M. Vana

Electron Emission from Polycrystalline Lithium Fluoride Induced by Slow Multicharged Ions

Total electron yields have been determined from electron emission statistics measured for impact of H+, Nq+ (q = 1, 5, 6) and Arq+ (q = 1, 3, 6, 9) on clean, polycrystalline lithium fluoride, by varying the impact energy from almost zero up to (10 × q) keV. Dependences of the electron emission statistics and yields on projectile charge and impact energy deviate strongly from the corresponding properties for clean metal surfaces, because of grossly different contributions of potential and kinetic emission and, in particular, a more efficient secondary electron emission if fast electrons are ejected from neutralising projectiles inside the LiF bulk.

Potential and kinetic electron emission from clean gold induced by multicharged nitrogen ions

Abstract Total electron yields have been determined from electron emission statistics measured for impact of N4+, N5+ and N6+ under normal incidence on clean polycrystalline gold. The projectile velocities were varied from 2 × 104 m/s, where potential emission (PE) is the exclusive electron emitting mechanism, over the threshold for kinetic emission (KE) at about (2–3) × 105 m/s, up to 106 m/s, where KE has become the dominant contribution. Separation of the respective PE- and KE contributions led to two remarkable results. (i) Comparison with measurements for inclined ion incidence from Hughes et al. (Phys. Rev. Lett. 71 (1993) 291) suggests that the resulting PE yields not only depend on the given “vertical impact velocity”, presumably due to different conditions for the subsurface part of PE. (ii) The KE yields increase the stronger with impact velocity the lower the primary ion charge, which is explained from a similar projectile charge dependence of the leading electron production mechanism for the KE process.

Precise total electron yield measurements for impact of singly or multiply charged ions on clean solid surfaces

Total electron yields γ for impact of singly or multiply charged ions (H+, He+, He2+, N3+, N4+, O5+, O6+) on clean polycrystalline gold have been accurately measured at impact energies from almost zero [exclusive potential emission (PE) range] up to 40 keV times projectile charge state q (dominant kinetic emission range). Impact energies above 10 q keV have been approached by postacceleration of ions via target biasing with up to −30 kV. Total electron yields for γ⩾3 have been derived directly from the related electron number statistics (ES) with total experimental errors of ±3%. Smaller values of γ have been determined from the related ES in conjunction with measurements of the respective primary ion, and ejected-electron currents, which caused somewhat larger experimental errors of typically ±5%. At higher impact velocity discrepancies arise between results from ES-based and current-based measurements of the total electron yield, respectively, because of systematic errors of the latter method due to pro...

Electron emission from polycrystalline lithium fluoride bombarded by slow multicharged ions

Abstract Total electron yields have been determined from electron emission statistics measured for impact of H+, Nq+ (q = 1, 5, 6) and Arq+ (q = 1, 3, 6, 9) on clean, polycrystalline lithium fluoride. Ion impact energies have been varied from almost zero up to 10 × q keV. The obtained total electron yields deviate considerably from available data derived via ion- and electron current measurements for LiF single crystal targets. Our results are explained by comparison with a recent model for MCI induced potential electron emission from clean metal surfaces, which has been properly adapted, available theory for kinetic electron emission from alkalihalide surfaces, and by considering also measured secondary electron yields for LiF. Dependences of the electron emission statistics and -yields on projectile impact energy and -charge differ strongly from corresponding properties for clean metal surfaces, which can be explained from the different roles of potential- and kinetic emission and, in particular, a relatively stronger contribution from secondary electron emission induced by fast electrons from finally neutralising projectiles inside the LiF bulk.

Distinction between multicharged fullerene ions and their fragment ions with equal charge-to-mass

Abstract Distinction between multiply charged fullerene ions C 60 q+ and their fragments with equal charge-to-mass ratio in a mixed cluster ion beam has been achieved by measuring the number statistics for the electron emission induced by impact of the respective ions on a clean metal surface. The new method is described with a practical example for a mixed beam of C 60 4+ and C 15 + . In addition, we show that this principle — that is, different electron emission characteristics for fullerene ions with different charges and masses but the same charge-to-mass ratio — can be utilized in any common ion mass spectrometer to distinguish various fullerene ions and their coinciding fragments by a judicious choice of the detector multiplier voltage.

