C. D. Moak

Charge-changing collisions of channeled oxygen ions in gold

Abstract Oxygen ions (4–40MeV) with various charge states have been used to study the effects of channeling upon emerging charge-state distributions and to determine the effect of channeling upon the charge-changing cross sections involved. At energies above 20 MeV, no true equilibrium is observed even for pathlength of 0.6 μm in Au channels. Instead we find quasi-equilibrium for charge states up to 6 + and strongly pathlength-dependent charge functions for 7 + and 8 + injection. The data are interpreted in terms of interaction with loosely bound outer electrons in Au.

Charge states of 15-140 MeV bromine ions and 15-162 MeV iodine ions in solid and gaseous media

Graphs are presented for use in estimating equilibrium charge distributions in Br and I ion beams as they penetrate various gases and solids. Tables of the basic data are included. Additional graphs show nonequilibrium charge distributions as functions of target thickness for several Br and I ion energies for H/sub 2/, He, and Ar targets. (auth)

Resonant-coherent excitation of channeled ions.

A first-principles calculation of the resonant-coherent excitation of planar-channeled hydrogenic ions is presented. The interplay between coherent interaction with the periodic crystal lattice potential and inelastic electron-electron collisions is shown to be crucial in both intraionic transitions and electron loss from the ion. The magnitude of resonant-coherent excitation is predicted to oscillate with the amplitude of the oscillations of the ion trajectory. Good agreement is found with experiments.

Resonant coherent excitation of O7+, F8+, and C5+ in the 〈100〉 axial channel in gold

Abstract Previous studies have shown that when an ion moves in an axial channel with a velocity, v , such that hKv/ d = ΔE ij , transitions are coherently induced. K is an integer, d is the longitudinal atomic spacing in the channel, and ΔE ij , is an ionic transition energy. Since the ionization cross section for an excited state is much greater than that for the ground state, the effect for 1s → 2p transitions is observed as a minimum in the surviving, one-electron charge-state fraction as the velocity is scanned through a resonance. Resonant coherent excitation (RCE) for the following one-electron ions, moving in the 〈100〉 axial channel in gold, are reported here: O 7+ , K = 2, 85.9 MeV; F 8+ , K = 2, 163.6 MeV; and C 5+ , K = 1, 81.6 MeV. The K = 2 resonances have a single narrow dip superimposed on a broad minimum, in contrast to the doublet minimum previously observed for lower Z ions. Comparison is made to predictions based on the positions of the Stark components deduced from the rainbow scattering theory. A similar comparison is made for the stronger and broader K = 1 resonance of C 5+ .

Resonant coherent excitation in planar channeling

Abstract Planar channeled ions (velocity = ν ) experience a coherent periodic perturbation of frequency ν = υ / d , where d is the distance along the ion path between planes orthogonal to the channeling plane. The velocity at which a given RCE harmonic, ( l , k ), occurs is tuneable with θ. This additional degree of freedom allows the measurement of (1) velocity dependence of the static and dynamic ( (wake) field on the ion, and (2) coincidences in velocity for more than one resonance. Using the enhanced ionization technique, we report on RCE for N 5+ and N 6+ in (100) planar channeling in Au.

ORIENTATION DEPENDENCE OF INTENSITY AND ENERGY LOSS OF HYPERCHANNELED IONS.

Abstract Energy loss measurements are reported for 21.6–60 MeV 127I and 3 MeV 4He ions transmitted through thin single crystals of Au and Ag along directions parallel or nearly parallel to 〈110〉 directions in the crystals. A systematic investigation of 21.6 MeV 127I ions in Ag indicates that for incidence angles within 0.12° of [O11] up to 10 percent of the transmitted ions have energy losses significantly lower than that for ions transmitted parallel to (111). The behavior of these ions is consistent with that expected for ions which are confined within one particular axial channel throughout the crystal. This specific type of axial channeling we call hyperchanneling. The data for hyperchanneled 21.6 MeV 127I in Ag are analyzed in detail and discussed in terms of a theory developed on the basis of potential energy contours obtained from the continuum potentials of the surrounding rows of atoms. Comparison of theory and experiment suggests that these ions undergo more multiple scattering than can be accou...

Channeling effects on the energy loss of high energy (20–80 MeV) 79Br and 127I ions in gold

Abstract Channeling effects on the energy losses of 20–80 MeV Br and I ions in thin (0.4 um) gold single crystals were investigated. When low-index crystal directions were aligned with the beam, a single energy loss group was observed which was significantly less than that obtained for polycrystalline gold of the same thickness. The fractional loss varied from 50 to 85% of the polycrystalline loss, depending on the energy, the species of the bombarding particle, and the crystal direction. With low-index planes parallel to the beam direction, two energy loss groups appeared, one having the normal loss and the other having a lower loss. Both the fraction of the ions channeled and the acceptance aperture into the channel were found to decrease with increasing ion energy. The fact that two distinct groups were observed implies that most of the channeled particles were focused almost immediately upon entering the crystal and remained in these channels during the remainder of their trajectories through the solid.

Nonequilibrium charge states of 60 MeV I-ions channeled in Au single crystals☆

Abstract Iodine ions (60 MeV) penetrating Au crystals in (100) planar channeling directions require ca 1200 A to reach equilibrium, implying a reduction of the charge changing cross sections. Differences in most probable charge of up to 3.6 units in 20 are observed.

Excited state populations and charge-exchange of fast ions in solids

Abstract Excited state populations and charge state fractions of 36 MeV C and 445 MeV Cl ions have been measured for a range of thicknesses of gaseous and solid C targets. Cross-sections for electron capture, loss, excitation and excited state quenching have been determined and these data are found to give a quantitative account of the Bohr-Lindhard density effect model. Implications to stopping power theory are considered.

Ion-beam depth-profiling studies of leached glasses

Ion-beam depth-profiling was carried out on three different glasses leached (or hydrated) in deionized water using /sup 1/H(/sup 19/F,..cap alpha gamma..)/sup 16/O nuclear reaction, secondary ion mass spectrometry (SIMS) and sputter-induced photon spectrometry (SIPS) techniques. The depth-profiles show an interdiffusion mechanism in which the sodium ions in the glass are depleted and replaced by hydrogen (H/sup +/) or hydronium (H/sub 3/O/sup +/) ions from the solution. The leaching behavior does not show significant difference whether the glass surface is fractured or polished. Problems of mobile ion migration caused by ion bombardment and loss of hydrogen during analysis are discussed.