Existence of a “Hot” Atom Mechanism for the Dissociation ofO2on Pt(111)

The dissociation of ${\mathrm{O}}_2$ on a Pt(111) surface was studied by variable temperature scanning tunneling microscopy at 150–106 K. The two oxygen atoms created by the dissociation appear in pairs, with average distances of two lattice constants. Since thermal random walk sets in only at around 200 K, with a diffusion barrier of 0.43 eV and a preexponential factor of ${10}^{-6.3\mathrm{}}{\mathrm{cm}}^2{\mathrm{s}}^{-1\mathrm{}}$, the distribution of distances at around 160 K evidences nonthermal processes during the dissociation. It is concluded that transient ballistic motion exists, where the short range traveled is in agreement with recent molecular dynamics studies.