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SIMS Theory: Secondary Ion Yields - Primary Beam Effects
Other factors affect the secondary ionization efficiencies in SIMS measurements. Oxygen bombardment increases the yield of positive ions and cesium bombardment increases the yield of negative ions. The increases can range up to four orders of magnitude.
Oxygen enhancement occurs as a result of metal-oxygen bonds in an oxygen rich zone. When these bonds break in the ion emission process, the oxygen becomes negatively charged because its high electron affinity favors electron capture and its high ionization potential inhibits positive charging. The metal is left with the positive charge. Oxygen beam sputtering increases the concentration of oxygen in the surface layer.
The enhanced negative ion yields produced with cesium bombardment can be explained by work functions that are reduced by implantation of cesium into the sample surface. More secondary electrons are excited over the surface potential barrier. Increased availability of electrons leads to increased negative ion formation.
The variability in ionization efficiencies leads to different analysis conditions for different elements as indicated on the periodic table.
