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The structure of the hafnium diboride (0001) surface was studied by impact-collision ion scattering spectroscopy. It was found that the clean surface, which is terminated by a flat (1×1) Hf layer at room temperature, turns into a surface with Hf adatoms after heating over 2000°C. This is probably caused by desorption of boron from the subsurface.

In order to investigate the mechanism of collision-induced electronic excitation, low-energy Ne + and Ne 0 have been scattered from an MgO(001) single-crystal surface together with clean and oxygenated polycrystalline Mg surfaces. One-electron excitation occurs efficiently during Ne 0 –Mg collision, which leads to (re)ionization of Ne 0 . At the MgO(001) surface, Ne 2+ ions are also created from Ne 0 incidence along the [110] azimuth where incident Ne 0 can make a sequential double collision on the aligned Mg 2+ ion rows, whereas Ne 2+ is not created in scattering along the [100] azimuth, where the Mg and O ions are arranged alternately. This result clearly shows that Ne 2+ is descended only from Ne + via one-electron excitation and not directly from Ne 0 via simultaneous two-electron excitation. From these facts, it is concluded that the unusually large Ne + yield from Mg should be ascribed to reionization of Ne 0 rather than autoionization of Ne**.

Vibrational frequency of H and D adsorbed on IVa and Va transition-metal carbide (TMC) (111) surfaces has been studied by high-resolution electron energy loss spectroscopy. Hydrogen (deuterium) was adsorbed dissociatively, showing the vibrational mode in an energy range of 110–120 (80–88) meV on ZrC(111), HfC(111) and NbC(111) surfaces, but at a much higher energy of 208 (149) meV on TaC(111). The former is similar to the case of H/TiC(111) consistent with three-fold hollow site adsorption. The latter seems too high to be explained by a variation of the force constant. It seems likely that on TaC(111), hydrogen is adsorbed on a different site from the other H/TMC(111) systems.

The atomic structure of magnesium oxide on the TiC(001) surface was investigated using time-of-flight impact-collision ion scattering spectroscopy. A hetero-epitaxial MgO layer was formed by thermal evaporation of magnesium onto the TiC(001) surface followed by exposure to oxygen at room temperature. From the time-of-flight spectrum which sees deeper layers from the top surface, it is found that the Mg and O atoms are located on the on-top site of the TiC(001) surface and the thickness of the MgO layer is restricted to two layers.

Low-energy (10 eV - 500 eV) He + ions are scattered from the LaB 6 (001) surface and the ejected electrons are detected. Two characteristic peaks are discernible in the electron energy spectra ; one is assignable to the surface state of the LaB 6 (001) surface resulting from Auger deexcitation of He * ( 3 S) descended from He + , and the other is due to autodetachment of temporary He - ions. These two peaks are very little broadened with increase of the velocity perpendicular to the surface, indicating that the electron emission occurs rather distant from the surface. On the other hand, marked energy broadening depending on the target species has been reported to occur if the electron is ejected via the Auger neutralization process. This is because Auger neutralization takes place even in a closer encounter with the individual surface atoms.

Low-energy (0.1–2 keV) helium and neon have been scattered from the Al(111) surface to investigate the mechanism of electronic excitation. Neutral He 0 and Ne 0 are (re)ionized during collision with Al as a result of one-electron excitation, whereas autoionization occurs preferentially for Ne + or Ne 0 incidence due to creation of two core vacancies. It is found that the Ne 2+ ion is formed during collision with Al and its intensity is largely dependent on the charge state of the primary beams; both single-scattering and double-scattering peaks of Ne 2+ emerge from Ne + incidence, whereas the double-scattering peaks of Ne 2+ dominates in Ne 0 incidence. This is because Ne 2+ originates from Ne + via one-electron excitation provided that the initial vacancy of Ne + exists in the 3dπ orbital and another vacancy is newly created in the 4fσ orbital after crossing the band. The predominance of one-electron excitation is characteristic of surface scattering and is basically caused by the band effect of energy-level crossing.

