Electron-impact ionization of hydrogen-like iron ions

Electron-impact ionization cross sections have been determined for hydrogen like iron ions at selected electron energies between 1.45 and 4.3 times the threshold energy. The cross sections were obtained by measuring the equilibrium ionization balance in an electron beam ion trap. This ionization balance is obtained from x-ray measurements of radiative recombination into the K-shell of hydrogen-like and bare iron ions. The measured cross sections are compared with distorted-wave calculations and several semiempirical formulations.

The measurement of the dielectronic recombination in He-like Fe ions

We have studied the dielectronic recombination process in He-like Fe ions and have obtained the resonant strengths of the KLn (3 less than or equal to, n less than or equal to, 5) resonances. This measurement was performed with the use of an electron beam ion trap by measuring the x-ray energy emitted from highly charged ions simultaneously with the electron beam energy scanned during the measurement. The total resonant strengths obtained are 5.0 x 10(-19), 2.1 x 10(-19) and 1.1 X 10(-19) cm(2) eV, for KLM, KLN and KLO, respectively.

Measurements of temperature dynamics of ions trapped inside an electron beam ion trap and evidence for ionisation heating

A technique is described whereby measurements of ions extracted from an electron beam ion trap can be used to deduce their temperature dynamics. The measured temperature dynamics shows the expected trend as a function of charge and also gives evidence for Landau-Spitzer heating, ionization heating and evaporative cooling.

Parity nonconservation in electron recombination of multiply charged ions

We discuss a parity nonconserving asymmetry in the cross section of KLL dielectronic recombination of polarized electrons on the hydrogenlike ions with Z less than or similar to 60. This effect is strongly enhanced because of the near degeneracy of doubly excited 2l2l(') states of opposite parity in He-like ions. For ions with Z similar to 30 the asymmetry is of the order of 10(-9). For Z approximate to 48 a level crossing takes place, leading to the PNC asymmetry of -1.3x10(-8), which is 10(8) times greater than the basic strength of the weak interaction in atoms.

Pulsed evaporative cooling of ion cloud in an electron beam ion trap

A technique for producing cold ensembles of trapped highly charged ions is described. The ions, trapped in an electron beam ion trap, can undergo a drastic contraction during the pulsed mode of evaporative cooling, if a truncated Boltzmann distribution is assumed. The underlying theory and the experimental results are presented.

The role of target polarization in electron-ion recombination

We investigate the role of dynamic polarization of the target electrons in the process of recombination of electrons with multicharged ions (polarizational recombination). Numerical calculations carried out for a number of Ni- and Ne-like ions demonstrate that the inclusion of polarizational recombination leads to a noticeable increase (up to 30%) in the cross sections for incident electron energies outside the regions of dielectronic resonances. We also present a critical analysis of theoretical approaches used by other authors to describe the phenomenon of polarizational recombination.

Dielectronic recombination in He-like titanium ions

Dielectronic recombination (DR) has been studied in highly charged He-like Ti ions using an electron beam ion trap. X-rays emitted from radiative recombination (RR) and DR were observed as the electron beam energy was scanned through the resonances. Differential DR resonant strengths were determined by normalizing the DR x-ray intensity to the RR intensity using theoretical RR cross sections. KLn (2 less than or equal to n less than or equal to 5) resonant strengths were determined for He-like Ti ions. The differential resonant strengths were calibrated without reference to any theoretical DR calculations while the electron energy scale was derived with reference to the well-known energy for ionization of the He-like and H-like ions from the ground state. Calibration in this way facilitates a more exacting comparison between theory and experiment than has been reported previously. To facilitate this comparison, total and differential theoretical resonance strengths were calculated. These calculations were found to be in good agreement with the measured results.

Systematic study of visible transitions in Ti-like high Z ions

The magnetic dipole transitions between fine structure levels in the ground term of Ti-like ions, (3d(4)) D-5(2)-D-5(3), were investigated by observation of visible and near-UV light for several elements with atomic numbers from 51 to 78. The wavelengths are compared with theoretical values we recently calculated. The differences between the present calculations and measurements are less than 0.6%. The anomalous wavelength stability predicted by Feldman, Indelicato and Sugar [J. Opt. Soc. Am. B 8, 3 (1991)] was observed. We attribute this anomalous wavelength stability to the transition from LS to JJ coupling and the asymptotic behavior of the transition energies in the intermediate coupling regime.

