Cesium diffusion at a tungsten surface

The diffusion of cesium at the surface of tungsten ribbons has been measured under ultrahigh vacuum conditions. A photoelectric method was used to determine the cesium surface concentrations which were in the range from about 5 × 10−4 to 2 × 10−2 monolayers. The measured changes in concentration with time and temperature were consistent, for a polycrystalline ribbon, with two bulk diffusion processes with activation energies of 1.7 ± 0.3 eV and 0.17 ± 0.03 eV. For a single crystal, it was found that limited bulk diffusion occurred with an activation energy of 0.21 ± 0.02 eV. The mean surface diffusion coefficient for cesium on a (110) tungsten surface over the temperature range from 550 °K to 850 °K was given by D = (0.23 ± 0.10 cm2 s−1) exp [−(0.57 ± 0.02 eV)/kT].