Room-temperature oxygen adsorption on NbC(111) and WC(1010) was studied by high-resolution electron energy loss spectroscopy and ultraviolet photoelectron spectroscopy. On NbC(111), oxygen is adsorbed dissociatively at the first stage. Above 2 X 10 -4 Pa s of exposure, a molecular species appears, which shows O-O stretching and M-O-O bending vibrations at 122 and 89 meV. These peaks disappear at 300°C. When the NbC(111) surface is saturated by dissociative N atoms prior to the oxygen adsorption, the molecular adsorption occurs from the first stage. This suggests that the molecular adsorption is caused by a site-blocking mechanism. On the other hand, oxygen is adsorbed molecularly on WC(1010) from the first. The UPS shows a broad peak at - 6.5 eV from the Fermi level, which is consistent with the 1π u and 3σ g orbitals of the molecular oxygen species.

The structures of oxygen-adsorbed and deuterium-adsorbed NbC(111) surfaces have been studied by impact-collision ion-scattering spectroscopy (ICISS) reflection high-energy electron diffraction (RHEED) and by computer simulation. At room temperature, both oxygen and deuterium are dissociatively adsorbed on the clean surface without long-range order, and an adsorbate atom goes into the threefold hollow site. The saturation coverage is more than 0.9 ML (monolayer) in both cases. The height of an adsorbate from the first layer is 1.10 A for oxygen and 1.00 A for deuterium, which corresponds to 2.13 A for the ONb distance and 2.08 A for the DNb distance. The ONb distance a little larger than the sum of the covalent radii (2.00 A) and the DNb distance is much larger than the sum of the covalent radii (1.64 A). Judging from the bond length and electronegativity, O and D atoms are supposed to have some negative charge.

Low-energy H + ions have been scattered from clean and oxygenated TiC(001) surfaces to investigate the mechanism of dynamical electronic transitions. The H + spectrum exhibits a marked surface peak of Ti, which is ascribed to reionization of neutralized H + rather than survival of the incident H + ions. Upon oxygenation, the H + (H − ) yield is increased (decreased) considerably; the surface peak for oxygen is almost absent in the H − spectra although it is remarkable in the H + spectra. These results are qualitatively elucidated on the basis of the model in which the electron transitions are mediated by the molecular orbital during a violent collision or a short-lived chemisorption state of hydrogen on the surface. The H 1s orbital has a strong antibonding character when colliding with the negatively charged oxygen atoms, by which the probabilities of neutralization, reionization, and negative ionization of the proton is largely affected after oxygenation.

In order to investigate the mechanism of electronic excitation during rare-gas ion scattering from surfaces, molecular orbital energies of various diatomic collision systems have been calculated on the basis of the discrete variational X α method. Doubly-excited autoionization is caused by the highly promoted 4fσ orbital of Ne colliding with the third-row elements of the Periodic System (NaAr), with decrease of the electron transition probability for heavier target atoms. By contrast, one-electron excitation leading to reionization is basically caused by the pseudo-crossing of the molecular orbitals. This is the reason why reionization is observed experimentally for a larger variety of projectile-target combinations compared to autoionization. Reionization may not be predicted sufficiently by the conventional treatment based on the diabatic correlation diagram. It is found that the probability of reionization is suppressed if the ionic level has a bonding character due to the interaction with the target d orbitals.

The mechanism of neutralization of and negative ionization from hydrogen ions is studied via experiments in which H + beams are incident on LiCl surfaces, combined with molecular-orbital-energy calculations. It is found that charge transfer and electron excitation occur due to the resonant-tunneling and electron-promotion mechanisms via a short-lived chemisorption state of hydrogen. The probability for H + to H − conversion at the LiCl surface depends on the species of the target atoms and is enhanced in collision with Li + by one order of magnitude relative to Cl − .

