Sticking and thermal desorption of O2 on Ag(001)

Abstract

The coverage dependence of the sticking coefficient and temperature programmed desorption kinetics for molecular adsorption of oxygen on Ag(001) are investigated both experimentally and theoretically. The nearly exponential decrease of the sticking coefficient as a function of coverage by about five orders of magnitude is explained within a kinetic lattice gas model to be the result of strong nearest- and next-nearest-neighbor repulsions. The latter also shape the desorption spectra which are calculated within the same model under the assumption that fast surface diffusion maintains the adsorbate in quasiequilibrium during desorption. Because the desorption rate is proportional to the sticking coefficient under these conditions,we find a massive decrease of the pre-exponential factor in the Arrhenius parametrization of the rate as a function of coverage which explains the peculiar shape of the desorption spectra. (C) 1997 American Institute of Physics

Anisotropy of the oxygen interaction with Ag surfaces

“…No EELS signal could be attributed to subsurface oxygen in accord with the large screening of the dynamical dipole moment for a subsurface site. On Ag(001) we noticed however [57] that upon dissociation a peak at 130meV forms which is more difficult to be sputtered away than ordinary oxygen adatoms (which vibrate at 33meV on Ag(001)). Such loss cannot be due to the formation of superoxide because it persists up to T = 350K.…”

“…Such effect dominates at low impact energy where the steering is most effective. To be more quantitative the functional dependence of so on impact energy and angle is generally described by the functional form : [55]. In the large coverage limit on the other hand an exponential decrease is found.…”

“…with selected impact energy azimuth for Ag(ll0) because of the anisotropic surface corrugation. Disso-and angle of incidence [20][21][22][23][24][33][34][35][36][37]. In particular two ciation is dways promoted by chemisorbed 0 2 and, when heating the PhD thesis were devoted to the study of this system at the University Of Genoa index surfaces [43, crystal, it prevails over desorption for Ag(llO), while the contrary holds for molecular dissociation.…”

Dissociative dynamics of spin-triplet and spin-singlet O2 on Ag(100)

“…Thermal desorption measurements show that dissociation is less effective than desorption for Ag͑100͒ ͑the dissociation probability is P diss =4ϫ 10 −3 at T s = 300 K͒, whereas the two processes are comparable on Ag͑110͒ ͑P diss = 0.63 at T s = 300 K͒. 9,10 The dynamics of molecular adsorption is studied combining molecular beam techniques with high resolution electron energy loss spectroscopy ͑HREELS͒. On Ag͑100͒, 9,11 the initial sticking probability increases monotonically with molecular incidence energy E i up to a maximum value of 0.7 and then starts to decrease for E i Ͼ 0.8 eV.…”

Role of molecular electronic structure in inelastic electron tunneling spectroscopy:O2on Ag(110)

“…Combining this technique with either electron-energy-loss spectroscopy ͑EELS͒, infrared spectroscopy, or thermal desorption spectroscopy ͑TDS͒, it is also possible to extract information on the energetics ruling the elementary gas/surface processes-activation energies, atomic and molecular adsorption energies, and reactive paths. Thanks to these kind of studies we know that O 2 dissociation on flat Ag surfaces is characterized by rather large activation energies 8,9 and, as a consequence only molecular adsorption is possible at crystal temperatures below 150 K. [10][11][12] The chemisorption of O 2 on the Ag͑110͒ surface has been particularly controversial because of the initial disagreement regarding the nature and orientation of the chemisorbed molecule. Density-functional calculations performed by Gravil et al 8 shed light on this controversy showing the existence of two distinct chemisorption states with essentially equal energies.…”