K. Kuroki

Potential sputtering of proton from hydrogen-terminated Si(100) surfaces induced with slow highly charged ions

A potential sputtering mechanism of hydrogen has been studied for impact of slow highly charged Xeq+ ions (<5 keV, q=4–12) on well-defined H-terminated Si(100) surfaces. It was found that the sputtering yields of protons are proportional to qγ (γ∼5), independent of the surface condition, that is, for both Si(100)2×1-H surface and Si(100)1×1-H surface. The yield for Si(100)1×1-H surface was ten times larger than that for Si(100)2×1-H surface, although the H coverage of the former is only twice the latter. Surface roughness is found to be the key parameter to vary the yield, and also to influence the energy distribution of sputtered protons. These findings are consistently explained with a pair-wise bond-breaking model induced by a double electron capture, where the classical over barrier process plays an essential role.

Strong charge state dependence of H+ and H2+ sputtering induced by slow highly charged ions

Abstract Secondary ion emission has been studied for very slow (∼ 0.01 νB) highly charged Ar and N ions bombarding C60 containing hydrogen as an impurity. It is found that the fragmentations of C60 are very rare even for Ar16+ bombardments. On the other hand, the sputtering of H+ and H2+ has been observed to increase drastically as a function of incident charge q like qγ (e.g., γ ∼ 4.6 for H+ sputtering by 500 eV Arq+).

High Rydberg and Auger electrons from fast projectile ions studied by zero-degree electron spectroscopy

Abstract Zero-degree electron spectra were measured with high resolution for 64 MeV S5+ ions on He and 64 MeV S12+ ions on He and C, and for 90 MeV Sc8+ ions on He. In 64 MeV S ion collisions, a series of intense lines have been observed, which are attributed to the Auger transitions from 1s22s22p53snl to 1s22s22p6 for S5+ ions on He, and to the Coster-Kronig transitions from 1 s 2 2 p nl to 1 s 2 2 s ϵ l l′ for S12+ ions on He or C foils. The angular-momentum (l) distribution of the S12+ ions in the Rydberg state with n = 9 has been obtained and it is found that the high-l components are significantly enhanced for the solid target. In 90 MeV Sc8+ +He collisions, three groups of intense lines have been observed, which are attributed to the transitions from 1s22s22p53s23p to 1s22s22p63s, 1s22s22p63p, and 1s22s22p63d. The last transition, in which three electrons are involved via electron correlation, is studied and compared with previous results for the isoelectronic ions Al+ and Ar6+.

Low-energy Rydberg electron spectra from 64 MeV Sq+ + He and C-foil collisions using zero-degree electron spectroscopy

Abstract Zero-degree electron spectra were measured with high resolution for 64 MeV Sq+ (q = 6, 11 to 13) ions on a He and C-foil target. Several peaks are observed in the electron energy range up to 2.5 eV for the 64 MeV S12+ + He collision, in addition to a series of autoionization electrons produced by the Coster-Kronig transition 1 s 2 2p( 2 P 3 2 )9l−1 s 2 2s ϵl′ . The charge state dependence of electron spectra for 64 MeV S11+.12+.13+ + He and S6+.12+.13+ + C were measured to identify the origin of the low energy electrons. No characteristic line is observed in the electron energy spectra following the 64 MeV S13+ beam excitation with He. This indicates that the electron excitation and capture process that could produce the Li- and Be-like initial configurations resulting in autoionizing Coster-Kronig transitions does not occur during a collision between 64 MeV S13+ ions and a He target at this collision energy. It is supposed that the electron spectra for S11+ shows a series of autoionizing Coster-Kronig transitions of 1s22p27l−1s22s2pϵl′, 1s22p29l−1s22s2pϵl′ and 1s22s2p9l−1s22s2ϵl′ for S11+ ions, which are energetically possible.

Metastable states produced with beam–capillary interaction

Abstract The formation and relaxation processes of hollow atoms and related excited states of ions produced with a Ni microcapillary thin foil have been studied employing various experimental techniques. The first stage of the charge transfer from the surface to the ions was studied by using visible light measurements. On the other hand, X-ray measurements revealed the core electronic configurations of ions at the last moment of the hollow atom evolution. The overall feature of the electron capture processes in the capillary was given by the charge state distribution measurements.

Potential sputtering of hydrogen atoms on various surfaces with slow highly-charged ions

Abstract Several recent findings on a new sputtering mechanism of hydrogen with slow highly-charged ions are discussed. The sputtering yields of protons were proportional to q∼5 for q≲10 independent of the surface condition for both untreated and well-defined surfaces, where q is the charge state of the ion. This q∼5 dependence started to level off for q≲10. The yield for the Si(100)1×1–H surface was ten times larger than that for the Si(100)2×1–H surface although the stoichiometric hydrogen abundance of the former is only twice that of the latter. The key quantity to govern the yield is proposed to be surface roughness, which also influences the energy distribution of sputtered protons. These findings were consistently explained with a pair-wise potential sputtering model involving two successive electron transfers which follow the classical over barrier mechanism.