CO coverages as low as 10(-3) monolayers-which may result from backgro
und partial pressures in the low 10(-11) mbar range-are found to affec
t island shapes, island densities, step-edge barriers for adatoms and
film roughness during homoepitaxial growth on Pt(111). The effects are
traced to preferential adsorption of CO molecules on low coordinated
Pt atoms.