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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