We report the transmission of low-energy (<10 eV) O+ ions through adso
rbed alkali-metal (Li, Na, K, and Cs) overlayers on an oxidized tungst
en substrate. We find that the O+ yield is attenuated by the alkali-me
tal 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 attr
ibute the attenuation of O+ in alkali-metal films for Theta>0.3 ML mai
nly to resonance electron tunneling from adsorbed alkali-metal atoms t
o excited states of oxygen.