M. Hasegawa

Specular reflection of fast ions at a single crystal surface

Abstract The interaction of fast ions with a solid surface is studied by the specular reflection of MeV H+ and He+ ions from clean (001) surfaces of SnTe single crystals. Oscillatory structure is observed in the energy spectra of the reflected He ions. This indicates that a part of the incident ions penetrate inside the crystal and travel for a few wavelengths of oscillatory motion in the (001) planar channel before reappearing at the surface. The position dependent stopping power is obtained to be proportional to E − 1 2 exp (−0.15x/a TF ) , where x is the distance from the surface. A large difference is observed in the charge state distribution between the specular reflection and the transmission through a self-supporting foil. This is attributed to the fact that the specularly reflected ions interact exclusively with the valence electrons. It is found that the velocity of convoy electrons emitted at the glancing-angle incidence of He ions is faster than that of the incident ions. This may be due to the acceleration of convoy electrons by the dynamical image potential induced by ions near the surface.

Surface channeling of MeV He+ ions on a SnTe single crystal

The scattering of 0.7 MeV He+ ions at a (001) surface of a SnTe single crystal is investigated under the condition of surface channeling. The observed azimuthal angle dependence of the scattered ion yield shows anormal peaks due to [100] and [110] surface channeling. The comparison of the observed results with computer simulations shows that the anormal peak is ascribed to the penetration of ions inside the crystal via surface steps.

Acceleration of convoy electrons produced by MeV He ions for glancing angle scattering from a clean surface

Abstract Dependences of the most probable energy of convoy electrons ontheergy of MeV H and He ions are studied for glancing angle scattering of the ions from a clean (001) surface of SnTe. The convoy electrons emitted at He scattering are faster than the projectile He ions, while at H scattering they are almost isotachic to the projectile H ions. The acceleration of convoy electrons at He scattering is explained as the result of polarization of valence electrons induced by the projectile He ion.

Secondary electron emission due to MeV He ions at glancing incidence angles on clean single crystal surfaces

Abstract The total secondary electron yields per ion, γ, are measured for MeV He + ions incident on clean (001) surfaces of SnTe and PbSe single crystals. The dependence of γ on the incidence angle of the ions is well fitted by 1/sin θ at larger θ. (θ is the angle of incidence with respect to the surface plane.) However, at θ ~ ψ a an abrupt decrease of γ is observed, where ψ a is the characteristic angle of planer channeling. The decrease is due to the specular reflection of ions at the single crystal surface.

Convoy electrons produced in specular reflections of MeV He ions at SnTe single crystal surface

Abstract Convoy electrons produced at specular reflections of 1 MeV He ions at a clean (001) surface of SnTe single crystal are measured in coincidence with reflected ions. It is found that convoy electrons are emitted outside the crystal on the outgoing trajectories of specularly reflected ions.

Convoy electrons produced at glancing angle scattering of MeV molecular ions from a single crystal surface

Abstract Convoy electrons produced at glancing angle scattering of MeV H 2 + , H 3 + , 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.

Stopping powers of metallic elements for high energy ions

Abstract Stopping powers of metallic elements have been measured for 55, 65 and 73 MeV protons and for 13 MeV/u 4He and 12C ions using a high resolution magnetic spectrograph. Analyzing experimental data for protons with the Bethe-Bloch formula, we deduced mean excitation energies for 10 metallic elements. The magnitude of Barkas correction was extracted from stopping power difference for 4He and 12C ions at the same velocity, which was found to be consistent with those measured in previous experiments.

Convoy electrons produced at glancing angle scattering of MeV HeH+ ions from a crystal surface

Abstract Convoy electrons produced at glancing angle scattering of MeV HeH+ ions from an atomically clean (001) surface of SnTe crystal are observed. Energy spectrum of the convoy electrons shows a peak broader than that at scattering of atomic projectiles and the most probable energy of convoy electrons at HeH+ scattering is larger than those at scattering of isotachic He ions. This acceleration of convoy electrons is qualitatively explained by the force due to surface wake induced by Coulomb exploding fragment He2+ and H+.

Thickness dependence of convoy electron yield at transmission of 50 MeV 3He2+ through carbon foils

Abstract The dependence of the convoy electron yield on target thickness is found at a transmission of 50 MeV 3 He 2+ ions through thin carbon foils (1 2 ), where the charge state equilibrium of the transmitted ions is not attained. It was found that the convoy electron yield is proportional to the fraction of He + in the transmitted ions. The probabilities of convoy electron production (per unit length), and thus the cross sections of ELC (electron loss to continuum) and of ECC (electron capture to continuum) and the mean free path for convoy electrons were estimated with the two-step model in which the convoy electrons are produced in the bulk and are transported out of the solid.

Production of convoy electrons by foil-excited He ions

Abstract Convoy electrons are studied, which are produced by the transmission of 50 MeV 3 He 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 (∼ 3 μg/cm 2 ), 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 3 He + ions produced at the upstream foil.