Site resolved adsorption dynamics of CO on Pt(111)

Site resolved sticking coefficients were measured by reflection absorption infrared spectroscopy for room temperature CO adsorbing on Pt(111) at 20 K where thermal diffusion was completely suppressed. CO adsorbed directly onto the top sites but not the bridge sites of the bare Pt(111) surface. Only through collisions between impinging CO and preadsorbed CO could bridge sites be occupied in the absence of thermal diffusion. Isotope studies revealed two mechanisms for collisional occupation of the bridge sites: one in which impinging CO collisionally converts preadsorbed CO from top sites into bridge sites, and another in which occupation of bridge sites by impinging CO is facilitated by a scattering collision with preadsorbed CO. Cross sections for these two processes were large, sigma(T --> B) = 11 +/- 2 Angstrom(2) and sigma(S) = 20 +/- 4 Angstrom(2).