Anomalous coverage dependence of secondary-ion emission from overlayers

We have observed an anomalous coverage dependence of sputtered Cs+ and Li+ yields from Cs and Li overlayers on Si(111) surfaces. The ion yield reaches a maximum and decreases at higher coverages even when the coverage is still less than a monolayer. We found that this phenomenon is directly related to the effect of the work function on the ionization probability. © 1984 The American Physical Society.