W. Heiland

Scattering of swift molecules, H2 and CO2, from metal surfaces

Abstract Molecular ions and neutral molecules are scattered at 500 eV and at grazing incidence from metal surfaces. H 2 + and CO 2 + are subject to charge exchange processes which lead in part to dissociation. In the case of CO 2 + evidence for negative molecular ion formation is found. When using neutral H 2 and CO 2 only in the case of CO 2 is charge capture observed, i.e. negative ions are found. Potassium “poisons” the dissociation of H 2 on Pd. These findings are supported by comparison of different metals, Ni(110), Pd(110) and Pd(110) covered with a monolayer of K. The dissociation probability of H 2 + and H 2 0 decrease at the K-covered surface, in the cases of CO 2 + and CO 2 0 an increase is found. CO 2 + on Pd(110) + K dissociates completely.

Dissociative scattering of hydrogen from Pd(110) and Pd(110) + K

Experiments on the scattering of fast molecular hydrogen (500 eV) at grazing incidence from Pd(110) are reported. Both molecular ions and neutrals are used. The ions are in part dissociated by charge capture. Also the neutral hydrogen is partly dissociated. The adsorption of potassium on Pd(110) reduces the dissociation of the neutral hydrogen considerably.

Scattering of carbon dioxide molecules from Pd(111) surfaces

Abstract Carbon dioxide positive ions and neutral molecules are subject to different charge exchange processes when scattered off clean or potassium-covered Pd(111) surfaces under grazing incidence. The time-of-flight method is a suitable tool to investigate these processes with respect to dissociation and different charge state formation. Using projectiles with primary energies of 250 up to 1500 eV we found not only a strong energy dependence but also a sizeable influence of initial charge state and work function on the molecular survival. In particular, potassium increases the dissociation probability whereas switching from CO + 2 to CO 2 projectiles leads to the opposite effect. Furthermore, in the case of CO 2 on Pd(111) +K we found evidence for the formation of negative molecular ions, which are known to play an important role in dissociative chemisorption.

Scattering of H at Ni surfaces:charge exchange and energy loss

Abstract Hydrogen is scattered at grazing incidence from Ni(111) and Ni(110) surfaces. Significant differences are found in the energy loss and the energy loss straggling as a function of the primary energy between 300 eV and 5keV. The energy losses of the different exiting charge states H0, H+, H− for incident H+ when scattered from Ni(110) are equal, so is the energy loss straggling for H0 and H−. The straggling of exiting H+ is enhanced. The charge state fractions as a function of the particle energy agree with previous findings, i.e. the H+ yield increases with energy and the H− yield exhibits a flat maximum at about 3 keV. A theoretical model which accounts for all three charge states and reproduces the experimental findings is proposed.

The interaction of small molecules with Pd and K covered Pd surfaces at energies from 200 keV to 6 keV

Abstract In the energy range of 200 eV to a few keV small molecules are scattered at grazing incidence from Pd(110) and Pd(110) covered with a monolayer of K. The experimental setup provides ions and neutrals as primary particles and the time-of-flight analysis of the scattered particles includes both ions and neutrals. Depending on energy and K coverage, dissociation and scattering of the molecules is found. At low energies and one monolayer K the survival of H 2 is very high and decreases with increasing kinetic energy. At the clean surface the dissociation is high and increases with increasing energy. The data analysis shows that at low energies electronic processes are most important for the fate of the molecules, whether they are neutrals or ions. With increasing kinetic energy, vibrational and rotational excitations of the molecules contribute increasingly to dissociation. Classical trajectories calculations reveal interesting details of these processes.

Scattering of fast N2 from Pd(111): A classical trajectory study

Molecular nitrogen is well known for its chemical inactivity. Experimental results for grazing incidence N2 scattering from Pd(111) surfaces in the keV range also reveal negligible influences of electronical processes on molecular fragmentation. Therefore, we carry out an appropriate classical treatment of this system. The N2–Pd(111) interaction is mediated by an analytical six-dimensional potential energy surface, based on ab initio density-functional-theory calculations and empirical data. The molecule-surface interaction seems to be strongly influenced by the azimuthal direction of incidence as well as the molecular axis orientation. Particularly, the fragmentation is found to be mainly due to vibrational excitation for highly indexed azimuthal directions, whereas for incidence along lowly indexed directions rotational excitation is more important.

Scattering of fast H2 molecules from Pd surfaces: classical trajectory simulations

Classical trajectories simulations have been carried out for H-2 molecules at energies from a few hundred eV up to a few keV scattering under grazing incidence from Pd surfaces of distinct crystallographic orientation. Emphasis is laid on the importance of collisional dissociation mechanisms for incidence along surface semi-channels and along highly indexed directions. The initial molecule orientation is found to be decisive for the dissociation probability. Distributions of the center-of-mass energy and the angular momentum of the scattering products suggest that collisional dissociation is rotationally induced for channeling conditions, the molecule axis being initially oriented at an angle with the beam. For off-channeling incidence, collisional dissociation is vibrationally induced, the molecule axis being initially perpendicular to the surface plane.

Energy loss spectra of H+ and H0 scattered off metal surfaces

Abstract We recorded energy spectra after scattering of H+ and H0 off Ni surfaces. The incident energy range was 0.5–5 keV, we used 5° incident angle and a scattering angle of 10°. Hence nearly all particles probe only the surface region and do not penetrate into the bulk. To account for the resulting spectra we modified our previously successfully applied model of the He/metal-surface interaction and adapted it to the H/metal-surface system. The main difference is the number of charge states involved. In the He case there are two “active” charge states (He+, He0) for the energy region under consideration, whereas for H we have to deal with three charge states (H+, H0 and H−).

THE BROADENING OF ENERGY SPECTRA OF ATOMS SCATTERED BY A SOLID SURFACE UNDER MOLECULAR ION BOMBARDMENT

Abstract The broadening of energy spectra of atoms scattered by a metal surface under molecular ion bombardment are calculated taking into account the molecular ions to suffer dissociative neutralization on the initial part of their scattering trajectory. The results of the calculations explain the experimental data for scattering of hydrogen atoms from aluminum, palladium and palladium covered by potassium surfaces under bombardment with H2+.

Interactions of fast N2 molecules with palladium surfaces

Abstract Time-of-flight measurements of N 2 and N + 2 scattered off clean and potassium covered Pd-surfaces indicate a collisional dissociation mechanism for kinetic energies of 250 up to 5000 eV and grazing incidence (5°). Therefore, this system is predestined to be described within the framework of Newtons mechanics. Classical trajectory calculations have been carried out using different types of interaction potentials. It turned out that results from simulations based on pair potentials are not in full agreement with the experimental data, so we performed additional calculations using semi-empirical potential energy surfaces. The importance of dissociation due to rotational and vibrational excitation respectively depends strongly on the initial molecular axis orientation as well as on the crystallographic azimuthal direction.