The rate coefficients for electron transfer from Ar and H-2 to Ar(q+) ions (3 less-than-or-equal-to q less-than-or-equal-to 6) have been measured using an ion-storage technique in a Penning ion trap. The ions were produced in the trap by K-shell photoionization of Ar atoms, using broadband synchrotron x-ray radiation. K-electron removal resulted in vacancy cascading, yielding a distribution of argon-ion charge states peaked near Ar4+. The stored ion gas had an initial temperature near 480 K. The basic data determining the rate coefficients k(Ar(q+)) are the storage time constants of each charge state in the trap, in the presence of a measured pressure of target gas. The results of the measurements (in units of 10(-9) cm3 s-1) are k(Ar3+, H-2) = 4.3(0.7), k(Ar3+, Ar) = 1.6(0.2), k(Ar4+, H-2) = 5.2(0.6), k(Ar4+, Ar) = 2.5(0.3), k(Ar5+, H-2) = 5.9(0.7), k(Ar5+, Ar) = 2.9(0.3), k(Ar6+, H-2) = 8.5(l.2), and k(Ar6+, Ar) = 2.5(0.3).

We have measured the distribution of photoion charge states resulting from vacancy cascades and electron shakeoff following K-shell photoionization of Ar2+ ions. The Ar2+ ions were produced by electron-impact ionization of Ar, stored in a Penning ion trap, and then photoionized using broadband synchrotron radiation. Time-reversed integration of coupled differential equations were used to account for contributions from electron capture with residual gas. The results are in reasonable agreement with a simple theoretical prediction.

Studies of electron transfer collisions of low energy stored ions with target atoms have been carried out as a function of the well depth of the Penning ion trap. The ions were produced by vacancy cascades following inner-shell photoionization by synchrotron radiation. The ion storage data, following an analysis based on the details of the ion production, storage, and collision processes in the trap, provides information for optimizing the accuracy of electron transfer rate coefficient measurements when several charge state are present. Low trap well depths exceeding a well-defined minimum value are the most useful.

Sequential photoionization has the potential of making available for study highly charged low energy ions. Synchrotron radiation was used to create a multicharged xenon ion target for further ionization by synchrotron radiation inside a Penning ion trap. Evidence of sequential photoionization was seen, though the yields were small. Improvements in the apparatus and radiation from third generation synchrotrons are expected to increase sequential photoionization yields significantly.

The feasibility of a precision measurement of the ground-state hyperfine structure of hydrogenlike sodium is considered, based on the use of a proposed rf-linac extreme-UV free-electron laser. The laser would be used for intensity-pumping population redistribution within the ground-state sublevels of the stored ions, through excitation of the 1s–2p transition near 1 nm. The magnetic dipole transition between the hyperfine-structure sublevels would be detected by changes in the mean number of scattered 1.2-keV photons following the low-frequency transition.

In recent years ion traps of various types have served in studies of collision interactions and spectroscopy of low energy multicharged ions. With large storage rings for multicharged heavy ions, dedicated to atomic physics research, now planned or coming into operation, it is useful to consider the role of ion traps in future experimental work. The capabilities and current limitations of multicharged ion research using traps are discussed, and a comparison with storage rings for the purposes of ion spectroscopy is made. Recent experimental investigations to develop techniques for the study of multicharged ions in traps using laser or synchrotron radiation are discussed, including a UHV pulsed gas source and an atom beam as ionization targets.

Resonant analog detection of ions stored in a Penning trap shows broadening of the ion resonances due to the presence of other ions in the trap. This broadening is reduced when certain m/q ratios are selectively removed. Linewidths have been studied with both axial and radial ion detection systems, for ions produced with different charge using synchrotron radiation photoionization, and for ions with different mass and charge produced using electron impact ionization. Linewidths increase when more ions are confined, apparently due primarily to space charge effects, but initial ion energy may also play a role. The multicharged ions produced using synchrotron radiation had significantly narrower linewidths, due to lower densities and energies.

The characteristics of radiation potentially available from proposed rf-linac free electron lasers1 are considered here for precision spectroscopy of highly-charged ions. Such ions are usually produced in environments not conducive to precision measurements, such as high temperature plasmas or fast ion beams. However, highly-charged recoil ions with eV energies have been produced by impact of fast, stripped ions on atomic targets2, and, using synchrotron radiation, multi-charged ions have been formed at thermal energies by K-shell photoionization followed by Auger emission and electron shake-off3. Using each of these cold ion sources, multi-charged ions have been stored under ultra-high vacuum conditions in Penning4,5 or radio-frequency6 ion traps, while maintaining the low energies at which the ions were generated. Long term storage at low energy in a near perturbation-free environment is consequently a reality for multi-charged ions, and experiments can be designed to exploit these properties for both spectroscopic7 and collision8 measurements. In particular, we discuss precision measurements on hydrogen-like multi-charged ions analagous to measurements on singly-charged alkalai-like ion hyperfine structures made using conventional pulsed laser techniques.9 Precision measurements of interest include, for example, studies of the Z-dependence of the Lamb shift in hydrogen-like and helium-like ions, and studies of the fine structure of helium-like ion n=2 states.

A distribution of argon ion charge states has been produced by inner shell photoionization of argon atoms using X-ray synchrotron radiation. These ions were stored in a Penning ion trap at moderate to very low well depths, and analog-detected yielding narrow charge-to-mass spectrum linewidths. Estimates of ion densities indicated that ion-ion collisional energy transfer should be rapid, leading to thermalization. Measurements using variants of this novel stored, multicharged ion gas are considered.

Synchrotron x rays have been used to produce a confined multicharged ion gas near room temperature. Comparison of charge-state-number observations characteristic of ion formation and of ion storage, together with measurements of Ar-to-${\mathrm{Ar}}^{q+}$ electron-transfer rate coefficients, provide information to estimate time constants for relaxation to thermal equilibrium and other stored-ion properties important to further development of the technique.