I. A. Sellin

Observation of high-order electric multipoles in convoy electron angular distributions

The authors have discovered multipoles of even order to about 10 in the multipole moments characterising angular distributions of convoy electrons ejected by swift ions during penetration of thin solids. They suggest the strongly transverse distributions observed reflect electron loss from excited n, l states inside the solid, which, though rapidly created and destroyed, progress toward finite dynamic equilibrium populations. Alternative interpretations are also discussed.

High-resolution electron time-of-flight apparatus for the soft x-ray region

A gas-phase time-of-flight (TOF) apparatus, capable of supporting as many as six electron-TOF analyzers viewing the same interaction region, has been developed to measure energy- and angle-resolved electrons with kinetic energies up to 5 keV. Each analyzer includes a newly designed lens system that can retard electrons to about 2% of their initial kinetic energy without significant loss of transmission; the analyzers can thus achieve a resolving power (E/ΔE) greater than 104 over a wide kinetic-energy range. Such high resolving power is comparable to the photon energy resolution of state-of-the-art synchrotron–radiation beamlines in the soft x-ray range, opening the TOF technique to numerous high-resolution applications. In addition, the angular placement of the analyzers, by design, permits detailed studies of nondipolar angular distribution effects in gas-phase photoemission.

Electric-octupole and pure-electric-quadrupole effects in soft-X-Ray photoemission

Second-order [ O(k(2)), k = omega/c] nondipole effects in soft-x-ray photoemission are demonstrated via an experimental and a theoretical study of angular distributions of neon valence photoelectrons in the 100-1200 eV photon-energy range. A newly derived theoretical expression for nondipolar angular distributions characterizes the second-order effects using four new parameters with primary contributions from pure-quadrupole and octupole-dipole interference terms. Independent-particle calculations of these parameters account for a significant portion of the existing discrepancy between experiment and theory for Ne 2p first-order nondipole parameters.

Beyond the dipole approximation: angular-distribution effects in valence photoemission

Angular distributions of valence photoelectrons showing effects due to higher-multipole photon interactions have been measured for the first time. Neon 2s and 2p photoemission exhibits effects beyond the dipole approximation throughout the 250 - 1200 eV photon-energy range studied. The results suggest that any photoemission experiment, on any sample, can be affected at relatively low photon energies, pointing to a general need for caution in interpreting angle-resolved-photoemission measurements.

On the angular distributions of electrons photoejected from fixed-in-space and randomly oriented molecules

Abstract New theoretical expressions are devised from a dynamical perspective for molecular photoionization cross sections differential in electron ejection angles which facilitate comparisons between theory and experiment and provide a convenient basis for ab initio calculations. The cross sections obtained for fixed-in-space molecules, including the lowest-order (nondipole) effects of retardation, are given in terms of invariant molecular body-frame transition moments and related normalized angular-distribution amplitudes which can be calculated employing interaction-prepared states without reference to specific scattering boundary conditions. Corresponding expressions for molecular dipole and nondipole anisotropy factors appropriate for randomly oriented molecules are obtained in closed forms involving expectation values of harmonic polynomials over the fixed-in-space body-frame angular-distribution amplitudes. The expressions are seen to be in the spirit of corresponding results for atomic photoionization anisotropy factors, to which they reduce in appropriate limits. Interpretations of recently measured angular distributions of photoelectrons from the K-shell of molecular nitrogen illustrate how the development relates measurements on randomly oriented molecules to those performed on fixed-in-space molecules. The theoretical formalism provides results in excellent accord with measurements of the molecular nitrogen K-shell dipole anisotropy factor, and accounts for the origins of large nondipole effects observed at relatively low photon energies (ℏω≤500 eV) in the measured angular distributions.

Photon energy dependence of ionization-excitation in helium at medium energies

We have measured the photoionization-excitation-to-photoionization ratio for (n = 2 - 6) at several photon energies from 90 to 900 eV. By extrapolating these values we could determine the asymptotic high-energy ratios for (n = 2 - 5) which agree with theoretical predictions. We show that the satellite-to-main-line ratios are consistent with experimental double-to-single photoionization ratios and agree well with recent measurements.

Yield of convoy electrons from solids

Abstract The number of convoy electrons emitted in the forward direction from solid targets of C, Al, Ag. and Au per incident 1–4 MeV/amu H, O, Si, and Ni projectile is reported. As expected, the yield is independent of target thickness and of the incident projectile charge state. We find that the yield is target ( Z ) 2 dependent, increases as Z 1 2.75 ( Z ) 1 is the atomic number of the incident projectile) and decreases as E 1 −2.25 ( E 1 is the incident projectile energy in MeV/amu).

Highly Ionized Ions

Publisher Summary This chapter reviews highly ionized ions and their characteristics. There are several known means of altering emergent ion charge state distributions to suit experimental needs, which are especially useful in producing excited states of very highly ionized ions while restricting the width of the corresponding charge state distributions. The simplest technique for altering emergent charge state distributions is to use electric or magnetic selection of the charge state of interest. Such a technique is usually combined with raising the beam energy toward the limit. A second technique takes advantage of very high charge states available from the scattering of energetic ions through angles greater than or approximately equal to a few tenths of a degree by collisions in heavy, usually gaseous, targets of sub-equilibrium thicknesses. Frozen charge state technique is used for studying the electronic excitation interactions of pure charge state ions. For highly ionized atoms, the terms Auger emission and autoionization (autoejection) may often be used interchangably, because inner shell vacancies, core excitations, and valence shell excitations are often indistinguishable. Single-particle valence shell excitation states do not, of course, autoionize. For autoionization to be energetically possible, an autoionizing state must be degenerate with a continuum state, that is, lie higher in energy than the ground state (or other excited states) of an ion in the next higher adjacent charge state.

Metastable autoionizing states

Abstract Research on the electron decay-in-flight of metastable states of elementary atomic systems of high Z was initiated at the Oak Ridge National Laboratory in 1970. This paper reviews our progress to date and comments on capabilities and limitations of the technique.

Production of soft X-ray emitting slow multiply charged ions: Recoil ion spectroscopy

Abstract A study of Ne L-shell vacancy production by S ∼14+ , Cl ∼12+ ions has shown copious production of NeIINeVIII excited states with ∼10 −18 cm 2 cross sections and recoil velocities which may permit significantly higher resolution spectroscopy than is possible with beam-foil methods.