  We report on new LEED, STM and ARPES studies of alkali/Si(111) previously
established as having a Mott insulating ground state at surface. The
observation of a strong temperature dependent Franck-Condon broadening of the
surface band together with the novel
$\sqrt{3}\times\sqrt{3}\to2(\sqrt{3}\times\sqrt{3})$ charge and lattice
ordering below 270 K evidence a surface charge density wave (SCDW) in the
strong e-ph coupling limit ($g\approx8$). Both the adiabatic ratio
$\hbar\omega_0/t\approx0.8$ and the effective pairing energy
$V_{eff}=U-2g\hbar\omega_0\approx-800$ $meV$ are consistent with the possible
formation of a bi-polaronic insulating phase consisting of alternating
doubly-occupied/unoccupied dangling bonds as expected in the Holstein-Hubbard
model.
