P.F. Dittner

COLLISION-INDUCED VIBRATIONAL EXCITATION AND DISSOCIATION OF HYDROGEN WITH ALKALI ATOMS AND IONS FROM 2 TO 50 eV.

Inelastic energy losses for Na0, Na+, K0, and K+ backscattered from H2 and D2 have been measured. At center‐of‐mass energies below15 ∼ eV for Na and ∼ 20 eV for K, the collisions yield vibrational excitations which are compared with the one‐dimensional exact classical calculations of Secrest. The vibrational excitation in the Na+–H2 and Na0–H2 system is in excellent agreement with three‐dimensional classical calculations of Cheng and Wolfsberg. Above these energies the molecules are dissociated but the energy loss distribution of the alkali projectile remains sharp and in the case of Na–H approaches the maximum loss possible at 50 eV.

Energy loss, angular distributions and charge fractions of low energy hydrogen transmitted through thin carbon foils

Abstract The energy loss, angular distributions and charge fractions of hydrogen transmitted through thin carbon foils in the energy range 200 ≤ E ≤ 3000 eV have been measured. The values of the stopping power constant equal 4.18 ± 0.30 and 4.38 ± 0.34 (eV)1/2 cm2/μg, where the two values are the result of using the most probable final energy and the average final energy, respectively. These values agree well with the theoretical value of 4.09 (eV)1/2 cm2/μg. The measured angular distributions were narrower than those predicted by the theory of Meyer except at the highest energies and thinnest foils. The fractions of particles emerging from the foil as positive ions, f+ , and negative ions, f −, as a function of the exit energy are given by f+ (%) = 3.0 E and f− (%) = 2.3 E, where E is in keV.

Molecular negative surface ionization of UF6

Investigations have been carried out to elucidate the process of formation of UF6− molecular ions on heated surfaces. On a carbon coated Pt surface the absolute efficiency of conversion of UF6 to UF6− was found to be 99+1−5% over a wide temperature range in accordance with anticipations based solely on measured thermionic work functions and the molecular electron affinity of UF6. The heat of adsorption of UF6− determined from residence time measurements is 33.6±1.1 kcal/mol on this surface. On clean Pt and other pure metal surfaces the ionization efficiency is orders of magnitude lower than predictions based on the Saha–Langmuir equation. The reasons for this behavior are discussed in terms of alternative dissociation processes on the surface.

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+ .

Energy and angular distribution of low energy H+ and D+ backscattered from polycrystalline carbon

Abstract The energy distributions of H+ and D+ backscattered from a polycrystalline graphite sample were recorded as a function of total scattering angle, angle of incidence, and for incident beam energies 200 i . The general shapes of the distributions are discussed qualitatively, and their variation with incident energy and total scattering angle are explained and compared with theoretical and other experimental results. The average energies E + of the distributions are found to increase relative to the single scattering energy, E k , with decreasing incident energy. E + /E k also increases with decreasing exit angle from the solid in a way which is slightly dependent upon the angle of incidence. The integrated intensities of the distributions are found to depend strongly upon the angle of incidence, with a normally incident beam producing a nearly cosine distribution of backscattered ions and grazing angles of incidence producing an intensity which peaks at an angle forward of the specular direction. Using charge fractions obtained previously for surface scattering from graphite and transmission through thin carbon foil, values of the particle reflection coefficient r n are obtained as a function of energy.

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.

Neutralization in surface scattering of low energy H+ by polycrystalline graphite☆

Abstract Backscattering H + spectra were obtained as a function of incident H + energy (600 E + of hydrogen backscattered as H + in the binary scattering edge was deduced. Models including only Auger neutralization at the surface failed to describe the results. A model including collisional neutralization and ionization which fits the data indicated that 95–98% of incident H + is neutralized by the collision while 2–9% of incident (neutralized) H + is re-ionized by the collision. The Auger neutralization at the surface is described by a characteristic velocity υ c −2 −10×10 6 cms −1 The empirical formula f + =5.1× 10 −10 υ f⊥ +1.6×10 −10 υ i⊥ using the initial (υ i⊥ ) and final(υ f⊥ ) perpendicular velocities gives are markably good fit to all data.

The histrap proposal: Heavy-ion storage ring for atomic physics

Abstract HISTRAP, Heavy-Ion Storage Ring for Atomic Physics, is a proposed 46.8-m-circumference synchrotron-cooling-storage ring optimized to accelerate, decelerate, and store beams of highly charged very-heavy ions at energies appropriate for advanced atomic physics research. The ring is designed to allow studies of electron-ion, photon-ion, ion-atom, and ion-ion interactions. An electron cooling system will provide ion beams with small angular divergence and energy spread for precision spectroscopic studies and also is necessary to allow the deceleration of heavy ions to low energies. HISTRAP will have a maximum bending power of 2.0 T m and will be injected with ions from either the existing Holifield Heavy Ion Research Facility 25-MV tandem accelerator or from a dedicated ECR source and 250 keV/nucleon RFQ linac.

Surgery of fast, highly charged ions studied by zero-degree auger spectroscopy

Abstract Zero-degree Auger spectra were measured for the projectile in collisions of oxygen and carbon on He with incident charge states of q = 2–5 and for energies from 5 to 30 MeV. Since the light target particle He acts selectively on the projectile ion, we refer to the present method as ion surgery. Apart from the one-electron processes of single excitation and single loss, two-electron processes such as transfer excitation and transfer loss are studied.