U. Imke

The O2Ag(111) interaction studied by 100–3000 eV glancing incidence O+2 scattering

Abstract We apply the technique of specular scattering of energetic beams from carefully flattened surfaces to investigate the dynamics of the O2Ag(111) adsorption/reaction system. The experiments were performed using 100–3000 eV beams of O+2 incident at 85° to the surface normal. The scattering products, O, O2, O− and O−2 were observed using time-of-night techniques. Part of the incident O+2 beam dissociates via charge transfer to dissociative or predissociative states of O2. Charge transfer to the ground electronic state of O2, if accompanied by the transfer of a second electron, results in O−2 formation. Above a threshold energy of 180 eV, the probability of formation of vibrationally cold O−2 is high (> 5%), indicating a large charge transfer probability at the surface and efficient access of the O−2 region of the adsorption/reaction potential energy surface (PES). At an incident beam energy of 2000 eV a threshold for O− production is observed. Classical trajectory calculations performed for the scattering of O2 from Ag(111) indicate that below 2000 eV no dissociation of the molecule occurs and that even about a fraction of a third of the molecules is vibrationally cold. Above 2000 eV collision induced dissociation of the O2 molecule is observed in the calculations that bears a striking resemblance to the experimentally observed O− yield. Therefore we propose that O−2 dissociates as a result of the impulsive collision with the surface, resulting in the observed O− production. We conclude from our results that O−2 is a precursor to dissociative adsorption and that the O− region of the PES is not readily accessible in our experiment, despite the presence of sufficient translational energy to overcome all activation barriers along the reaction coordinate.

O2− formation at Si(001)

A 450 eV−3 keV O2+ beam incident at 5° to a carefully flattened Si(001) surface was used to probe the adiabatic potential energy surface for the O2Si(001) adsorption/reaction system in the limit of low coverage. Over the full incident energy range, O2- an are formed as scattering products. We argue that the O2− product correlates with an O2−-like region on the potential energy surface and therefore propose that an O2−-like species is an intermediate or precursor in the dissociative adsorption of O2 on Si(001). Classical trajectory calculations indicate that a large amount of collision induced dissociation occurs at all energies.

Ion scattering as a probe of intermediate states in adsorption and reaction at surfaces

Abstract A direct correlation is established between the results of 480 eV CO + 2 -, N + 2 -, CO + + -, and 490 eV O + 2 -Ni(110) specular scattering experiments and known adsorption behaviour of the corresponding neutral molecules on Ni(110). Most importantly, we observe CO − 2 and O − 2 ions in our scattered product distributions. This observation demonstrates that negative ions that are precursors to dissociative chemisorption can be exposed and directly observed using high energy beam scattering techniques.

Charge transfer and dissociation in scattering of O2+ from Ag(111)

Abstract Neutralization and harpooning leading to negative molecular ions has been observed in collisions of O2+ at 1400 eV from clean Ag(111). The experiments are performed for specular scattering at grazing angles of incidence (85°). Scattered positive ions are not observed. O, O2, O− and O2− products are observed using time-of-flight techniques. The degree of dissociation for the neutral molecules is large, and in contrast to the small degree for negative ions. The results show that grazing incidence high energy beams can be used to probe chemical interactions at surfaces.

Interaction of fast molecular ions with surfaces

Abstract The scattering of molecular ions from surfaces has in general the same aspects as atomic scattering: nuclear and electronic interactions. For the nuclear or elastic part we have with molecules the additional channels of vibrational and rotational excitation, which are very important for the scattering of molecules at thermal energies, and may contribute to the dissociation of fast molecules. For the electronic or “inelastic” part molecules provide an additional channel for charge exchange processes, i.e. neutralization into dissociative states known as dissociative attachment in gas phase interaction. Experimental and theoretical results of the interaction of diatomic molecules with clean and Cs covered surfaces show that further improvements of present day charge exchange theories are needed. The results include the influence of the molecular axis orientation on the neutralization in the case of H+2 and the influence of work function changes on the molecular survival and neutralization in case of H+2, N+2 and O+2.

The interaction of fast N2+ ions with a Ni(111) surface

Abstract The reflection of nitrogen molecular ions from a nickel surface has been studied at energies between 200 eV and 4400 eV. In the scattered beam neutral atoms and molecules and atomic and molecular ions are detected. From the energy spectra of the atoms the relative kinetic energy distributions of the fragments are obtained. These give information on the processes leading to dissociation. The main mechanism for dissociation at low primary energies is charge capture. At higher energies vibrational and rotational excitation dominates.

Neutralization and dissociative attachment in molecular ion-surface interaction: Scattering of N2+ and O2+ from Ni(111)

The influence of the symmetry of molecular states on the neutralization and dissociation of molecular ions is demonstrated experimentally for O2+ and N2+ scattered from Ni(111).

A study of dissociative attachment in H2+−Al(110) surface scattering at low ion energies (300-900 eV)

During the interaction of molecular ions with a metal surface charge exchange processes occur which change the status of the molecule. One of the processes causes the dissociation of the molecule. The process is identified as dissociative attachment by capture of electrons from the conduction band of the metal.

Investigation of the dynamics of the O2/Si(001) adsorption system by small-angle ion-surface scattering

Abstract The small non-adiabaticity associated with the slow collision of a 450 eV to 3 keV O 2 + beam incident at 5° to a carefully flattened Si(001) surface was used to probe the adiabatic potential energy surface for the O 2 /Si(001) adsorption/reaction system in the limit of low coverage. The scattering products include O 2 − and O − . From the observation of efficient O 2 − formation and th dependence of the negative-ion yields on beam energy, we infer that O 23 − is an intermediate or precursor in the dissociative chemisorption of O 2 on Si(001). The total yield of negative ions (O 2 − and O − ) is high in comparison with similar experiments on th O 2 /Ag(111) system. This may be explained by a reduced reneutralisation probability on the exit trajectory due to the band gap of Si.

Negative molecular ion formation and work function changes: Experimental results for CO and CO2 scattering from nickel (+potassium) surfaces

The scattering of CO+ and CO+2 at grazing incidence from Ni(111)+K and clean Ni(111), Ni(110) surfaces produces CO, CO2 and dissociated species. The observation of negative species O− and CO−2 is strongly dependent on the K coverage or work function of the surface. The dissociation of CO+ (CO) is weakly changed by the presence of K, whereas in the CO+2 (CO2) case dissociation via CO−2 → CO + O− is strongly increasing with K coverage.