Robert D. DuBois

Electron emission in heavy-ion-atom collisions

1. Introduction.- 2. Overview of Ionization Mechanisms.- 3. Theoretical Work.- 4. Experimental Methods.- 5. Ionization by Bare Projectiles.- 6. Ionization by Dressed Projectiles.- 7. Multiple Electron Processes.- 8. Concluding Remarks.- A. Summary of Analytic Cross Section Formulae.- B. Kinematic Effects on Electron Emission.- C. References to Double Differential Cross Section Studies.- References.

Protein fragmentation due to slow highly charged ion impact

We present first results of experiments on the fragmentation of biomolecules using highly charged heavy ions. Fragmentation and modification of oligopeptides, such as dimerization and attachment of ionic salt components, have been observed by means of mass spectrometry. Plasmid DNA molecules were imaged with an atomic force microscope after ion irradiation and profound molecular damage was found.

Energy-loss measurements for single and multiple ionization of argon by positron impact

Experimental data, as a function of projectile energy loss, are presented for single, double and triple ionization of argon by 750 eV positrons. These differential ionization yields are for positrons scattered into a forward cone of angles between 0° and 17° and for energy losses up to 85% of the initial energy. The single ionization data are shown to be in good agreement with distorted wave Born calculations. For the angular range investigated the percentage of double ionization is found to increase rapidly for the first 80 eV of energy loss. The ratio then slowly approaches a constant value that is in good agreement with the ratios of total cross sections for photoionization of the M shell. The present energy-loss data also demonstrate the importance of outer versus inner-shell ionization in the double and triple ionization channels.

Positron and electron impact double ionization of argon: How 1st- and 2nd-order mechanisms influence the differential electron emission

Double-to-single–ionization ratios for electron emission as a function of the angle are measured for 200, 500, and 1000 eV positron and electron impact on argon. Both the sign of the projectile charge and the impact energy are shown to influence the angular dependences. By combining these ratios for positron and electron impact, information about how first- and second-order double-ionization mechanisms interfere and contribute to the total differential electron emission at different collision velocities is obtained.

Differential ionization measurements using positron impact

Abstract This paper describes techniques being used at the University of Missouri-Rolla for the purpose of obtaining differential ionization information for positron impact. Data are presented for single and multiple ionization of argon as a function of energy loss by positrons scattered in the forward direction. The impact energy is 750 eV. The data and their ratios are used to provide insights into which ionization channels are active. For large energy losses the double ionization ratios are found to be very similar to those obtained for photon impact energies where only outer shell ionization is possible

Ground-state excitation of heavy highly-charged ions

We have studied the excitation of H-like and He-like uranium (U91+ and U90+) in relativistic collisions with gaseous targets by observing the subsequent x-ray emission. The experiment was conducted at the ESR storage ring of the GSI accelerator facility in Darmstadt, Germany. The measurements were performed with a newly developed multi-phase target at different collision energies. This enabled us to explore the proton (nucleus) impact excitation as well as the electron impact excitation processes in the relativistic collisions. The large fine-structure splitting in uranium allowed us to unambiguously resolve excitation to different L-shell levels. Moreover, information about the population of different magnetic sublevels has been obtained via an angular differential study of the decay photons associated with the subsequent de-excitation process. The experimental results are compared with calculations performed within the relativistic framework including excitation mechanisms due to both protons (nucleus) and electrons.

Electron loss and capture from low-charge-state oxygen projectiles in methane

Absolute cross sections for single- and double-electron loss and single- and multiple-electron capture of 15?1000?keV oxygen projectiles (q =??1, 0, 1, 2) colliding with the methane molecule are presented. The experimental data are used to examine cross-section scaling characteristics for the electron loss of various projectiles. In addition, a modified version of the free-collision model was employed for the calculation of the single- and total-electron-loss cross sections of oxygen projectiles presented in this work. The comparison of the calculated cross sections with the present experimental data shows very good agreement for projectile velocities above 1.0 au. The comparison of the present single-electron-capture cross sections with other projectiles having the same charge shows good agreement, and a common curve can be drawn through the different data sets.

Target-Scaling Properties for Electron Loss by Fast Heavy Ions

Electron loss by fast heavy ions resulting from interactions with dilute gaseous targets is discussed. Of particular interest is how the cross sections scale as the target nuclear charge increases. Various theoretical models that have been proposed are discussed and compared with available experimental data. It is shown that none of these models yield good agreement with data but that by combining ideas and concepts contained within different models agreement can be obtained.

Polarization and anisotropic emission of K-shell radiation from heavy few electron ions

The population of magnetic sublevels in hydrogen-like uranium ions has been investigated in relativistic ion–atom collisions by observing the subsequent X-ray emission. Using the gas target at the experimental storage ring facility we observed the angular emission of Lyman-α radiation from hydrogen-like uranium ions. The alignment parameter for three different interaction energies was measured and found to agree well with theory. In addition, the use of different gas targets allowed for the electron-impact excitation process to be observed.

Projectile Charge Effects in Differential Ionization by Positrons and Electrons

Differential data for single and double ionization of argon by positron and electron impact are presented and compared. For single ionization, coincidences between scattered projectiles, ejected electrons and recoil ions are measured as functions of the projectile scattering angle and energy loss. Differences associated with the sign of the projectile charge are indicated with regard to the scattering angle, the energy loss, and the relative intensities for binary and recoil events. Ejected electron‐recoil ion coincidences are also measured as a function of the observation angle along the beam direction. From these, double to single ionization ratios are determined and compared. Differences in the magnitudes and angular dependences for positron and electron impact are attributed to the interference between the TS‐1 and TS‐2 double ionization mechanisms.