Impact Energy Region Research Articles

Absolute cross section measurements for the scattering of low- and intermediate-energy electrons from PF3. II. Inelastic scattering of vibrational and electronic excitations.

As a sequel paper to our study of the elastic scattering for electron collisions with phosphorus trifluoride, PF3 molecules, we report absolute inelastic differential and integral cross sections (DCS and ICS) of vibrational excitations for the compound fundamental vibrational modes v13 (v1 + v3), v24 (v2 + v4), and their sum in the impact energy range of 2.0-10 eV and over a scattering angle range of 20°-130°. The measured angular distributions of scattered electron intensities for the present inelastic scattering are normalized to the elastic peak intensity corresponding to the DCSs of He. These vibrational excitation measurements demonstrate the presence of resonances around 2 eV and also around 6-10 eV. In addition, a generalized oscillator strength analysis is applied to derive oscillator strength f0-values and (unscaled Born) ICSs from the corresponding DCSs measured for the low-lying optically allowed 8a1-1 → 7e (σ*) excitation band, which is assigned as the Jahn-Teller splitting and 8a1-1 → 4s Rydberg transition at impact energies of 100, 200, and 300 eV, over a scattering angle range of 1.0°-15°. The f0-values obtained in the present study are compared with the results of previous photoabsorption and pseudo-optical measurements. The unscaled Born ICSs are compared with the binary-encounter f-scaled Born ICSs estimated over a wide impact energy region from the excitation thresholds.

Ab initio study of charge transfer in low energy collisions of B4+ with H

The charge transfer processes in collisions of B4+(1s) ions with H(1s) atoms are investigated by using the quantum-mechanical molecular orbital close-coupling method with electron translational effects included in the impact energy region of 10-5 − 20 keV/u. Molecular data with high accuracy are calculated using ab initio method. Our calculations clarified the controversy in the total and state-selective cross sections at low energies. The treatment of the core electron will influence the accuracy of the calculated molecular structure and then sensitively influence the charge transfer cross sections in the low energy region. The rotational couplings play an important role in the state-selective cross sections at energies above 50 eV/u, but weakly influence the total cross sections for this collision system.

Resonance excitation of autoionizing states in alkali atoms by electron impact

In the work, the main results on excitation and decay processes of autoionizing states in alkali atoms are considered. As follows from the data on excitation dynamics of subvalence shells in an impact energy region from the lowest excitation threshold up to 600 eV, at threshold energies their electron excitation results exclusively from the formation of negative-ion resonances. At mean impact energies, the cascade population processes due to resonance excitation and subsequent radiative decay of the upper states play an essential role for the low-lying autoionizing states. By comparing with theoretical data, the absolute excitation cross sections for autoionizing states and negative-ion resonances were determined. The absolute autoionization cross sections as well as the autoionization contribution to the total single ionization cross section were also determined. Keywords: atom, ion, autoionization, resonances, ionization, effective cross section.

Electron-atom bremsstrahlung: Double-differential cross section and polarization correlations

The leading-order electron-atom bremsstrahlung is investigated within the rigorous relativistic approach based on the partial-wave representation of the Dirac wave functions in the external atomic field. Approximating the atomic target by an effective local potential, we calculate the double-differential cross section and the polarization correlations in a wide range of the impact energies. Connection between the bremsstrahlung at the hard-photon end point of the spectrum and the continuum-threshold limit of the radiative recombination is studied. A detailed analysis of the screening effect and the energy dependence of the polarization correlations is presented, with the main focus on the high impact energy region.

Benchmark Integral Cross Sections for Electron Impact Excitation of the n = 2 States in Helium

In this paper, we present integral cross sections (ICS) for electron impact excitation of the n = 2 levels in helium in the impact energy range of 23.5 eV to 35 eV. The ICS of each final state, 23S, 21S, 23P and 21P, has been determined by integration of the angular differential cross sections (DCS) over all of 0° to 180°, where those DCS were obtained from both our previous experiments and the extrapolation using the convergent close coupling calculation. The present experimental ICS for the optically allowed 21P transition state are also compared with those obtained from the BEf-scaling method. Very good agreement between the experimental and BEf-scaled 21P ICSs is generally found in the measured impact energy region.

