Post-collision Interaction Research Articles

Post-collision interaction after electron impact measured by (e,2e) coincidence technique

Auger electron line-shapes after electron impact inner-shell ionization of Argon at 350eV primary electron energy have been studied by coincidence spectrometry. Emitted Auger electrons are detected in coincidence with the scattered electrons and the energy of the very slow PCI inducer ejected electron was calculated from energy conservation. The background, caused by outer-shell electrons was measured and then removed from the coincident spectrum. The effect of PCI is studied on the background-free diagram Auger spectrum. A systematic deviation was found from the line-shape given by the semi-classical approach.

Dissociation dynamics of anionic and excited neutral fragments of gaseous SiCl4 following Cl 2p and Si 2p core-level excitations

The state-selective dissociation dynamics for anionic and excited neutral fragments of gaseous SiCl4 following Cl 2p and Si 2p core-level excitations were characterized by combining measurements of the photon-induced anionic dissociation, x-ray absorption and UV/visible dispersed fluorescence. The transitions of core electrons to high Rydberg states/doubly excited states in the vicinity of both Si 2p and Cl 2p ionization thresholds of gaseous SiCl4 lead to a remarkably enhanced production of anionic, Si− and Cl−, fragments and excited neutral atomic, Si*, fragments. This enhancement via core-level excitation near the ionization threshold of gaseous SiCl4 is explained in terms of the contributions from the Auger decay of doubly excited states, shake-modified resonant Auger decay, or/and post-collision interaction. These complementary results provide insight into the state-selective anionic and excited neutral fragmentation of gaseous molecules via core-level excitation.

Effect of electron correlations on the excitation of neutral states of zinc in the autoionizing region: A photon emission study

Electron impact excitation of the $4p\phantom{\rule{0.2em}{0ex}}^{3,1}P_{1}$, $4d,5d,6d\phantom{\rule{0.2em}{0ex}}^{3}D_{1,2,3}$, and $4d,5d\phantom{\rule{0.2em}{0ex}}^{1}D_{1}$ states of the neutral zinc atom was studied by measuring photon decay intensities in the energy region of the $3{d}^{9}4{s}^{2}4p$ autoioionizing states. Two phenomena, both involving strong electron correlations and an active role of one of the $3d$ electrons in an intermediate scattering step, have been observed. The first is $3d$ core-excited negative-ion resonances, which dominate the energy region between 10.5 and $11.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The second is postcollision interaction (PCI) in the decay of autoionizing states which populates higher excited states at incident electron energies above $11.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The low energies of both the scattered and ejected electrons after near-threshold excitation of $3{d}^{9}4{s}^{2}4p$ autoionizing states enable a large transfer of orbital angular momentum, up to two units, by the PCI.

On the L 2 photoelectron spectroscopy main line of Cu metal measured in coincidence with the Coster–Kronig preceded Auger decay

The present many-body theory shows that the L 2 photoelectron spectroscopy (PES) main line of Cu metal measured in the sudden limit in coincidence with the L 2-L 3M 4,5 Coster–Kronig (CK) preceded Auger decay satellite peak does not show any peak KE shift and asymmetric line shape change compared to the singles (noncoincidence) one. The peak KE shift and asymmetric line shape change between the two PES main lines observed by Creagh et al. [C.A. Creagh, S.M. Thurgate, R.P. Craig, C.P. Lund, Surf. Sci. 412 (1999) 297] are not due to the post-collision interaction (PCI) between a slow CK electron and a fast photoelectron (or a fast Auger electron) they attributed to. The inherent PCI between a slow CK electron and a fast Auger electron in the CK preceded Auger decay final state does not manifest in the coincidence L 2 PES main line of Cu metal, even if there is one. The discrepancies between the two PES main lines are due to the inelastic L 3 photoelectrons collected either in coincidence with the elastic Auger electrons emitted by the L 3-VV Auger decay or in coincidence with the inelastic ones. The inelastic L 3-photoelectron background appropriate, respectively for the singles L 2 PES main line and the coincidence one, should thus be subtracted. It is shown that the inherent PCI between a slow CK electron and a fast Auger electron in the CK preceded Auger decay final state is absent in Fe metal irrespective of photon excitation energy because the M 4,5 hole created by the CK decay delocalizes prior to the L 3-hole decay so that the L 2 photoelectron and the CK electrons are decoupled from the L 3-hole decay.

(e,2e)study of Ca in coplanar symmetric geometry

The triple differential cross section of $\mathrm{Ca}(4{s}^{2})$ is calculated by using the distorted wave Born approximation with exchange and with exchange and post-collision interaction. The calculations have been carried out at different impact energies ($4--58.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the first ionization potential of Ca). The present calculations are in close agreement with the recent $(e,2e)$ experimental studies [A. J. Murray, Phys. Rev. A. 72, 062711 (2005)]. The calculations show an improvement in agreement with the experiment as compared with the earlier theoretical works.

