HIGH-ORDER COMMENSURATE STRUCTURES OF CF4 ON CU(110) FROM INTERACTIONPOTENTIAL CALCULATIONS

Potential energy calculations are performed in order to interpret the high-order commensurate (13x18) and (2x18) structures of the CF4 layer adsorbed on Cu(110) determined from helium diffraction experiments. W e find that the most stable geometry at 0 K is a low order commensurat e (2x2) phase containing two CF4 molecules which are dipod oriented in the Cu troughs. However, several other configurations close to this s table structure with tilted dipod and tripod orientations yield total binding energies per molecule which are only 10 meV (i.e., less than 5 %) weaker. Due to the lack of additional information on the potential accuracy, we find that the minimization procedure has difficulty discr iminating unequivocally between structures with a large number of mole cules per unit cell and for which changes in molecular orientations do not sensitively modify the total energy in the cell. In order to reco ver the experimental structure we propose a (2x18) phase with 18 CF4 m olecules per unit cell, deduced from the optimization calculation and leading to an electronic corrugation above the surface which is compat ible with the measured helium diffraction profile. (C) 1997 American I nstitute of Physics.