting, for example, fluxes with densities of 1 × 109 < v < 3 × 1012cm −2 s−1, with the ribbon temperature varied in the range 1800 < T < 2200 K. Remarkably, at a constant ribbon temperature the Cs flux remained constant for many hours at a time. After an exposure to the atmosphere, the Cs-saturated rhenium ribbon continued to emit cesium in UHV conditions. The activation energies of Cs atom diffusion into rhenium, Es1 = 5.65 eV, and out of it, E1s = 5.85 eV, have been determined.