S. B. Elston

Observation of high-order electric multipoles in convoy electron angular distributions

The authors have discovered multipoles of even order to about 10 in the multipole moments characterising angular distributions of convoy electrons ejected by swift ions during penetration of thin solids. They suggest the strongly transverse distributions observed reflect electron loss from excited n, l states inside the solid, which, though rapidly created and destroyed, progress toward finite dynamic equilibrium populations. Alternative interpretations are also discussed.

Yield of convoy electrons from solids

Abstract The number of convoy electrons emitted in the forward direction from solid targets of C, Al, Ag. and Au per incident 1–4 MeV/amu H, O, Si, and Ni projectile is reported. As expected, the yield is independent of target thickness and of the incident projectile charge state. We find that the yield is target ( Z ) 2 dependent, increases as Z 1 2.75 ( Z ) 1 is the atomic number of the incident projectile) and decreases as E 1 −2.25 ( E 1 is the incident projectile energy in MeV/amu).

Production of soft X-ray emitting slow multiply charged ions: Recoil ion spectroscopy

Abstract A study of Ne L-shell vacancy production by S ∼14+ , Cl ∼12+ ions has shown copious production of NeIINeVIII excited states with ∼10 −18 cm 2 cross sections and recoil velocities which may permit significantly higher resolution spectroscopy than is possible with beam-foil methods.

Observation of intense low energy autoionization lines in the wings of the forward peak from fast ion-atom collisions

In the ejected electron spectra from collisions of C2+, O4+ and Si10,11,12+ with argon toms at energies of 2–4 MeV/amu, a series of intense lines has been observed, which originates from doubly excited autoionizing states of Belike projectiles. These lines correspond to electron energies between 2 and 20 eV in the projectile frame. For kinematic reasons these lines can only be detected near the forward direction.

Studies of Collision Interactions and Kinetic Energy Distributions of eV Recoil Ions Inside Penning Traps

Measurements and techniques are described which lead to charge transfer collision studies of low energy multi-charged recoil ions with target gas atoms and molecules. The recoil neon ions are produced by impact ionization of neon atoms using 1 MeV amu-1 Cl+5 projectile ions stripped to a mean charge state near 10e. The recoils with e ≤ q ≤ 7e are confined by the electric and magnetic fields of a Penning ion trap. The number and mass-to-charge ratio of the stored ions is determined by resonant axial excitation and cyclotron resonance. Mean storage time constants > 100 ms limited by charge transfer are observed at pressures in the 10-8 torr range. Estimates of mean stored ion energy are made by examining the ion number stored as a function of axial well depth. Preliminary charge transfer reaction rate constant results are presented. Possibilities for precision spectroscopy on multi-charged stored recoil ions are also discussed.

Radiative Lifetimes and Oscillator Strengths for Allowed Transitions in Li-like and Be-like Si

The lifefimes of the (2p)2P3/2 level in Si XII and the (2p2)3P2,1,0, (2s2p)1P1 and the (2p2)1D2 levels in Si XI have been determined using the beam-foil method. It is proposed that existing discrepancies between experimental and theoretical f values for the (2s2)1S0-(2s2p)1P1 transition in some ions of the Be sequence may be the result of the inability of the experimentalist to account for strong cascading from the (2p2)1S0 level.

Studies of neonL-shell excitation by impact of highly ionized heavy ions

Passage of foil-excited 1.4 MeV/A S and 1.1 MeV/A Cl ions of neon charge state ∼ 12+ through neon gas targets at pressure ∼100 mTorr has been found to be accompanied by copious production of Ne II–VIII excited states. Comparable excitation cross-sections ∼10−18 cm2 are found for a large number of levels belonging to all of these charge states and corresponding to principal quantum numbersn=2, 3, 4. Vacancy distributions very similar to those found in beam-foil excitation of ∼1MeV neon beams are found. Because the Ne recoil velocities are small compared to the fast beam velocities characteristic of the beam-foil source, it is possible to reduce both Doppler shifts and spreads by 3–4 order of magnitude for equivalent collimation. It has also been found that there is an excitation cross-section change of a factor ∼5 for a corresponding projectile charge state change from 6+ to 12+, that efforts to classifyK x-ray satellite spectra byLshell vacancy labels (KL0,KL1,...) are probably inaccurate due to extensive population ofn≧3 spectator levels, that both the recoil ion and beam-foil spectra exhibit few lines withn≧4, and that for the allowed transitions studied here, collisional excited states quenching effects due to the ∼100mTorr target gas pressures used are negligible.

Measured multipole moments of continuum electron transfer angular distributions

Abstract The velocity space distribution of electrons emitted near the forward direction from collisions involving fast, highly stripped oxygen ions with gaseous and solid targets is presented and described in terms of multipole moments of the ejected charge distribution, which permits direct comparison with recent theory. The measurements are produced by employing position-sensitive electron detection to combine emission angle definition with conventional electrostatic spectrometry. Agreement obtained between theory and distributions observed for binary continuum electron loss processes coupled with a similar multipole content observed with solid targets suggests a model of convoy electron production dominated by electron loss from the projectile within the bulk of the target. Further, the connection between multipoles of the projectile electron emission distribution in single collisions and the state of excitation of that projectile excited states may provide the basis for a probe of the state of ions traversing bulk solid matter.

Electron Capture to the Continuum at Asymptotically High Velocities

The velocity distrubutions of electrons ejected from gaseous He targets into the forward direction by 15-18 au velocity bare O, Ne and Ar nuclei are measured. The shape of the resulting ECC (electron capture to the continuum) cusps is studied in detail by three fitting methods and compared to theory. The results favor theoretical treatments which include higher-order ejected electron partial waves to describe the observed asymmetry.

Velocity dependence of the widths of electron-loss cusps in the range 7-12.5 au

The velocity dependence of the full width at half maximum of projectile electron-loss cusps is investigated. For Siq+, Oq+ and Cq+ projectiles traversing He, Ne and Ar targets in the velocity range 7-12.5 au, the FWHM is found to be independent of the ion velocity. These results disagree with theoretical predictions predating those of Day and restrict the velocity dependence of the anisotropy parameter beta in Days theory (1980).