Interaction of slow multicharged ions with solid surfaces: current concepts and new information on slow electron emission

Abstract Slow multicharged ions (MCI) approaching a solid surface are transiently converted into highly excited complexes, involving a potential energy possibly much larger than the kinetic projectile energy. De-excitation of such complexes proceeds within less than one pico-second via a manifold of interrelated reaction channels, giving rise to characteristic signatures as slow and fast electron emission, and X-ray radiation. For impact on LiF, also potential-energy dependent ejection of neutral and ionized target particles has been observed. Before their close contact with a metal surface, MCI are converted by multiple capture of target electrons into the so-called hollow atoms which rapidly decay by autoionization, at the surface loose outer electrons due to shielding, and eventually become de-excited inside the solid. At insulator surfaces, MCI neutralization and de-excitation is less well understood. Recently measured electron number statistics for MCI impact on clean metal (Au) as well as insulator (LiF) surfaces deliver interesting new information.

On the formation of hollow atoms in front of an insulating LiF surface

Abstract KLL Auger spectra of hydrogenic (1s) N, O and Ne ions impinging on an insulating LiF(100) single crystal are presented. Beam energy and incident angle have been varied such that the lowest possible velocity towards the target is achieved, at the same time varying the velocity parallel to the target by a factor of 10. Similarities and differences between the LiF spectra and spectra measured on (semi) conducting Si(100) and Al(110) surfaces are discussed.

Ion-induced electron emission from solid surfaces: information content of the electron number statistics

Abstract Measured electron number statistics resulting from the impact of singly and multiply charged atomic, cluster and fullerene ions on atomically clean metal (polycrystalline gold) and insulator (lithium fluoride) surfaces depend in a characteristic manner on the given projectile-target combinations and projectile impact energies. Comparison with model statistics delivers, apart from the respective total electron yields, further information on the total numbers and single-electron emission probabilities of the complete electron ensemble taking part in the emission process. For potential emission induced by slow multicharged ions on gold, the present analysis provides new insights on hollow atom formation and decay in front of the surface, in excellent agreement with recently calculated hollow atom autoionization rates.

Projectile charge dependence of kinetic electron emission from clean gold

Total electron yields for impact of H+, He+, He2+, C5+, N+ ÷ N6+, O5+ and O6+ on clean polycrystalline gold up to impact energies of 40 keV times the projectile charge have been determined from both the related electron number statistics and the respective ion- and ejected electron currents. The results depend on the amount of projectile potential energy deposited until close surface contact as well as on potential- and kinetic projectile energies deposited below the surface. The first contribution can be satisfactorily explained by the classical over-barrier model for potential electron emission. The second contribution results mainly from kinetic electron emission which involves nonlinear screening of the projectile-configuration dependent interaction with the metal electron gas.

Electron emission for impact of slow fullerene ions on clean gold

Electron emission for impact of singly and multiply charged fullerene ions Cnq+ (15 ≤ n ≤ 60; 1 ≤ q ≤ 5) on atomically clean polycrystalline gold has been studied at impact energies far from below threshold up to 250 eV/amu by measuring the number statistics of emitted electrons, from which accurate absolute total electron yields have been determined. In contrast to impact of atomic ions the fullerene charge q apparently has no influence on the total electron yield, i.e. no potential emission takes place. There is also no clear threshold for the electron emission which becomes measurable already around 10 eV/amu, above which it rises faster than linear with the ion impact velocity. Isotachic Cnq+ ions cause electron yields in direct proportion to their size n, with 250 eV/amu C60q+ producing on average more than 25 electrons. These results are tentatively explained by assuming that either the fullerene ions are completey shattered at the gold surface and individual carbon atoms produce kinetic emission, or electrons originate from thermionic emission of projectiles and their fragments heated up in the surface collision.