Exposure of a Ru(0001) surface covered by submonolayer quantities of Cs to O-2 leads to the emission (with very low probabilities) of O- ions. The yield of O- reaches a maximum at Cs coverages around 0.3-0.4 monolayers (1 ML is equivalent to an absolute coverage theta(Cs) = 0.33), and in this range is much higher for low (similar to 260 K) than for high sample temperatures. These effects are traced back to the varying degree of ''metallization'' of the Cs overlayers as substantiated by metastable deexcitation spectroscopy (MDS) experiments.
