ONE-DIMENSIONAL DELOCALIZED ADSORBATE BLOCH STATES ON A SEMICONDUCTORSURFACE - C2H4 SI(001)-(2X1)/

We report the observation of a 1D molecular adsorbate band structure c aused by direct adsorbate-adsorbate interactions on a semiconductor. F or ethylene chemisorbed on Si(001)-(2 x 1) two adsorbate-derived elect ronic states, 1b(3u) and 1b(2g), are found to delocalize along the Si- Si dimer rows. The corresponding bands can be followed in reciprocal s pace from the <(Gamma)over bar> point into the third surface Brillouin zone. Density functional calculations for ethylene adsorbed on a sili con slab reproduce the experimental 1D band structure and allow us to identify the origin of the lateral interactions.