Complex angular momentum analysis of low-energy electron elastic scattering from lanthanide atoms

Electron attachment to the lanthanide and Hf atoms resulting in the formation of stable excited lanthanide and Hf anions as Regge resonances is explored in the near-threshold electron impact energy region, $E<1.0$ eV. The investigation uses the recent Regge-pole methodology wherein is embedded the electron-electron correlations together with a Thomas-Fermi--type model potential incorporating the crucial core-polarization interaction. The near-threshold electron elastic total cross sections (TCSs) for the lanthanide and Hf atoms are found to be characterized by extremely narrow resonances whose energy positions are identified with the binding energies (BEs) of the resultant anions formed during the collision as Regge resonances. The extracted BEs for excited lanthanide anions are contrasted with those of the most recently calculated electron affinities (ground state BEs). We conclude that the BEs for the ${\mathrm{Pr}}^{\ensuremath{-}}$, ${\mathrm{Tb}}^{\ensuremath{-}}$, ${\mathrm{Dy}}^{\ensuremath{-}}$, ${\mathrm{Ho}}^{\ensuremath{-}}$, ${\mathrm{Er}}^{\ensuremath{-}}$, and ${\mathrm{Tm}}^{\ensuremath{-}}$ anions of O'Malley and Beck [Phys. Rev. A 79, 012511 (2009)] are not identifiable with the electron affinities as claimed. Formation of bound excited anions is identified in the elastic TCSs of all the lanthanide atoms including Hf, except Eu and Gd. The imaginary part of the complex angular momentum $L$, $\mathrm{Im}L$ is used to distinguish between the shape resonances and the bound excited negative ions. These results challenge both experimentalists and theoreticians alike since the excited anions are very weakly bound, but mostly tenuously bound ($\mathrm{BEs}<0.1$ eV). Shape resonances and Ramsauer-Townsend minima are also presented.

Electron emission from tungsten induced by slow, fusion-relevant ions

Tungsten has recently been introduced as a new wall material for fusion, because it exhibits favourably low sputtering yields and a very low tritium retention as compared to the commonly used graphite wall and divertor tiles. We measure total electron emission yields due to impact of slow singly and multiply charged ions (deuterium, helium and carbon) on sputter-cleaned polycrystalline tungsten surfaces by using a current method in combination with a retarding grid. Results are presented in the eV to keV impact energy region as typical for fusion edge plasma conditions and discussed in terms of potential and kinetic electron emission.

Similarities and differences between electron and positron scattering from molecules

In this paper a comparative study is carried out between total cross sections (TCS) for electron impact with those for positron impact using both experimental and theoretical procedures. The method used for the measurements is a retarding potential time-of-flight (RP-TOF) while the theoretical procedure is the Continuum Multiple Scattering (CMS) model. The experimental energy range is 0.2–1000 eV while that for the CMS is 1.5–100 eV. For polar molecules, the increase in TCS observed below 2 eV is expected due to the dipole induced long range interaction that has been observed only for some molecules for electron impact. In the energy range 3–60 eV electron TCS are characteristically larger than positron TCS due to resonances in the former. Some peculiar cases have been studied whereby positron TCS become unexpectedly greater than electron TCS at energies below 10 eV. The first Born approximation has been observed to hold for the impact energy region above a few hundred of eV for all molecules studied in this laboratory, albeit with different threshold energies.

Excitation of autoionizing states in potassium by electron impact

The ejected-electron excitation functions for the 3p 5 3d4s 4 P 3/2 , 4 DJ lowest quartet and 3p 5 3d4s( 1 P) 2 P 3/2 upper doublet autoionizing states in potassium have been measured with an energy resolution of 0.2 eV over the electron impact energy region from the lowest excitation threshold up to 500 eV. The detailed picture of the excitation dynamics of the 4 P 3/2 , 4 D j and 2 P 3/2 levels in the near-threshold impact energy region has been achieved due to the use of a small increment of the incident electron energy. An analysis of the data, including the consideration of resonance and cascade excitation processes, has been performed on the base of available experimental and theoretical data on the 3p 6 -excitation in potassium atoms.

The role of cascade processes in electron-impact excitation of the (3p54s2)2P3/2, 1/2 autoionizing levels in potassium

The cascade population of the (3p54s2)2P3/2, 1/2 lowest autoionizing levels in potassium has been studied to shed light on recent experimental observations of a noticeable enhancement in the cross sections around 22 eV incident electron energy. The ejected-electron excitation functions of both levels have been re-measured in the range of 20–28 eV, and calculations of the decay parameters for the 3p54s4p levels have been performed in single-configuration Hartree–Fock and configuration-interaction approaches. The analysis of these results, together with other available experimental and theoretical data on the 3p subshell excitation in potassium, revealed a relatively small contribution (<3%) of radiative transitions from the (3p5 4s4p1P)2P, 2D levels to the apparent (including cascade contributions) excitation cross sections of the (3p54s2)2P3/2, 1/2 levels. However, the decay of negative-ion resonances associated with the upper doublets in the 3p54s4p and 3p53d4s configurations may lead to a significant cascade population of the (3p54s2)2P3/2, 1/2 levels in the impact energy region 21–28 eV.

