Projectile Electron Research Articles

Polarization dependence in inelastic scattering of electrons by hydrogen atoms in a circularly polarized laser field

We theoretically study the influence of laser polarization in inelastic scattering of electrons by hydrogen atoms in the presence of a circularly polarized laser field in the domain of field strengths below 107V/cm and high projectile energies. A semi-perturbative approach is used in which the interaction of the projectile electrons with the laser field is described by Gordon–Volkov wave functions, while the interaction of the hydrogen atom with the laser field is described by first-order time-dependent perturbation theory. A closed analytical solution is derived in laser-assisted inelastic electron–hydrogen scattering for the 1s→nl excitation cross section which is valid for both circular and linear polarizations. For the excitation of the n=2 levels simple analytical expressions of differential cross section are derived for laser-assisted inelastic scattering in the perturbative domain, and the differential cross sections by the circularly and linearly polarized laser fields and their ratios for one- and two-photon absorption are calculated as a function of the scattering angle. Detailed numerical results for the angular dependence and the resonance structure of the differential cross sections are discussed for the 1s→4l excitations of hydrogen in a circularly polarized laser field.

High Resolution Auger Projectile Electron Spectroscopy of Li-like Ions Produced by Electron Capture in Collisions of He-like Ions with Gaseous Targets

We have recently build a new experimental station with a beam line dedicated to atomic collision physics at the 5 MV TANDEM accelerator of the Institute of Nuclear and Particle Physics at the National Research Center "Demokritos” in Athens, Greece. A complete zero-degree Auger projectile spectroscopy apparatus composed of a single stage hemispherical spectrograph and a 2-dimensional position sensitive detector combined with a doubly differentially pumped gas target has been set up for high resolution studies of electrons emitted from projectile ions excited in collisions with target atoms. With the new setup we have started a systematic isoelectronic investigation of K-Auger spectra emitted from pre-excited ions in collisions with gas targets using novel techniques. Here, we present some of our first new results involving collisions of carbon He-like ions with target gases. These results are expected to lead to a deeper understanding of the neglected importance of cascade feeding of metastable states in collisions of ions with gas targets and further elucidate their role in the non-statistical production of excited three-electron states by electron capture, recently a field of conflicting interpretations awaiting further resolution.

Energy loss of tens keV charged particles traveling in the hot dense carbon plasma

The energy loss of charged particles, including electrons, protons, and α-particles with tens keV initial energy E0, traveling in the hot dense carbon (C) plasma for densities from 2.281 to 22.81 g/cm3 and temperatures from 400 to 1500 eV is systematically and quantitatively studied by using the dimensional continuation method. The behaviors of different charged particles are readily distinguishable from each other. Firstly, because an ion is thousands times heavier than an electron, the penetration distance of the electron is much longer than that of proton and α-particle traveling in the plasma. Secondly, most energy of electron projectile with E0 < 100 keV deposits into the electron species of C plasma, while for the cases of proton and α-particle with E0 < 100 keV, about more than half energy transfers into the ion species of C plasma. A simple decreasing law of the penetration distance as a function of the plasma density is fitted, and different behaviors of each projectile particle can be clearly found from the fitted data. We believe that with the advanced progress of the present experimental technology, the findings shown here could be confirmed in ion-stopping experiments in the near future.

Investigation of the electron capture process in semiclassical plasma

Abstract In this work, the process of electron capture in partially ionized plasma is considered. Electron-atom interaction was described by the effective interaction potential, which takes into account the screening effect at large distances and the diffraction effect at the small distances. The results of numerical calculations of the electron capture radius, differential cross-section for different values of the coupling and density parameters are presented. The differential cross-section was obtained on the basis of perturbation theory and also by solving of the equation of motion of the projectile electron.

First results from the Jordan COLTRIMS imaging system

A Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) apparatus has been constructed and operated cooperatively by the Atomic and Molecular Physics Laboratory in the Physics Department at the Hashemite University, Jordan, and the Atomic and Molecular Physics Group at the 4.75MeV Van de Graaff Accelerator Facility at the University of Jordan. This COLTRIMS imaging system allows the investigation of the dynamics of various ion impact induced atomic and molecular interactions and fragmentations. The performance of the COLTRIMS system was tested by carrying out preliminary studies of two collision systems. In the first study, electron capture processes in the He2+–Ne collision system at 0.25 and 1.0MeV impact energies were investigated. Significant simultaneous electron capture and target excitation has been observed for the 0.25MeV as opposed to the 1.0MeV impact energy. In the other study, projectile single electron loss and simultaneous single target ionization in the O+–He collision system was studied at three different projectile impact energies: 0.6, 1.45 and 2MeV. Enhancement of the electron–electron contribution to projectile electron loss with increasing collision energy has been observed.

