We measured the ion-induced secondary electron emission coefficients gamma i of (111)-, (110)-, and (100)- oriented MgO single crystals under panel-like conditions using breakdown measurements rather than ion-beam experiments. Plasma-surface interaction removes a surface layer with different, and in the case of MgO worse secondary electron emission properties thus improving gamma _i. The gamma _i -value of 0.5 found for MgO (111) is substantially higher than the values found for the other orientations. For MgF_2 layers measurements indicate the formation of a thin MgO-like layer, the removal of which leads to a reduction of the gamma _i -values.
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