Attenuation of low-energy (<10 eV) O+ ions in transmission through adsorbed alkali-metal layers

We report the transmission of low-energy (<10 eV) O+ ions through adsorbed alkali-metal (Li, Na, K, and Cs) overlayers on an oxidized tungsten substrate. We find that the O+ yield is attenuated by the alkali-metal overlayers. For Theta>0.3 ML, we derive attenuation cross sections of (18+/-2)X10(-15) cm(2) for Cs, (13+/-1)X10(-15) cm(2) for K, (5+/-1)X10(-15) cm(2) for Na, and (3.3+/-0.3)X10(-15) cm(2) for Li. We attribute the attenuation of O+ in alkali-metal films for Theta>0.3 ML mainly to resonance electron tunneling from adsorbed alkali-metal atoms to excited states of oxygen.