Caesiated Surfaces and Negative Ion Emission

The strong enhancement of the negative ion emission produced by caesium atoms is used now during analyses by secondary ion mass spectrometry more and more frequently. In most experiments, the caesium coverage results from the implantation into the target of the caesium positive ions that are producing the sputtering of the target itself [1,2,3]. After removal of a given thickness of the sample, an equilibrium is reached where the concentration of the implanted elements is controlled by the target sputtering yield. But, it is likely that the caesium concentration obtained that way is generally not sufficient to reach the saturation of the emission process. Another procedure consists of a chemisorption of neutral caesium atoms while the sample is simultaneously bombarded by a noble gas ion beam; thus the surface coverage can be adjusted by controlling the densities of either the primary ion beam or the neutral caesium jet [4,5].