Single-electron capture processes in slow collisions ofHe2+ions withO2,NH3,N2, andCO2

Using the translational energy-gain spectroscopy technique, we have measured the energy-gain spectra and absolute total cross sections for single-electron capture (SEC) in collisions of ${\mathrm{He}}^{2+}$ ions with ${\mathrm{O}}_{2}$, $\mathrm{N}{\mathrm{H}}_{3}$, ${\mathrm{N}}_{2}$, and $\mathrm{C}{\mathrm{O}}_{2}$ at laboratory impact energies between 25 and $400\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{amu}$. The measured spectra for the ${\mathrm{He}}^{2+}\ensuremath{-}{\mathrm{N}}_{2}$ and $\mathrm{C}{\mathrm{O}}_{2}$ collision systems show that the dominant reaction channel is due to dissociative transfer ionization (i.e., SEC accompanied by ionization of the molecular target ion). In the case of the ${\mathrm{He}}^{2+}\ensuremath{-}\mathrm{N}{\mathrm{H}}_{3}$ collision system, nondissociative single-electron capture into $n=2$ states of ${\mathrm{He}}^{+}$ with production of $\mathrm{N}\mathrm{H}_{3}{}^{+}$ in the ground state is predominantly populated. These processes are observed to be the dominant reaction channels over the entire impact energy region studied and at laboratory scattering angles between 0\ifmmode^\circ\else\textdegree\fi{} and 8\ifmmode^\circ\else\textdegree\fi{}. The energy dependence of total cross sections for SEC are also measured and found to slowly increase with increasing impact energies. The measured cross sections are also compared with the available measurements and theoretical results based on the Demkov and Landau-Zener models.

Electron emission and molecular fragmentation during hydrogen and deuterium ion impact on carbon surfaces

Molecular fragmentation and electron emission during hydrogen ion impact on graphite surfaces has been investigated in the eV to keV impact energy region typical for fusion edge plasma conditions. As a target surface graphite tiles for the Tokamak experiment Tore Supra in CEA-Cadarache/France and highly oriented pyrolytic graphite (HOPG) have been used. For both surfaces studied, the experimentally observed threshold for electron emission is at about 50 eV/amu impact energy. Electron emission from the high conductivity side of the carbon tile is 15–20% less as compared to its low conductivity side, whereas results for HOPG are generally between these two cases. Deuterium and hydrogen ions are almost equally effective in liberating electrons from graphite when comparing results for the same impact velocity. Surface-induced dissociation of deuterium ions D 3 + upon impact on Tore Supra graphite tiles, in the collision energy range of 20–100 eV, produced only atomic fragment ions D +. The other possible fragment ion D 2 + could not be observed.

Scaling relation in the collision of hydrogenlike ions with antiprotons

The impact excitation and ionization of hydrogen (H)-like ions by antiprotons are studied by solving the time-dependent Schr\"odinger equation with a time-dependent generalized pseudospectral method in a wide scaled impact energy region (1 to 1000 keV). The validity region of the scaling relation is investigated by comparing the time-dependent calculations with the first Born approximation results. The numerical error and the validity region of the time-dependent generalized pseudospectral method have been explored. Based on the present study, we conclude that (i) the scaling relation of the ionization and excitation cross sections is valid for high-Z H-like ions or in the high-impact-energy regime for low-Z H-like ions; (ii) for given H-like ions, the scaling relation works better for the impact excitation to the dipole allowed transition than for the impact excitation to the dipole forbidden transition; and (iii) the time-dependent generalized pseudospectral method is not valid in a very weak perturbation system, which can be fairly studied by the time-independent methods. We also discuss the physical originations of the numerical observations.

Edge plasma-relevant ion–surface collision processes

Graphite tiles or carbon-coated structures are largely used as wall or divertor materials in magnetically confined fusion devices. Laboratory work on plasma–wall interaction has so far mainly focused on obtaining physical and chemical sputtering yields for such materials. Much less is known on electron emission and molecular fragmentation and reactions during ion impact on graphite surfaces, despite the fact that both processes might play a decisive role in plasma–wall interaction. We therefore have measured such data for fusion-relevant ion species (e.g., Hn+, n=1–3 and Cz+, z=1,2,4,5) in the eV to keV impact energy region typical for fusion edge plasma conditions. As a target surface graphite tiles from the Tokamak experiment Tore Supra in CEA-Cadarache/France have been used. Implications of the experimental results for plasma edge modeling are also discussed.

Emission energy distribution of secondary ions produced through the electronic sputtering process under heavy ion bombardment

The emission energy distribution of secondary ions has been systematically measured for a SiO2 bombarded by Si, Cu and Ag ions at an impact energy region of MeV, where the electronic stopping power dominates over the nuclear stopping power. The obtained emission energy for atomic Oq+ and Siq+ (qe: electric charge) ions depends not only on projectiles but also on the electric charge qe. The energy distribution is asymmetric extending to a high-energy side. Conversely, the energy distribution for cluster ions is very narrow compared with that for the atomic ions. The mean energy for the cluster ions hardly depends on the size of clusters. However, in the case of the Cu projectiles, the mean energy of the (SiO2)x series (x: number of molecules) is overall less than those of the Si(SiO2)x and SiO(SiO2)x series. These results combined with the yield dependence on the electronic stopping power show that the atomic ions are produced from the hot track region and forced to move by the electric repulsive force. The cluster ions are formed in the periphery of the track region.

