Trajectory and collision related neutralisation in low energy He+ ion scattering

A recent Surface Science Letter by MacDonald and Martin (Surface Sci. 111 (1981) L739) presents new experimental data on the neutralisation of He⁺ ions scattered from a Ni surface, which is interpreted in terms of a large collisional contribution to the neutralisation, not compatible with the usual Auger neutralisation picture. With the aid of new calculations, it is shown here that these data are consistent with an Auger neutralisation model involving local ion-atom charge exchange.     

Oscillations in scattered 4He+ ion yield curves from HgCdTe

Ion scattering spectra using ⁴He⁺ from Te and from epitaxial HgCdTe on CdTe were measured. A nearly smooth featureless curve was obtained for ⁴He⁺ → Te in the metal, but in the HgCdTe the ⁴He⁺ → Te curve showed oscillatory fine structure similar to that obtained from Sn. Apparently the Te molecular orbital energies have changed allowing quasiresonant interactions between ion and atom. Ion scattering spectrometry may provide bonding information as well as elemental analysis capability.     

Mechanism of electron exchange between low energy He+ and solid surfaces

A general picture on the mechanism of electron exchange between low energy He⁺ ions and solid surfaces is proposed on the basis of experiments on three-dimensional angle resolving ion scattering spectroscopy in which not only He⁺ but neutral He is used as a projectile.     

Crystallographic effects in low energy ion scattering due to local ion-atom neutralisation

New experiments on 1 keV ₄He⁺ ion scattering from Ni {100} and Ni {100} (√2 × √2)R45°?O surfaces show azimuthal anisotropies attributable to variations in ion neutralisation probability for different ion trajectories relative to the position of the surface atoms. These effects are shown to be compatible with a simple localised ion-atom neutralisation mechanism. The results indicate that local neutralization is an important process in substrate shadowing in low energy ion scattering studies of adsorbate structures.     

Inelastic effect in low energy ion-surface collisions: He+Au,Ag

Energy distributions of He⁺ ions scattered by Au and Ag surfaces are measured by an ISS system with high energy resolution, at a scattering angle of 90° and at incident ion energies ranging from 277 to 977 eV. It is found that the observed peak energies deviate toward the low energy side by several electron-volts with respect to the calculated elastic single collision energies. Both the deviation Q' and the inelastic loss energy Q are plotted as a function of incident ion energy for the Au surface.     

Crystal effects in the neutralization of He+ ions in the low energy ion scattering regime

Investigating possible crystal effects in ion scattering from elemental surfaces, measurements of the positive ion fraction P+ are reported for He+ ions scattered from single and polycrystalline Cu surfaces. In the Auger neutralization regime, the ion yield is determined by scattering from the outermost atomic layer. For Cu(110) P+ exceeds that for polycrystalline Cu by up to a factor of 2.5, thus exhibiting a strong crystal effect. It is much less pronounced at higher energies, i.e., in the reionization regime. However, there a completely different angular dependence of the ion yield is observed for poly- and single crystals, due to massive subsurface contributions in nonchanneling directions.     

He4-He4 interaction potential from low energy total cross section measurements

Total relative and absolute cross sections for He⁴-He⁴ have been measured between 110 and 1100 m/s velocity of the primary beam at scattering chamber temperatures of 5.0 and 77 K. Our measurements are sensitive to the low energy repulsive and the attractive part of the potential. The resulting potential is able to describe scattering measurements of other authors; it is also in very good agreement with ab initio calculations of Bertoncini and Wahl.     

Influence of the chemisorption state on the charge distribution of low energy Si scattered from I adatoms

The ion fractions of 5 keV Si⁺ ions singly scattered from iodine adatoms adsorbed on Al(1 0 0), Si(1 1 1) and pre-oxidized Si(1 1 1) were measured with time-of-flight spectroscopy. A considerable ion yield was observed, which did not change significantly with exit angle or I coverage. The mechanism of ion formation is assigned to valence electron resonant charge transfer (RCT) assisted by promotion of the Si ionization level. The yields are smaller than those of Si scattered from Cs adatoms, however, which suggests that electron tunneling from the occupied chemisorption states of the I adatom provides an additional neutralization channel.     

Surface hydrogen detection by low energy He ion scattering spectroscopy

Hydrogen adsorbed on a Si(100) surface can be detected by angle-resolved LEIS (low-energy ion scattering spectroscopy) which uses a ⁴He⁺ ion beam with incident energies around 1 keV. The scattering peaks from the surface hydrogen are restricted to narrow scattering angles from 0° to 15°, which is in agreement with those expected by the binary elastic collision model.     

Vibrational and rotational energy transfer of CH+ in collisions with 4He and 3He

A quantum mechanical investigation of vibrational and rotational energy transfer in cold and ultra cold collisions of CH⁺ with ³He and ⁴He atoms is presented. Ab initio potential energy calculations are carried out at the BCCD(T) level and a global 3D potential energy surface is obtained using the Reproducing Kernel Hilbert Space (RKHS) method. Close coupling scattering calculations using this surface are performed at collision energy ranging from 10^-6 to 2000 cm^-1. In the very low collision energy limit, the vibrational and rotational quenching cross sections of CH⁺ in collisions with He are found to be of the same order of magnitude. This unusual result is attributed to the large angular anisotropy of the intermolecular potential and to the unusually small equilibrium value of the Jacobi R coordinate of the He–CH⁺ complex. As for the He–N₂ ⁺ collision, we also find a strong isotope effect in the very low collision energy range which is analyzed in terms of scattering length and the differences between these two collisions are also discussed.     

