A comparison of secondary electron yields from accelerated water cluster ions (M/z<50 000) striking Al2O3 and copper surfaces

Abstract

A comparison of the yields of secondary electrons (γ) ejected by water cluster ions, with M/z ratios from 18 to 49 000 and kinetic energies from 40 to 250 kV, striking copper and aluminum oxide surfaces is detailed. For clusters striking Al₂O₃ with M/z in the range 21 000–49 000, threshold velocities of 1×10⁶ cm/sec were observed, lower than previously measured values from Cu. Threshold for secondary electron emission for cluster ions occurs when the estimated depth of penetration, based on standard stopping power formulae, is less than one atomic layer. For the oxide surface it is shown that the electron yields do not obey an additive relationship. The yields are always less than one would predict by summing the electron yields from each water molecule in the cluster. Lack of additivity is also shown in the yields resulting from small water clusters striking Cu. Electron yields from both surfaces are compared to a model which relates γ to the energy deposited in the target within the electron escape depth. For the oxide surface, a reduced electron yield can be qualitatively explained by the formation of a positive charge at the surface which can retard the escape of secondary electrons. Some implications for the practical use of these two surfaces as detection dynodes in high molecular weight mass spectroscopy are discussed.