Electron impact excitation of the , , and states of CO at 10.0, 12.5 and 15.0 eV impact energies

We have measured the differential and integral cross sections for electron impact excitation of the , , and states of CO in the 10-15 eV impact energy region. These measurements fill the gap which existed between the low-energy (threshold to about 12 eV) data of Zobel et al and the intermediate energy (20-50 eV) results of Middleton et al. Recent electron time-of-flight measurements are utilized to place the relative cross sections (deduced from energy-loss spectra) on the absolute scale. The cross sections are in excellent agreement with the results obtained by Zobel et al at the overlapping impact energies and always smoothly extrapolate to their low-energy results. Extrapolation to the intermediate energy domain and comparison with the results of Middleton et al is not so clear. Comparison with theoretical differential and integral cross sections of Sun et al and the integral cross sections of Morgan and Tennyson reveals angular and energy dependences which resemble those found for the experimental results but there are serious discrepances concerning the absolute magnitudes.

Collision Strengths from a 29-State R-Matrix Calculation on Electron Excitation in Helium

A previously reported 29-state R-matrix electron scattering calculation including the n ≤ 5 states of neutral helium is extended to include higher-incident electron energies and also (N + 1)-electron symmetries with orbital angular momentum L ≤ 9. Total collision strengths for many LS transitions among the lowest 19 (n ≤ 4) states are presented for the first time. All results include where necessary an estimate of the contribution to the total collision strength from those partial waves with L > 9 which are not explicitly included. Data are presented in several formats. Collision strengths for all transitions from the ground state are presented in tabular form for energies from threshold to 33.9 eV, using a mesh fine enough to resolve the main resonances in the excitation threshold impact energy region. Then, thermally averaged effective collision strengths are tabulated for all converged transitions in a range of electron temperatures from 2000 to 30,000 K. Finally, the energy-dependent collision strengths together with comparisons with a previous 19-state R-matrix calculation are plotted graphically for all transitions.

Measurements of Ionization Cross Sections of 4d-Electrons in Xenon by Electron Impact

Partial ionization cross sections for 4 d -electrons in xenon by electron impact have been measured by means of a combination of the electron-energy-loss spectroscopy and the Auger electron spectroscopy in the impact energy region from 70 eV to 4 keV. The cross sections increase with the impact energy rapidly from the threshold and display a sharp maximum, whose value is (17.0 ± 3.5) × 10 -18 cm 2 , around 120 eV impact energy, revealing an existence of a resonance. The cross sections arise again from a minimum at about 200 eV and exhibit a broad maximum of (14.5 ± 3.0) × 10 -18 cm 2 around 500 eV. Absolute scale of the cross section is determined using the generalized oscillator strengths for the 4 d -ionization, which are measured by the electron-energy-loss spectroscopy. Relative cross sections are measured as a function of the impact energy using an Auger line assigned to the N 5 O 2,3 O 2,3 ( 1 S 0 ) line. Experimental results are compared with a previous experimental result given by a different proc...

Double- and Single-Charge Transfer in Collision of C6+Ion with He Atom at Low Impact Energies

Total double- and single-charge transfer cross sections are calculated for the collisions of C 6+ ion with He atom in the impact energy region 0.1≤ E ≤10 keV/amu. The perturbed-stationary-state method in the impact parameter approximation is used with a basis set of SCF-CI molecular orbitals. There are many curve crossings among the adiabatic potential curves. A diabatic treatment is, however, found to be useful for the most of curve crossings. The single-charge transfer mainly occurs through the avoided crossing near 5 au, while the double-charge transfer takes place via a two-step one-electron transfer or a direct process. Hence the double-charge transfer cross section obtained(∼10 -16 cm 2 ) is about a tenth of the single-charge transfer cross section in these impact energies.

Shape of the "cusp" in He2+-He and He+-He collisions in the 0.6-1.6-MeV impact-energy region.

The shape of the distribution of electrons ejected into the forward direction in the He/sup +/-He and He/sup 2+/-He collisions in the 0.6-1.6-MeV impact-energy region are investigated. In both cases the anisotropies observed in the measured double-differential electron spectrum are analyzed by the help of a series expansion and a fitting procedure for a better characterization of the cusp. The theoretical peak shapes are also analyzed by this method. In the case of He/sup 2+/ the agreement between the measured and the calculated cusp shapes is especially poor.