CHARGE-EXCHANGE PROCESSES IN LI+ AND HE+ ION-SCATTERING FROM ALKALI ADSORBATES ON CU(110)

Measurements have been made of the 1 keV Li+ --> Cs, Li+ --> Cu, He+ --> Na, He+ --> K, He+ --> Cs and He+ --> Cu ion scattering yields from submonolayer coverages of Na, K and Cs on Cu(110) using a 150-degrees scattering angle and 13-degrees incidence relative to the surface normal. The Li+ --> Cs data indicate that there is a steady decrease in the scattered ion fraction as the work-function decreases throughout the full submonolayer coverage range, a result which is quantitatively inconsistent with the semi-classical resonant charge transfer theory in either local or average work-function pictures. Possible origins of this discrepancy are discussed. He+ ion scattering results indicate that the nature of the He+ ion-surface charge exchange may also be dependent on the alkali overlayer coverage, and it is proposed that resonant charge transfer, as well as Auger neutralisation, becomes important as the work-function falls. The presence of a substantial inelastically scattered background in the He+ ion scattering spectra from the alkali-covered surfaces (which may be associated with plasmon creation) is, however, evidence that trajectory dependent neutralisation is far less effective in Na and K overlayers than in the Cu substrate.