We report measurements of the initial adsorption probability S-0 as we
ll as its coverage dependence S(theta(CO)) of CO on clean and oxygen-p
recovered Ir(110) for adsorption temperatures T-s below the onset of C
O2 formation. The impact energy (1.3<E(0)24 kcal mol(-1)), the oxygen
precoverage theta(O), and the surface temperature (85<T-s<200 K) were
varied. The initial adsorption probability for CO on the clean surface
decreases linearly with increasing impact energy E-0, and is independ
ent of T-s. The shape of the S(theta(CO),E-0) curves points to non-act
ivated precursor-mediated adsorption kinetics for low impact energies;
for E-0 greater than or equal to 20 kcal mol(-1), direct adsorption e
vents also contribute to the overall adsorption probability. For high
impact energies (E-0>3 kcal mol(-1)), S-0(theta(0)) decreases almost l
inearly with increasing oxygen precoverage. However, for low impact en
ergies (E-0<3 kcal mol(-1)), S-0(theta(0)) remains constant at 0.92+/-
0.03 independent of theta(0). The results suggest a physical site bloc
king mechanism. No significant temperature dependence of S-0(theta(0))
was observed below the onset of CO oxidation. (C) 1997 Elsevier Scien
ce B.V.