R.T. Short

Sequential photoionization of ions using synchrotron radiation and a Penning ion trap

Abstract 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.

Photoion Auger-electron coincidence measurements near threshold

Abstract The vacancy cascade which fills an atomic inner-shell hole is a complex process which can proceed by a variety of paths, often resulting in a broad distribution of photoion charge states. We have measured simplified argon photoion charge distributions by requiring a coincidence with a K-LL or K-LM Auger electron, following K excitation with synchrotron radiation, as a function of photon energy, and report here in detail the argon charge distributions coincident with K-L1L23 Auger electrons. The distributions exhibit a much more pronounced photon-energy dependence than do the more complicated non-coincident spectra. Resonant excitation of the K electron to np levels, shakeoff of these np electrons by subsequent decay processes, double-Auger decay, and recapture of the K photoelectron through postcollision interaction occur with significant probability.

Properties of cold ions produced by synchrotron radiation and by charged particle impact

Argon recoil ions produced by beams of 0.8 MeV/u Cl/sup 5 +/ have been detected by time-of-flight (TOF) techniques in coincidence with the loss of from one to five projectile electrons. Recoil-ion energies have been determined to be more than an order of magnitude higher than those of highly-charged ions produced by unmonochromatized synchrotron radiation. Charge-state distributions, however, show similarities, suggesting that loss of projectile electrons corresponds, in some cases, to inner-shell target ionization producing vacancy cascades. In an essential improvement to the usual multinomial description of ionization in the independent-electron-ejection model, we find the inclusion of Auger vacancy cascades significantly alters the description of the recoil ion spectra corresponding to projectile-electron loss. These conclusions are consistent with impact parameters inferred from determinations of mean recoil energy. 11 refs., 5 figs.

Production of highly-charged recoil-ion beams at very low energy

Abstract An apparatus for production of an intense, charge-state separated, recoil-ion beam of very low energy and good collimation has been constructed. In a test experiment, in which 30 MeV C15+ projectiles were used to produce recoil ions, the feasibility of the method was demonstrated. The mass to charge ratio of the beam is selected by means of a Wien-filter, and the beam energy can be varied between 2qeV and 1q keV with a constant charge-state resolution. Presently, the angular definition of the beam is 2.5°, but it can be impoved by at least a factor of 2. Very low energy Ar-ion beams, of intensities usable for secondary slow collision experiments, can be created for charge states ranging from one to nine. For example, beams of Ar4+ and Ar6+ of intensities 2.5 × 104 and 5 × 103 s−1, respectively, can be furnished at energies of 10 q eV, while the corresponding numbers for 2 q eV beams ar approximately five times lower.

Research with stored ions produced using synchrotron radiation

Abstract 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 radiation inner-shell photoionization of atomic and molecular gases

Abstract A time-of-flight (TOF) spectrometer has been used to measure the average kinetic energies of near-thermal atomic and molecular ions, as well as energetic molecular ion fragments, produced by inner-shell photoionization of atomic and molecular gas targets using X-rays from the National Synchrotron Light Source (NSLS). While atomic and molecular ions acquire very little energy from the initial photoionization event and subsequent vacancy cascades, multiply-charged molecular ions decompose rapidly, producing fragment ions which have kinetic energies determined by their configuration at the moment of decomposition. The strong effects observed suggest that this technique may be extended to the study of configurations of free molecules as well as the orientation of molecules adsorbed on surfaces.

Angular distributions of single- and double-electron capture in very-slow Ar6+-Ar collisions

Abstract We have measured state-resolved angular distributions of one- and two- electron capture in 32 to 800 eV Ar 6+ -Ar collisions. The experimental energy-gain spectra show that single-electron capture mainly populates the 5s, 5p and 4f levels. We observe detailed structures in the corresponding angular distributions, but a final interpretation has to await a quantitative analysis of the collision dynamics. We tentatively ascribe the main features in the angular distribution of true double-electron capture at Q 26 eV (4s4f and 4s5s) and Q 42 eV (3d4d) to processes involving two consecutive one-electron transitions. For the transfer ionization process, we measure a Q -value of ∼ 26 eV, which we assign to autoionizing 4s5s (or 4s4f) levels. The 4s5s, 4s4f, and 3d4d levels all reside above the first ionization limit of Ar 4+ but we find that the 3d4d level stabilizes through radiative decay.

A new facility for capture-state resolved measurements of angular distributions in very slow collisions between highly charged ions and atoms

We have constructed, and performed the first experiments with, a new facility for studies of charge exchange in very slow collisions between highly charged ions and neutral atoms. State-selective angular differential cross sections for single-electron capture have been measured for laboratory collision energies down to 4.75q eV, where q is the charge state of the projectile. A very slow beam, with an energy spread of ~0.2q eV, is collimated to a maximum angular divergence of ±0.5° before it interacts with a gas-jet target. The post-collisional energy-, charge-, and angular-distributions are then analysed by means of a cylindrical energy analyzer and a two-dimensional position-sensitive detector. The relative energy resolution is measured to be 1.6% and the angular acceptance is ±9°. In the first experiment, we determined the dominating electron transfer mechanisms for single-electron capture, transfer ionization, and true double capture in 0.5-20 eV/amu Ar4+-Ar collisions by means of comparisons with multichannel Landau-Zener angular differential cross sections.