Fully differential study of ionization in p + H2 collisions near electron—projectile velocity matching

We have performed a kinematically complete experiment on ionization of H2 by 75 keV proton impact for electrons ejected with a speed close to the projectile speed. The fully differential data are compared to a three-body distorted wave and a continuum distorted wave—eikonal initial state calculation. Large discrepancies between experiment and theory, as well as between both calculations, are found. These probably arise from a strong coupling between the ionization and capture channels, which is not accounted for by theory.

Role of the kinematics of probing electrons in electron energy-loss spectroscopy of solid surfaces

Inelastic scattering of electrons incident on a solid surface is determined by the two properties: (i) electronic response of the target system and (ii) the detailed quantum-mechanical motion of the projectile electron inside and in the vicinity of the target. We emphasize the equal importance of the second ingredient, pointing out the fundamental limitations of the conventionally used theoretical description of the electron energy-loss spectroscopy (EELS) in terms of the ``energy-loss functions''. Our approach encompasses the dipole and impact scattering as specific cases, with the emphasis on the quantum-mechanical treatment of the probe electron. Applied to the high-resolution EELS of Ag surface, our theory largely agrees with recent experiments, while some instructive exceptions are rationalized.

Total and state-to-state electron capture and excitation cross-sections for Li+, Be and colliding with at low-to-intermediate energies

The electron capture process plays an important role as a diagnostic tool for measuring the temperature, plasma rotation, and impurity densities of plasma in tokamaks. In this work we report the electron capture and excitation cross-sections for Li+, Be and colliding with atomic hydrogen in the collision energy range 0.25–25 keV/amu. For this, we solve numerically the time dependent Schrödinger equation by using a finite difference approach. We model the ion projectile interaction with the target using a pseudopotential obtained within a Hartree–Fock method. We use classical trajectories, obtained self-consistently, for the projectile at collision energies lower than 2 keV/amu and a straight line trajectory at high collision energies. We report new results for the total, and 4 state projectile electron capture cross-section, as well as the n = 2-state target excitation cross-section. We find a good agreement between our cross-section results when compared with available theoretical and experimental data found in the literature. Finally, we find that the electron capture probability, as a function of the impact parameter, shows Stückelberg oscillations at low collision energies for the n = 2 of Be+ and n = 3 of in the radial range (large impact parameters). Our results assess the validity of the adiabatic basis set at low collision energies and confirm the use of a finite difference method as an accurate approach to study a time-dependent process in charge exchange collisions. A discussion of our results is provided.

Inelastic scattering of electrons by metastable hydrogen atoms in a laser field

The inelastic scattering of fast electrons by metastable hydrogen atoms in the presence of a linearly polarized laser field is theoretically studied in the domain of moderate field intensities. The interaction of the hydrogen atom with the laser field is described by first-order time-dependent perturbation theory, while the projectile electrons interacting with the laser field are described by the Gordon-Volkov wave functions. An analytic expression is obtained for the differential scattering cross section in the first-order Born approximation for laser-assisted inelastic e-H(2s) scattering for the 2s-nl excitation. Detailed analytical and numerical results are presented for inelastic scattering accompanied by one-photon absorption, and the angular dependence and resonance structure of the differential cross sections is discussed for the 2s -4l excitation of metastable hydrogen.

Use of Gas and Foil strippers for the production of He-like ionic beams in both pure ground state (1s2) and mixed states (1s2, 1s2s) for zero-degree Auger Projectile Electron Spectroscopy

SynopsisWe present the unique benefits of gas and foil pre- and post- stripping in the production of pure ground state (1s2), as well as mixed (1s2, 1s2s) state He-like beams. The availability of such beams in TANDEM accelerators is of vital importance in studies concerning the population mechanisms of the 1s2s2p 4P metastable states produced in collisions of these He-like beams with gaseous targets.

