R. Souda

Neutralization and electronic excitation during low-energy H and He scattering from LiF

Abstract Low-energy H + and He + ions, together with their neutrals in the ground state, have been scattered from a polycrystalline LiF surface in order to investigate the mechanism of ion neutralization and electronic excitation. The surface peak of F obtained with He + incidence is composed of three subpeaks assignable to elastic scattering and inelastic scattering due to single and double electron-hole pair excitation. In the case of H + scattering, the surface peak of F consists only of the elastic peak, and a background due to multiple scattering from the deeper layers is remarkable. Almost the same H + spectrum without the surface peak is obtained for H 0 incidence. These results are analyzed on the basis of molecular-orbital energy calculations. It is found that the F 2p orbital has antibonding character with respect to the He 1s orbital during collision and, hence, one or two F 2p electrons can be excited preferentially. In the case of H + scattering from F − , on the other hand, the H 1s state appears in the band gap due to antibonding interaction with the F 2p orbital, leading to excitation of the H 1s electron rather than the F 2p valence electrons.

Formation of monolayer graphite at the WB2(0001) surface

The formation of monolayer graphite on the WB 2 (0001) surface has been investigated by scanning tunneling microscopy (STM). STM observation shows that two kinds of terrace exist, which correspond to those with planar and puckered boron in the third layer. Monolayer graphite (MG) was formed on the WB 2 (0001) surface, but the two surface regions exhibit different abilities for MG formation. The ab initio cluster calculations suggest that MG prefers to be formed on the terrace with puckered boron in the third layer.

Segregation of Eu implanted at the MgO(100) surface

Abstract The surface segregation of Eu implanted at the MgO(100) substrate has been investigated using time-of-flight coaxial impact-collision ion scattering spectroscopy (TOF-CAICISS) and reflection high-energy electron diffraction (RHEED). It is found that Ca impurities, which are included originally in the bulk of MgO in an amount of 210 ppm by weight, are segregated to the surface after the implantation of 200 keV Eu + ions. Segregation of the implanted Eu occurs when the substrate is annealed at 1000°C in ultrahigh vacuum (UHV; 5 × 10 −10 Torr). In the process of Eu segregation, Eu is located at the substitutional position of Mg, in addition to the interstitial position of the MgO lattice. This indicates that there are two types of pathway for the segregation of Eu. At the last stage of Eu segregation, it is seen that Eu is concentrated at the outermost surface layer, protruding from the original MgO plane by 0.3 A.

Positive ionization of hydrogen during scattering and sputtering from clean and passivated Si(111) surfaces

Abstract The mechanism of positive ion formation has been investigated by H+2 (H+) and He+ bombardment of the clean and hydrogenated/oxygenated Si(111) surfaces. It is found that the primary H+2 (H+) ions are equilibrated (neutralized) immediately at the surface, and the scattered and sputtered H+ ions are caused by reionization during asymmetric diatomic collision with Si at the surface. The resulting H+ ion preferentially accommodates the dangling-bond electron via a transient chemisorption just before leaving the surface, which is evidenced by the fact that the H+ yield is largely increased due to passivation of the dangling bond. Thus, a unified picture of positive ion formation is obtained between scattering of reactive ions and sputtering of secondary ions. These findings also indicate that charge exchange during scattering of low-energy proton gives a unique insight into surface femtochemistry.

Surface segregation of implanted ions: Bi, Eu, and Ti at the MgO(100) surface

Abstract Surface segregation of Bi, Eu, and Ti implanted at the MgO (100) surface has been investigated by using time-of-flight coaxial impact-collision ion scattering spectroscopy (CAICISS) and reflection high energy electron diffraction (RHEED). It is observed that implanted-Bi is concentrated at subsurface layers of MgO. On the other hand, it is found that Eu segregates to the outermost surface layer by annealing at 1000°C, in addition to the simultaneous segregation of Ca which is included in the bulk as impurity of 40 ppm. During the implantation of Ti at the MgO substrate, it is found that C makes inroads upon the substrate. Surface segregation of Ti itself is not observed by annealing the substrate from 400°C to 1000°C. It is concluded that nature of surface segregation of these systems primarily depends on the size of the implanted ions.