Molecular beam study of the desorption of cesium ions from tungsten crystals

Abstract

The rates of desorption of Cs + from W(100) and W(110) crystals have been measured by the modulated molecular beam method. The Cs coverage was extremely low (≲ 5 × 10 11 cm −2), thereby insuring that: (a) the desorption rate of Cs atoms was negligible relative to that of Cs ions; (b) the desorption rate depended primarily on the interaction of Cs with W rather than the interaction of Cs with Cs. Assuming that the dependence of the Cs + desorption rate on coverage n and temperature T may be represented by the form R + = nν + exp( − χ0 + kT ) , we determined ν + and χ0 + from the measured rates. For W(100) in the range 900–1250 K, ν + = (1.0 ± 0.2) × 10 13 sec −1 and χ0 + = 2.05 ± 0.02 eV; for W(110) in the range 880–1050 K, ν + = (1.7 ± 0.4) × 10 12 sec −1 and χ0 + = 1.77 ± 0.02 eV. The results are compared with existing experimental data and theoretical predictions. The predictions of the image potential model appear to be consistent both with the binding energies inferred from the present results and with the dipole moments inferred from previous contact-potential measurements for Cs on W(100) and W(110) crystals.