A new device for the study of electron-ion interactions: The Belfast EBIT

An electron beam ion trap ( EBIT) has been designed and is currently under construction for use in atomic physics experiments at the Queen's University, Belfast. In contrast to traditional EBITs where pairs of superconducting magnets are used, a pair of permanent magnets will be used to compress the electron beam. The permanent magnets have been designed in conjunction with bespoke vacuum ports to give unprecedented access for photon detection. Furthermore, the bespoke vacuum ports facillitate a versatile, reconfigurable trap structure able to accommodate various in-situ detectors and in-line charged particle analysers. Although the machine will have somewhat lower specifications than many existing EBITs in terms of beam current density, it is hoped that the unique features will facilitate a number of hitherto impossible studies involving interactions between electrons and highly charged ions. In this article the new machine's design is outlined along with some suggestions of the type of process to be studied once the construction is completed.

Charge exchange and dissociative processes in collisions of slow He2+ ions with H2O molecules

Experimental and theoretical studies of one-electron capture in collisions of He2+ ions with H2O molecules have been carried out in the range 0.025-12 keV amu(-1) corresponding to typical solar wind velocities of 70-1523 km s(-1). Translational energy spectroscopy (TES), photon emission spectroscopy (PES), and fragment ion spectroscopy were employed to identify and quantify the collision mechanisms involved. Cross sections for selective single electron capture into n=1, 2, and 3 states of the He+ ion were obtained using TES while PES provided cross sections for capture into the He+(2p) and He+(3p) states. Our model calculations show that He+(n=2) and He+(n=3) formation proceeds via a single-electron process governed by the nucleus-electron interaction. In contrast, the He+(1s) formation mechanism involves an exothermic two-electron process driven by the electron-electron interaction, where the potential energy released by the electron capture is used to remove a second electron thereby resulting in fragmentation of the H2O molecule. This process is found to become increasingly important as the collision energy decreases. The experimental cross sections are found to be in reasonable agreement with cross sections calculated using the Demkov and Landau-Zener models.

Collision mechanisms in one-electron capture by slow H and He- like ions of C, N and O in H and H-2

Translational energy spectroscopy (TES) has been used to study state-selective one-electron capture by H and He-like ions of C, N and O in both H and H-2 within the range 250-900 eV amu(- 1). The main collision mechanisms leading to state-selective electron capture have been identified, their relative importance assessed and compared, where possible, with theoretical predictions and with any previous measurements based on photon emission spectroscopy. For one-electron capture in H-2, the relative importance of contributions from non- dissociative and dissociative capture as well as from two- electron capture into autoionizing states is found to be strikingly different for the cases considered. Our TES measurements in atomic hydrogen provide an important extension of previous measurements to energies below 1000 eV amu(-1) and show that, as the impact energy decreases, electron capture becomes more selective until only a single n product channel is significant. These product main channels are well described by reaction windows calculated using a Landau-Zener approach. However, the same approach applied to the more complex energy- change spectra observed in H-2 is found to be less successful.

State-selective one-electron capture by slow state-prepared N2+(P-2) ground-state ions in collisions with hydrogen atoms

The technique of double translational energy spectroscopy.(DTES), recently successfully developed in this laboratory for use with targets of atomic hydrogen, has been used to study one-electron capture by ground-state N2+(2s22p)(2)p(0) ions in collisions with hydrogen atoms at energies within the range 0.8-6.0 keV. Cross sections for the formation of the main excited product channels have been determined. The measurements allow a re-evaluation of our previous TES measurements carried out with N2+ primary beams containing an admixture of metastable N2+(2s2p2)(4)p ions. The main findings of these earlier measurements are confirmed and the DTES measurements now remove any ambiguity in interpretation of the experimental data. While recent theoretical studies correctly predict the two main N+ D-3(0) and P-3(0) product channels, the quantitative agreement with experiment is only partially satisfactory.

High harmonic generation in the relativistic limit

The generation of extremely bright coherent X-ray pulses in the femtosecond and attosecond regime is currently one of the most exciting frontiers of physics - allowing, for the first time, measurements with unprecedented temporal resolution(1-6). Harmonics from laser - solid target interactions have been identified as a means of achieving fields as high as the Schwinger limit(2,7) (E = 1.3 x 10(16) V m(-1)) and as a highly promising route to high-efficiency attosecond (10(-18) s) pulses(8) owing to their intrinsically phase-locked nature. The key steps to attain these goals are achieving high conversion efficiencies and a slow decay of harmonic efficiency to high orders by driving harmonic production to the relativistic limit(1). Here we present the first experimental demonstration of high harmonic generation in the relativistic limit, obtained on the Vulcan Petawatt laser(9). High conversion efficiencies (eta> 10(-6) per harmonic) and bright emission (> 10(22) photons s(-1) mm(-2) mrad(-2) (0.1% bandwidth)) are observed at wavelengths <4 nm ( the 'water-window' region of particular interest for bio-microscopy).