Laser-assisted inelastic scattering of electrons by helium atoms

The differential cross section for electron-impact excitation of helium atoms in the presence of a linearly polarized laser field is calculated. The interaction of the laser field with both the projectile electron and the target atom is treated in a fully nonperturbative way, while the electron-atom interaction is treated within the first Born approximation. We are interested in studying two cases where the photon energy of the laser field is chosen to be far from resonance and when it matches with $2{\phantom{\rule{0.16em}{0ex}}}^{1}S\ensuremath{\longrightarrow}2{\phantom{\rule{0.16em}{0ex}}}^{1}P$ and $3{\phantom{\rule{0.16em}{0ex}}}^{1}S\ensuremath{\longrightarrow}3{\phantom{\rule{0.16em}{0ex}}}^{1}P$ transition frequencies. The agreement between perturbative and nonperturbative results is good, except close to the resonance where the perturbative cross sections diverge while the nonperturbative ones predict no maximum of the cross sections. Another interesting effect is the presence of an avoided crossing of the Floquet pseudoenergies at resonance.

Classical treatment of Li2++Ar and He2++Ar collisions

SynopsisClassical Trajectory Monte Carlo calculations are carried out for Li2++Ar and He2++Ar collisions, motivated by recent experiments on these systems. Cross sections for electron capture, projectile electron loss and target multiple ionization processes are evaluated and compared to the experimental values in the 75-500 keV/amy impact energy range.

Multiple ionization of neon atoms in collisions with bare and dressed ions: A mean-field description considering target response

We investigate projectile-charge-state-differential electron removal from neon atoms by impact of ${\mathrm{He}}^{2+}$, ${\mathrm{Li}}^{3+}$, ${\mathrm{B}}^{2+}$, and ${\mathrm{C}}^{3+}$ ions at intermediate projectile energies $(25\phantom{\rule{0.16em}{0ex}}\text{keV}/\text{u}$ to $1\phantom{\rule{0.16em}{0ex}}\text{MeV}/\text{u})$. The many-electron problem is described with an independent electron model in which active electrons at both collision centers are propagated in a common mean-field potential. Response to electron removal is taken into account in terms of a time-dependent screening potential, and a Slater-determinant-based method is used for the final-state analysis. Total cross sections for net recoil ion production, multiple ionization, and capture channels are mostly in good agreement with published experimental data. Results from equicharged bare and dressed ions are compared and the net recoil ion production cross section is broken down into contributions associated with different final projectile charge states in order to shed light on the role of the projectile electrons.

Total projectile electron loss cross sections ofU28+ions in collisions with gaseous targets ranging from hydrogen to krypton

Beam lifetimes of stored ${\mathrm{U}}^{28+}$ ions with kinetic energies of 30 and $50\text{ }\mathrm{MeV}/\mathrm{u}$, respectively, were measured in the experimental storage ring of the GSI accelerator facility. By using the internal gas target station of the experimental storage ring, it was possible to obtain total projectile electron loss cross sections for collisions with several gaseous targets ranging from hydrogen to krypton from the beam lifetime data. The resulting experimental cross sections are compared to predictions by two theoretical approaches, namely the CTMC method and a combination of the DEPOSIT code and the RICODE program.

Electron emission in ionization of He and Ne by fast dressed oxygen ions and projectile-charge-state dependence

The double-differential cross sections (DDCS) of low-energy electrons emitted at forward, backward, and perpendicular directions are reported for collisions of 3.75 MeV/u ${\mathrm{O}}^{q+}$ ($q=5$, 6, 7, 8) projectiles with He and Ne targets. The measured DDCS are found to be deviating from the ${q}^{2}$ dependence throughout the entire energy region. The effect of projectile electrons, for the dressed ions, as a function of the impact parameter is clearly noticeable for large as well as low-impact parameter collisions. We also present a theoretical calculation based on the prior form of the continuum distorted wave--eikonal initial state approximation, in which the projectile-active electron interaction is modeled with the Green-Sellin-Zachor potential. This particular representation of the potential has been proven to give good qualitative results for projectiles with residual electrons. In addition to the total DDCS, the individual contributions from target ionization, projectile electron loss, and simultaneous ionization processes are also calculated. The total DDCS obtained from these calculations are shown to be in excellent agreement with the experimental observations.

