SUPEREFFICIENT DIFFUSION OF CESIUM ATOMS INTO RHENIUM COVERED BY A 2DGRAPHITE FILM

Superefficient diffusion (SED) of cesium atoms into (<10(1)over bar 0> )Re covered by a 2D graphite film has been demonstrated. Indeed, if a flux of Cs atoms of density v = 1 X 10(13) cm(-2) s(-1) is incident on uncoated rhenium at T=1800 K, the efficiency of their diffusion into the bulk is extremely low, similar to 10(-10). If, however, Cs atoms a re deposited at 1800 K on a rhenium surface covered by a film of graph ite, the efficiency of their diffusion into the bulk rises dramaticall y to values comparable to unity, similar to 0.1-0.5. The mechanism of the SED involves efficient migration of Cs atoms from the surface unde r the graphite islands in the adlayer on the metal, and is based essen tially 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 serv e as a novel efficient source of Cs+ ions (or Cs atoms) capable of emi tting, for example, fluxes with densities of 1 x 10(9) < v < 3 x 10(12 ) cm(-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 a tmosphere, the Cs-saturated rhenium ribbon continued to emit cesium in UHV conditions. The activation energies of Cs atom diffusion into rhe nium, E(s1) = 5.65 eV, and out of it, E(1s) = 5.85 eV, have been deter mined.