Using a novel combination of fundamental and harmonic polarizations, w
e have used optical second-harmonic (SH) generation to probe the nonli
near currents induced parallel to the surface plane of a Si(001) cryst
al upon adsorption of Na, K, and Cs. For all three alkalis, we observe
abrupt thresholds in SH intensity and relative SH phase shift at the
same coverage of very nearly one-sixth of a monolayer. We speculate th
at these thresholds correspond to insulator-metal transitions, and tha
t a recently observed low-coverage (2 x 3) structure is a Mott-Hubbard
-like insulator.