Abstract

The system CO/Pt(111) was studied by means of LEED, thermal desorption and work function measurements. At 170 K a sqrt (3 )� sqrt(3)/R30� structure at theta = 1/3 is continuously transformed with increasing coverage into a c4 � 2 structure at theta = 1/2 and finally into a hexagonal close-packed layer with saturation at about theta = 0.68. Due to a decrease of the adsorption energy by about 4 kcal/mole at theta = 0.5 (=7.5 � 1014 molecules/cm^2) adsorption is completed at this coverage at room temperature. The initial adsorption energy is about 33 kcal/rnole. A strong tendency for disordering far below room temperature manifests itself with the LEED patterns and with the "Delta phi" data. The work function at first decreases, exhibits a (temperature-dependent) minimum at theta = 1/3, attains nearly the value of the clean surface at theta = 1/2 and again exhibits a second (shallow) minimum around theta = 0.6. A detailed discussion reveals that the observed effects may be explained by assuming the occupation of two different adsorption sites at theta less-than-or-equals, slant 0.5 with different dipole moments (presumably bridge (A) and threefold coordinated (B)) whose adsorption energy differs by only 0.5 kcal/mole. At low temperatures at theta = 1/2 sites A and at theta = 1/3 sites B are preferentially occupied whereas their small energy difference favours disordering at increasing temperature.

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