Binding and charge transfer associated with alkali metal adsorption on single crystal nickel surfaces

Abstract

Work function and thermal desorption energy measurements of alkali metal coated single crystal Ni surfaces are reported and correlated with previously reported surface crystallography measurements. It is shown that the minimums in the work function versus coverage curves do not generally correspond to significant changes in surface structure. Initial dipole moments calculated from the work function measurements are 7.4 ± 0.5, 7.2 ± 0.5 and 3.2 ± 0.3 Debye units for Na on Ni(111), (100) and (110), respectively, and 3.2 ± 0.3, 5.3 ± 0.3 and 7.0 ±0.5 Debye units for Na, K and Cs on Ni(110), respectively. A quantum mechanical theory developed by Gadzuk predicts dipole moments in good agreement with these experimental values for alkali metal coated Ni. The thermal desorption energies corresponding to the initial stage of adsorption measured for Na on Ni(111), (100), and (110) were 2.54, 2.52 and 2.18 ± 0.1 eV, respectively, and 2.18, 2.56 and 2.72 ± 0.1 eV for Na, K and Cs on Ni (110), respectively. The shapes of the desorption energy and frequency factor versus coverage curves were related to the surface crystallography. Finally, correlation of the desorption energy with the work function measurements emphasized the importance of ionic bonding at low coverages.