Analysis of sodium on glass surfaces disturbed by ion-beam induced absorption currents

Glass discs form capacitors during ion-beam sputtering. The charge transfer to the surface under bombardment generates a build-up of surface potentials. These surface potentials cause a change in the concentration of sodium on the surface of flat Na2O.n-SiO2 discs by a drift of mobile Na+-ions. The ion beam mills the surface. Sputtered sodium atoms are partly excited and emit at lambda = 589 nm the NaI-radiation. An abrupt change of the surface potential produces a characteristic transient in the photon current/time plot. These transients are caused by absorption currents or the currents of residual charge consisting of mobile Na+-ions. These currents generate a change in the concentration of sodium or other mobile cations on/in the glass surfaces. The time dependence of the Na-concentration changes is reflected in the IBSCA (ion beam spectrochemical analyzer)-plot and these intensity/time changes can be calculated according to a model. Other surface analysis methods which cause a charge transfer to the glass surface, may show a similar behaviour. Experimental results indicate that the disturbing surface potentials can be avoided in principle if coincident ion and electron beams are applied. The best suitable experimental conditions can be selected by IBSCA. (author)