Molecularly chemisorbed intermediates to oxygen adsorption on Pt(111): A molecular beam and electron energy-loss spectroscopy study

High translational energy adsorption of oxygen on the (111) surface of plat inum was examined with electron energy loss spectroscopy (EELS) and molecul ar beam techniques. EEL spectra indicate that over an incident energy range of 0.2-1.37 eV and on a Pt(111) surface held at 77 K, oxygen adsorbs in an associative chemisorbed state-yielding to the dissociated state only after sufficient substrate heating. Simple direct dissociation appears negligibl e for all incident kinetic energies studied. At near-zero surface coverages , exclusive population of the peroxolike molecular precursor is observed fo r adsorption at these high translational energies, while both superoxolike and peroxolike forms are detected for low energy adsorption (0.055 eV). Thi s peculiarity represents evidence that translational energy is effective in differentially populating reaction intermediates and provides better quant ification of potential energy barriers to dissociation. We estimate the act ivation barrier for dissociation from the peroxolike precursor to be approx imately 0.29 eV. Initial adsorption probability measurements over a wide ra nge of surface temperatures and high incident kinetic energies corroborate a molecular chemisorption mediated mechanism. (C) 1999 American Institute o f Physics. [S0021-9606(99)70132-7].