The elastic scattering of 33 MeV He particles from Ni,Cu and Zn

Differential cross sections have been measured for the elastic scattering of 33 MeV ³He particles from targets of ^58,60,62,64Ni,^63,65Cu and ^64,66,68Zn over the angular range 5° to 175°. The data have been analysed with the optical model, with particular emphasis on the investigation of the ambiguities in the real potential, the form of the imaginary potential and the use of a spin-orbit term.     

Energy spectra of 6–32 keV neutral and ionized Ar and He scattered from Au targets; ionized fractions as functions of energy

The neutralization of ions is an important aspect of low energy ion scattering for surface analysis. Electrostatic energy analyzers (ESA) have been used almost exclusively in such work, and information on charge neutralization efficiencies is needed for quantitative interpretation of ESA data. In the past, the occurrence in low energy ion spectra of surface peaks and low backgrounds due to scattering from inside the solid has been attributed to preferential neutralization of ions which penetrate beyond the surface. In the work to be described, a time-of-flight technique was used to measure energy spectra of both neutral and ionized Ar and He scattered at 90° from a polycrystalline gold target. Incident energies of 6–32 keV were used. The energy spectra of neutral Ar scattered from polycrystalline gold exhibit sharp surface peaks, and double scattering shoulders, over this entire energy range. For He there is a gradual downward slope toward lower energy rather than a sharp surface peak. The behavior in both cases is attributed to large scattering cross-sections which cause a loss of beam particles during penetration. A calculation using a $\text{1}\text{r}{}^2$ potential illustrates this effect as a function of energy for helium. In the present experiments we find that the ion fraction of scattered argon does indeed depend on depth of penetration. This is in contrast to the behavior of He and H at higher energies, e.g. 100 keV, in which cases the charge state depends on emergent velocity but not on depth of penetration. The characteristic shapes of ion scattering spectra in this energy range appear to result from both neutralization and beam attenuation inside the target.     

The elastic scattering of low energy electrons from Xenon

Calculations are presented of angular distributions for electrons elastically scattered from xenon in the energy range 5.5 to 10 eV. The potential describing the xenon atom is determined by the X_α-approximation. The relative dependence with energy of the differential cross section at 30° scattering angle has been calculated in the range 30 to 200 eV. The theoretical results are in very good agreement with experimental data.     

Sputtering of SiC with low energy He and Ar ions under grazing incidence

The effect of low energy sputtering under grazing incidence upon the surface composition of SiC was investigated by Auger electron spectroscopy. The energy of the sputtering projectiles (He, Ar) varied from 200 to 1500?eV. Peak shifts to the higher energies with increasing argon ion energy were observed for all silicon and carbon Auger transitions. These shifts were explained by enhanced damage of the surface region within the sampling depth of the Auger electrons. The insensitivity of the Auger peak position to the energy of helium ions indicates that the damage state in the surface region does not change with the increasing energy of helium ions. An increase of the carbon concentration with the decrease of the argon energy was observed. The experiments were accompanied by dynamic Monte Carlo simulations by the TRIDYN code.     

Elastic and inelastic scattering components in the angular intensity distribution of He scattered from graphite

The angular intensity distribution of thermal energy He beam scattered from highly oriented pyrolytic graphite (HOPG) surface has been measured by means of supersonic molecular beam scattering technique in the wide surface temperature range. To separate the elastic and inelastic scattering components, simple analysis method has been developed by applying the classical binary collision theory of the hard cube model (HCM). From the extracted elastic scattering component in the scattering distribution, the Debye temperature of the HOPG surface has been derived as 590 ± 30 K. On the basis of the HCM analysis for the extracted inelastic scattering components of He beam, the effective mass for the HOPG surface has been found to be 72 u (six carbon atoms).     

The polarization energy of normal liquid 3He

We present new quantum Monte Carlo calculations of the ground-state energy of liquid ³He using backflow and 3-body wavefunctions and twist-averaged boundary conditions. We estimate the effects of the 3-body potential and compare to experimental measurements of the unpolarized equation of state and the magnetic susceptibility. There is still a large discrepancy with respect to the magnetic susceptibility, requiring either improved nodal surfaces or going beyond the fixed-node method.     

TDHF study of the He+ collision on atomic He targets at the8Be ground state energy

Experimentally the⁸Be ground state resonance has been studied in He⁺ collisions on atomic He atoms. The nuclear resonance manifests itself by satellite resonance lines corresponding to different electron configurations of the Be ion. Experimentally a large probability for the emission of one electron has been deduced. We study the atomic He⁺+He collision within a model in which the evolution of the electron wavefunction is treated dynamically in the TDHF scheme, and the motion of the nuclei is treated classically. In agreement with experiment we find a large probability for one electron to be emitted into the continuum during the lifetime of the⁸Be ground state resonance.     

Theory of ion neutralisation scattering from surfaces

A method for the calculation of the probability of neutralisation of an ion, which is scattered from the surface of a solid is presented. It assumes the ion to move along a classical trajectory and solves for the time evolution operator for the electronic system. For one electron Hamiltonians the solution can be carried out exactly. Results are presented for scattering from a semi-infinite linear chain.