Secondary electron emission induced by 5-30-keV monatomic ions striking thin oxide films

An experimental investigation was made of how ion velocity and angle of incidence affect the magnitude and uniformity of secondary electron yield from thin amorphous oxide films. Flat target surfaces of Al2O3 and BeO in a high vacuum of 10-8--10-9 torr were bombarded with singly-charged positive ions of Li, Na, K, Rb, Cs, and Ni isotopes. Each of these elements has a different yield versus velocity response. The data are consistent with Parilis and Kishinevskii's prediction of a common velocity threshold. The estimate of the threshold velocity is 5.5 x 106 cm/sec. For a given element within experimental error, isotopes of different masses but with the same impact velocity produce identical average secondary electron yields. Furthermore, the statistical variances of their secondary electron frequency distributions also are identical. These results demonstrate the absence of an isotope effect on secondary electron emission. In general, secondary electron yield is not linear with ion velocity. Additional results were obtained for MgO and Ta2O5 surfaces. It was found that secondary electron yield depends strongly on the electronic shell structure of a projectile ion, that is, on its atomic number Z. The data can be interpreted qualitatively using the Fano--Lichten--Barat electron promotion model for ion-atom collisions when the Z's are unequal. A statistical model of secondary electron emission is described in which each ion-atom collision below a surface may result in the release of one or more electrons. In this model the random sequence or order in which target atoms are struck, the unequal amount of electronic excitation generated during collisions of a projectile ion with different kinds of target atoms, and the electronic properties of an oxide surface are all factors which shape the frequency distribution of observed secondary electrons