鈴木 康文

Energy losses of 1-MeV H-2+ ions passing through carbon foils of 2-8 mug/cm2 thickness have been measured. The observed energy loss increases linearly with the target thickness and shows an offset, 0.4 keV, at zero thickness. The stopping power derived from the observed energy losses is 1.15 times as large as the sum of the stopping powers for two single H+ of the same velocity. Stopping powers for H-2+ ions and diprotons are calculated with the first Born approximation. The calculated stopping power for the diproton agrees quite well with the one observed. This indicates that the observed H-2+ ions traverse the target as diprotons after the loss of the binding electrons upon entrance into the foil and recapture target electrons at the exit surface. The observed energy-loss offset at zero thickness also supports this scenario.

Scattered H+ fragments resulting from glancing angle incidence of MeV HeH+ ions on the clean (001) surface of a SnTe crystal are distributed on an ellipse in the E-phi plane, where the angle phi is measured in the plane parallel to the surface and E is the energy of the fragment. The kinetic energies released upon dissociation derived from the diameters of the ellipses are smaller than those calculated from the explosion of fully ionized fragments. Based on a simplified model, we have simulated the dissociative scattering of HeH+ ions at the surface, where the dissociation takes place via excitation of HeH+ ions and the charges of fragments change along the trajectory. It is concluded from the simulation that the cross section for dissociation in HeH+-electron collisions is of the order of 10(-16) cm2.

Angular and energy distributions of scattered fragments have been measured at glancing angle incidence of MeV HeH+ ions on a clean (001) surface of SnTe crystal. The distributions show that the internuclear vectors between fragments tend to align with the beam direction and the kinetic energy of the fragments acquired by the dissociation is smaller than that released at He2+-H+ explosion. The alignment of the internuclear vectors is explained in terms of a surface wake potential due to dynamic response of surface electrons to the charges of fragments. The observed kinetic energy indicates that most of the HeH+ ions are excited to some excited states without loss of electrons and dissociate into pairs of He and H ions on the incoming trajectories and the loss of electrons of the fragments does not occur until the internuclear distance of the pairs becomes a few times of the ground state one.

Convoy electrons are studied, which are produced by the transmission of 50 MeV He-3(2+) ions through two thin foils of carbon, where the downstream foil is exposed to the beam transmitted through the upstream foil. By eliminating the convoy electrons emitted from the upstream foil by applying an electric field, the convoy electrons produced at the downstream foil are studied. The yields of convoy electrons produced at the downstream foil, which has a thickness of less than the mean free path for convoy electrons (approximately 3-mu-g/cm2), are about twice as large as those emitted from a single foil of the same thickness. The increase is attributed to the convoy electrons produced by electron loss to the continuum (ELC) of Rydberg states of He-3(+) ions produced at the upstream foil.

Convoy electrons produced at glancing angle scattering of MeV H2+, H3+, HeH+ molecular ions from a clean (001) surface of SnTe single crystal are studied. Acceleration of the convoy electrons is observed similarly to the glancing angle scattering of atomic ions. The acceleration is larger than that observed at glancing angle scattering of the atomic ions with the same velocity which constitute the molecular ion. The experimental results are explained in terms of the surface wake induced by Coulomb exploding fragments.

Yields of secondary electrons from the (001) surface of SnTe are studied at the impact of 0.3-0.8 MeV/amu H+, H-2+ and H-3+ ions. The yield per ion upon impact of the H(n)+ ion is more than n times as large as that of isotactic proton impact. The observed molecular effects in the yields are explained as being the result of increased stopping power for fragment protons and of electrons released from the molecular ions upon their dissociation. It is also shown that the scaling relation, gamma-3 = 2-gamma-2 - gamma-1, remains roughly constant at oblique incidence angles up to 10-degrees to the surface, where gamma(n) is the yield at H(n)+ incidence.

Anomalies in the yield of ion-induced secondary electrons from single-crystalline target are studied. They are (1) disappearance of channelling dips in the angular distribution of the yield of secondary electrons in MeV light ion channelling, and (2) an increase of the yield at angle of incidence on a surface smaller than about 10-degrees. These are interpreted in terms of ion trajectories affected by crystal periodicity and effective mean escape length for secondary electrons in a solid.

Total secondary electron emission yields, gamma, were measured at the bombardment of Li+, Na+ and K+ ions with energies ranging from 15 to 65 keV on clean (001) surfaces on SnTe and PbSe crystals under UHV conditions. The yields are explained in terms of a kinetic electron emission model. From the orientation dependence of gamma, the ratio of the stopping power for channelling to that for random ions is deduced for a few low-index channels. Anomalous dependence of gamma on the thickness of PbSe layer during in situ expitaxial growth on SnTe(001) is found, and this is explained as the result of dechannelling of ions at misfit dislocations on the PbSe/SnTe interface.

Yields of convoy electrons are measured at the transmission of 50 MeV3He2+ ions through thick foils of C, Al, Ti, Ni, Mo, Ag, Ta and Au, Au foils coated with a thin C layer (< 11-mu-g/cm2) and thin carbon foils (1.2-100-mu-g/cm2). It is shown that the convoy electron yield is proportional to the He+ fraction in the beam transmitted by the foil. The results are discussed with the two step model of convoy electron production.

Good single crystals of PbS and PbTe are grown epitaxially on the (001) surface of SnTe. Misfit dislocations are observed on the interface of a PbTe/SnTe bicrystal but not on a PbS/SnTe interface by transmission electron microscopy. These observations are confirmed by detection of ion scattering from misfit dislocations in energy spectra of ions at RBS/ channelling of MeV He ions.