H.F. Krause

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.

Channeling heavy ions through crystalline lattices

Publisher Summary In recent years, considerable progress has been made in the study of collisions involving multiply charged ions and electrons. Areas that have been opened to investigation include excitation, ionization, radiative recombination, and dielectronic recombination. All of these processes occur when swift ions impinge on gases and solids. In the past 15 years, many groups have used the channeling effect that occurs in crystalline solids to create conditions under which such processes may be studied relatively easily at electron densities in the excess of 1023/cm3. This chapter summarizes the essential points required to define the “channeling effect” and the unique collision environment for channeled particles. Most recently, channeling techniques are used in the study of atomic collision phenomena involving energetic and heavier multiple-charged projectile ions, even uranium projectiles moving at relativistic energy. For heavier ions moving at much higher speed, cross sections for electron impact phenomena are greatly reduced. In this case, X-ray techniques are also more easily applied to probe specific excited states of heavier projectiles. The chapter focuses on the studies that have occurred within the past 15 years and emphasizes the study of motion, atomic collisions, and ionic electronic states as ions move through crystal channels.

Three-step resonant photoionization spectroscopy of Ni and Ge : ionization potential and odd-parity Rydberg levels

In preparation of a laser ion source, we have investigated multi-step laser ionization via Rydberg and autoionizing states for atomic Ni and Ge using a mass separator with an ion beam energy of 20 keV. For both elements resonant three-step excitation schemes suitable for modern Ti:sapphire laser systems were developed. Rydberg series in the range of principal quantum numbers 20 n 80 were localized, assigned and quantum numbers were allocated to the individual resonances. Ionization potentials (IP) were extracted from fits of the individual series and quantum defects of individual levels were analysed for confirmation of series assignment. For Ni the ionization potential could be extracted with significantly increased precision compared to literature with a value of EIP (Ni) = 61 619.77(14) cm −1 . Also, at least one notable autoionizing state above the first IP was discovered for both elements, and the different ionization schemes via Rydberg or autoionizing states were compared with respect to line shape, ionization efficiency and selectivity.

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.

Low energy chemical sputtering of ATJ graphite by atomic and molecular deuterium ions

We present experimental chemical sputtering results for D+, D2+ and D3+ ions incident on ATJ graphite in the energy range 5–60 eV D−1, and compare them with simulations for deuterated amorphous carbon impacted by neutral D, D2 and D3. The measured methane yields/D for the different species compared at the same energy/D diverge below about 60 eV D−1, the incident triatomic molecular ions leading to the largest yields/D, and the atomic ions to the smallest, reaching a factor of two difference at 10 eV/D. The measured yields/D are in reasonable agreement with molecular dynamics simulations over the entire calculated energy range. The model surfaces were prepared by D, D2 and D3 impacts in a way that mimics the experiment. For D2 incident at energies below 15 eV/D, the simulations show a strong dependence of the sputtering yields on the vibrational state of the incident projectile.

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.

Recent ORNL measurements of chemical sputtering of ATJ graphite by slow atomic and molecular D ions

We describe here an ORNL Physics Division research activity whose focus is the investigation of chemical sputtering of graphite by atomic and molecular D ions at very low energies that have so far been unexplored. Our initial experimental approach is based on the use of a quadrupole mass spectrometer (QMS) which samples the partial pressure of selected mass species produced in a scattering chamber when the incident ion beam strikes a graphite sample. While most of the ORNL results obtained to date are for D2+ ions incident on ATJ graphite, preliminary results are shown for D+ projectiles incident at energies down to 10 eV D−1, and for D3+ ions incident at 10 and 4.5 eV D−1. The possibility of obtaining complementary information using a time-of-flight approach is discussed as well.

Implications of heavy-ion-induced satellite X-ray emission IV: Chemical effects in L X-ray satellites of molybdenum compounds and alloys

The L X-ray satellites produced by 1MeV/uClq+ bombardment of a series of molybdenum compounds and alloys were examined using a high-resolution (7 eV at 2.3 keV) crystal spectrometer. The normalized emission spectra exhibited variations in the intensity distribution of the LnαMm and LnβMm satellites for different targets. Using Mo metal as a reference or subtrahend, amplification of the intensity variations was obtained by calculating difference spectra. Select regions of these difference spectra displayed systematic variations that were correlated to the valence-electron density of the material. These results clearly demonstrate that heavy-ion-induced L X-ray satellites may be used to probe the chemical environment of intermediate Zelements. In addition, the alloy results may provide a basis for the application of this technique to a variety of materials problems.

Emittance characterization of a hot-cavity laser ion source at Holifield Radioactive Ion Beam Facility

The first investigation of the transverse emittance of a hot-cavity laser ion source based on all-solid-state Ti:sapphire lasers is presented. The emittances of (63)Cu ion beams generated by three-photon resonant ionization are measured and compared with that of the (69)Ga and (39)K ion beams resulting from surface ionization in the same ion source. A self-consistent unbiased elliptical exclusion method is adapted for noise reduction and emittance analysis. Typical values of the rms and 90% fractional emittances of the Cu ion beams at 20 keV energy are found to be about 2 and 8 pi mm mrad, respectively, for the ion currents of 2-40 nA investigated. The emittances of the laser-produced Cu ion beams are smaller than those of the surface-ionized Ga and K ion beams.