The deexcitation rate of metastable S-3 He* atoms was measured during the g
rowth of Na, K, and Cs films on a Cu(100) surface. As a consequence of the
reduced work function, which is caused by adsorption of small amounts of al
kali metals, the He* deexcitation mainly takes place via Auger deexcitation
. Since the incident He* atoms interact only with the vacuum side tails of
the electronic states of the surface atoms, the deexcitation rate for this
mechanism reflects the electronic surface density of states. At coverages o
f about 0.4 monolayer a dramatic increase in the deexcitation rate was obta
ined, which is attributed to the subsequent filling of the alkali metal val
ence band and thus the onset of metallization.