The segregation of W impurities at the TiC(001) surface has been investigated by using impact collision ion scattering spectroscopy and Auger electron spectroscopy. The topmost layer of TiC(001) is strongly enriched with W accompanied by carbon vacancies (20–25%) in their nearest neighbors, while no marked enrichment of W occurs in the deeper layers. The oxygen is incorporated by the (001) surface irrespective of the existence of W segregants, whereas its uptake rate is moderately enhanced by W, due probably to oxygen diffusion into the bulk mediated by the carbon vacancies.

Low-energy Li+ ion scattering is employed to study surface segregation of dilute transition-metal impurities (0.6 wt% of W, Ta, Zr, Nb and Mo) in TiC by applying two scattering angles so as to perform the layer-by-layer analysis of the atomic concentration. A marked face dependence of the impurity distribution is observed in the first five layers. At the close-packed (100) surface, the first layer is strongly enriched with the segregants substituted for the Ti atoms with the concentration of as large as 90 at%, though the deeper layers exhibit no marked enrichment of the segregants. At the TiC(111) surface, on the other hand, the segregants distribute not only at the topmost layer but also in the deeper layers at least up to the fifth layer with almost the same concentration of about 6 at%.

A new method for a computer simulation of impact-collision ion scattering spectroscopy (ICISS), a three-dimensional N-atom (3DNA) model, has been developed as an extension of the three-dimensional three-atom (3D3A) model [W. Hayami, R. Souda, T. Aizawa, Y. Ishizawa, Surf. Sci. 303 (1994) 247]. This 3DNA model can include arbitrary number of atoms, and the scattering cross-section of the system at an arbitrary scattering angle is analytically calculated as a function of impact parameter only, so that the computational time is as short as that of the 3D3A model. As an example, an ICISS spectrum of a four-atom system has been calculated, showing a different critical angle from the 3D3A model when two shadowcones overlap on the target atom.

On the basis of neutralization and inelastic scattering of low-energy D+ ions, the bond nature of Cs and CsCl adsorbed on the Si(100)2 × 1 surface has been investigated. In a low-coverage regime ( < 0.2 ML), Cs is adsorbed ionically on Si(100) and subsequent oxygen adsorption readily causes the ionic CsO bond. CsCl is dissociatively adsorbed on the surface in a low-coverage regime due to the preferential reaction of Cl with the Si dangling bond and the resultant Cs is ionically bonded to Si.

Low energy D+ scattering (E0 = 100 eV) is employed to explore the bond nature of the TiO2(110) surface with and without alkali-metal (AM) adatoms. The D+ ions are found to be neutralized almost completely in the course of scattering from Ti, while D+ survives neutralization during scattering from Na, K and Cs adatoms in the low-coverage regime (< 0.5 ML). On the basis of the hole diffusion picture of resonance neutralization, the absence of the Ti surface peak is indicative of covalency in the Ti-O bond, while the presence of the marked surface peaks shows that the AMs are perfectly ionized. With increasing coverage, an ionic-to-neutral transition of the AM overlayer takes place at 0.5 ML coverage. Above this coverage, the AM surface peak markedly decreases in intensity because partial filling of the AM s state gives rise to resonance neutralization of D+.

The surface phonon dispersion relationship and electronic structure of a graphite monolayer on (111) and (001) surfaces of ZrC were studied by high-resolution electron energy loss and angle-resolved ultraviolet photoelectron spectroscopy. The graphite monolayer on the (111) surface shows larger softening in phonon structures and decreased β band initial state energy than on the (001) surface. These phenomena were discussed in the viewpoint of charge transfer from the substrate to the graphite overlayer and it was revealed that there was much more charge transfer for the C/ZrC(111) system than for the C/ZrC(001) system.

The phonon dispersion relations of monolayer graphite on Pt(111), NbC(111) and NbC(001) were measured by electron energy loss spectroscopy. The monolayer graphite on Ni surfaces has been reported to show large softening, while that on Pt(111) was found to indicate little softening and rather resembled pristine graphite. Similarly, the monolayer graphite on NbC(111) indicated fairly large softening but that on NbC(001) did not. This difference is discussed in relation with the interface structure.