SELF IMAGING OF SURFACE-IONIZATION ION SOURCES - WHERE DO THE IONS COME FROM

Images of the ion emitting regions of solid state ion sources have been produced using the ions emitted from the sources during operation to help elucidate the chemistry and physics of surface ionization. Examples are presented of a ceramic negative ion source producing perrhenate anions and of a zeolite ion source producing cesium cations, both of which are used as ion emitters in SIMS guns. In both cases it is shown that the ions originate from the surface of the ceramic or zeolite matrix, and not from interfacial regions between the matrix and the metal support structure. It is argued that for these two systems the gas phase ions are formed predominantly by direct sublimation of preformed ions from the hot surface of the matrix, due to the established fact that the ions observed in the gas phase are also known to exist in the solid, while it is improbable that the neutral species exist in the solid in appreciable concentrations. It is further shown that conventional surface ionization filament designs introduce asymmetric voltage gradients in the ion lens, leading to a loss in focusing. This can be corrected by using an indirectly heated ion source that has no voltage gradient across the face of the ion emitting region.