It was noted that Na, when deposited onto cleaved (111)-(2 x 1) surfaces, induced a (2 x 1) to (1 x 1) transition at a monolayer coverage of about 50%. Angle-resolved direct and inverse photo-emission data revealed that the surface was semiconducting. The measured energy dispersion of the empty Na-induced surface state was consistent only with calculations that favored a 3-fold hollow site for the position of adsorbed Na.

B.Reihl, S.L.Sorensen, R.Dudde, K.O.Magnusson: Physical Review B, 1992, 46[3], 1838-41