G.C. Ball

Resonant coherent excitation (RCE) of higher-order resonances for 28Si13+ ions channeled in a thin Si crystal

Abstract Resonant coherent excitation of hydrogen-like 28Si13+ ions, channeled in a thin Si crystal, has been observed for Δn = 1, 2 and 3 transitions (K-shell to L-, M- and N-shells, respectively). In addition, selection of ions which come close to the string, permitted observation of the Stark splitting of the 2s2p states of the Δn = 1 resonance.

CHANNELING OF 7.6 MEV HE IN YBCO

Abstract Channeling dips for 7.6 MeV He ions in YBa 2 Cu 3 O 7− x have been measured for all constituents to investigate possible anomalies in thermal vibrations near the transition to superconductivity. Quantitative agreement with simulations is obtained for all the dips with a single set of structural parameters. Angular scans for crystal temperatures in the range 30–220 K confirm the presence of an anomaly at T c for the width of CuYBa backscattering dips, which can be interpreted as a freeze-out of Cu vibrations at the transition to superconductivity. A detailed temperature scan reveals for the first time that the change in width takes place over a temperature interval of ∼15 K below the transition temperature, in agreement with recent EXAFS measurements of relative Y–Cu vibrations. Our data for oxygen are not conclusive but may indicate a similar small anomaly.

Dielectronic recombination for He-like 79Br ions channeled along the 〈110〉 direction in Ni

Abstract Dielectronic recombination for He-like 79Br has been measured for channeling along a 〈110〉 axis in Ni. The KLL resonance has a FWHM of 14% of the beam energy and an area corresponding to a density of about 6 electrons per atom.

Use of the 7.6 MeV 16O(α, α) resonance in studying the anomalous channeling behaviour of YBa2Cu3O7 − x near Tc

Abstract By using the intense 16 O( 4 He, 4 He) elastic scattering resonance at 7.3–7.6 MeV, we have been able to extend the earlier investigations [1–4] of the channeling behaviour in YBa 2 Cu 3 O 7 − x around T c to include the backscattering signal from the oxygen sublattice. We have also developed a computer code enabling us to simulate c -axis angular scans for each atomic sublattice as a function of depth. These simulations reproduce very well the observed oxygen angular scans at 70 K (below T c ) and at 100 K (above T c ), with magnitudes of the thermal vibrational amplitudes which are in reasonable agreement with neutron-scattering data. Concerning small changes at T c , as observed in earlier channeling investigations, we have concentrated on the analysis of oxygen scans. A direct fit of the simulations to our measurements indicates an increase above T c of about 0.5 pm in the amplitude of oxygen vibrations perpendicular to the c -axis, or a smaller increase combined with a static displacement. Within the estimated uncertainties, this result is consistent with the neutron data. The oxygen (O4) atoms on the CuO rows along the c -axis contribute a wider channeling dip than the other oxygen atoms (O1, O2 and O3), which form pure oxygen rows along the c -axis. Hence, the magnitude of the O4 vibrational amplitude is important only for the yield in the shoulders of the combined oxygen dip and it can be determined independently from the fit to measurements. Our results do not confirm the large change of this amplitude at T c suggested by Remmel et al. [8].

Dielectronic recombination (DR) in He‐like 79Br channeled along the 〈110〉 direction in Au

We have measured dielectronic recombination for 79Br33+ ions channeled along a 〈110〉 axis in a thin Au crystal. The resonance width and strength agree with predicted values for a Fermi gas with a density corresponding to the average electron density for the 〈110〉 channel in Au.