Dressed projectile charge state dependence of differential electron emission from Ne atom

We study the projectile charge state dependence of doubly differential electron emission cross section (DDCS) in ionization of Ne under the impact of dressed and bare oxygen ions. Experimental DDCS results measured at different angles are compared with the calculations based on a CDW-EIS approximation using the GSZ model potential to describe projectile active-electron interaction. This prescription gives an overall very good agreement. In general a deviation from the q2-law was observed in the DDCS. The observations crudely identify the dominance of different projectile electron loss mechanisms at certain electron energy range.

Electron emission in collisions of dressed ions with He atoms

We present theoretical and experimental results for angular and energy distributions of electrons emitted in collisions of B2+ with He atoms. In particular, we analyze the cusp formed by projectile Electron Loss to Continuum (ELC) and Electron Capture to Continuum (ECC). The cusp shape dependence on the electronic initial state is discussed and compared with our previous measurements on Li+ + He. The projectile incident energy was about 414 keV/u and the electron detection angle was varied between 0° and 150°. The experimental data is compared with three-body quantum Continuum-Distorted-Wave calculations.

Atomic Physics with Accelerators: Projectile Electron Spectroscopy (APAPES)

The new research initiative APAPES (http://apapes.physics.uoc.gr/) has already established a new experimental station with a beam line dedicated for atomic collisions physics research, at the 5 MV TANDEM accelerator of the National Research Centre "Demokritos" in Athens, Greece. A complete zero-degree Auger projectile spectroscopy (ZAPS) apparatus has been put together to perform high resolution studies of electrons emitted in ion-atom collisions. A single stage hemispherical spectrometer with a 2-dimensional Position Sensitive Detector (PSD) combined with a doubly-differentially pumped gas target will be used to perform a systematic isoelectronic investigation of K-Auger spectra emitted from collisions of preexcited and ground state He-like ions with gas targets using novel techniques. Our intention is to provide a more thorough understanding of cascade feeding of the 1s2s2p 4P metastable states produced by electron capture in collisions of He-like ions with gas targets and further elucidate their role in the non-statistical production of excited three-electron 1s2s2p states by electron capture, recently a field of conflicting interpretations awaiting further resolution. At the moment, the apparatus is being completed and the spectrometer will soon be fully operational. Here we present the project progress and the recent high resolution spectrum obtained in collisions of 12 MeV C4+ on a Neon gas target.

Bare- and Dressed-Ion Impact Collisions from Neon Atoms Studied Within a Nonperturbative Mean-Field Approach

We study electron removal processes in collisions of bare and dressed doubly charged ions with neon atoms in the 20 keV/u to 1 MeV/u impact energy regime. The many-electron problem is represented by a single mean field, which in the case of dressed-ion impact includes the projectile electrons. Moreover, the same basis is used to propagate all active orbitals thereby ensuring orthogonality at all times and allowing for a final-state analysis in terms of standard Slater determinantal wave functions. The same approach was used in a recent work for B2+ -Ne collisions [Phys. Rev. A 88 012712], in which we examined the role of the projectile electrons for target-recoil-charge-state production. The present study expands on that work by considering additional collision channels and comparing results of equicharged dressed and bare ions in order to shed more light on the role of the projectile electrons.

Theoretical analysis on fully differential cross sections for C6+ impact ionization of helium

Fully differential cross sections (FDCS) are calculated within a four-body model for single ionization of helium by C6+ impact at the incident energy of 100 MeV/a.u. (atomic unit). The results are compared with experimental data and other theoretical predictions. It is shown that our results are in very good agreement with experiment for three small momentum transfers in the scattering plane; however, some significant discrepancies are still present at the largest momentum transfer in both the scattering plane and the perpendicular plane. In actuality, the problem has not been explained by the theory during the last decade. Accordingly, the contributions of different scattering amplitudes to FDCS are analyzed. It is found that for the largest momentum transfer the cross section arising from a destructive interference of the three amplitudes is much smaller than the experimental data. However, the cross section due to the constructive interference of two scattering amplitudes between projectile—ionized electron interaction and projectile—passive electron interaction almost approaches the experimental data.