DWBA-G calculations of electron impact ionization of noble gas atoms

We perform calculations of electron impact ionization of noble gas atoms within the distorted wave Born approximation (DWBA) corrected by the Gamow factor (G) to account for the post-collision interaction. We make an extensive comparison with experimental data on He 1s2, Ne 2s2, 2p6 and Ar 3p6 under kinematics characterized by large energy transfer and close to minimum momentum transfer from the projectile to the target. For all atoms, good agreement between theory and experiment is achieved. In the case of Ar, the disagreement of experimental data with theory reported earlier by Catoire et al (2006 J. Phys. B: At. Mol. Opt. Phys. 39 2827) is reconciled.

Projectile interactions in theoretical triple differential cross sections for simultaneous excitation–ionization of helium

The importance of projectile interactions in triple differential cross sections (TDCS) is explored for the problem of simultaneous excitation–ionization of helium by electron impact using a new approach that we call the four-body distorted wave model (4DW). The 4DW model includes all projectile interactions, namely initial- and final-state projectile–target interactions, and the post-collision interaction between the two continuum electrons. Results are presented for an incident electron energy of 500 eV and are compared to experimental data, as well as a second-order R-matrix theory and the first Born approximation. Results for absolute TDCS ratios of ionization without excitation to excitation–ionization are also presented.

(e, 2e) triple differential cross-sections for ionization beyond helium: the neon case at large energy transfer

We report new coplanar (e, 2e) results for ionization of the He 1s and Ne 2p and 2s subshells under kinematics which have remained rather unexplored to date and characterized by large energy transfer and close to minimum momentum transfer from the projectile to the target. The experimental results obtained in two laboratories are used as a sensitive test of a number of state-of-the-art available theoretical models for multi-electron atoms. The He results are in excellent agreement with convergent close-coupling predictions as well as with distorted-wave + Gamow and hybrid distorted-wave + R-matrix models. In the Ne case, an overall satisfactory agreement with experiments is obtained for all considered models. The importance of post-collisional interaction effects is demonstrated whereas second-order effects in the projectile-target interaction at small distances are shown to be negligibly small. Remaining discrepancies between theories and experiments are discussed.

Postcollision interaction upon electron excitation of the 5sns 1 S 0 and 5snd 3 D j levels of the cadmium atom

The optical excitation functions (OEFs) for two series of spectral lines of the Cd atom originating from the 5sns1S0 (n = 6−11) and 5snd3D1, 2, 3 (n = 5 and 6) levels excited by an ultramonoenergetic (ΔE1/2 < 0.05 eV) electron beam with energies exceeding the single ionization threshold are presented. In the energy range from 10.8 to 12.9 eV, the energy dependences of the excitation cross sections of the studied spectral transitions exhibit the effect of postcollision interaction of slow scattered and fast emitted electrons. This process leads to an additional population of the initial levels of the spectral transitions due to the capture of a scattered electron into an excited atomic level. The energies and widths of the electronic decay of autoionizing states are estimated in the classical approach by two methods, namely, by the least squares method and by direct calculation. Calculations are performed using approximate formulas valid for different relations between the postcollision shifts of the OEF maxima and the binding energies of the atomic levels. The terms of the autoionizing atomic states responsible for the maxima observed in the OEFs of the spectral transitions are determined.

Dynamics of electron emission in double photoionization processes near the krypton 3d threshold

Two-electron emission following photoabsorption near the Kr 3d threshold is investigated both experimentally and theoretically. On the experimental side, electron/electron coincidences using a magnetic bottle time-of-flight spectrometer allow us to observe the complete double photo ionization (DPI) continua of selected Kr2+ final states, and to see how these continua are affected by resonant processes in the vicinity of the Kr 3d threshold. The analysis is based on a quantum mechanical approach that takes into account the contribution of three different processes: (A) Auger decay of the inner 3d vacancy with the associated post-collision interaction (PCI) effects, (B) capture of slow photoelectrons into discrete states followed by valence multiplet decay (VMD) of the excited ionic states and (C) valence shell DPI. The dominant process for each Kr2+(4p−2) final state is the photoionization of the inner shell followed by Auger decay of the 3d vacancies. Moreover, for the 4p−2(3P) and 4p−2(1D) final ionic states an important contribution comes from the processes of slow photoelectron capture followed by VMD as well as from double ionization of the outer shell involving also VMD.

Post-collision interaction measured by coincidence spectrometry in the electron impact Auger process

Auger electron lineshapes after electron impact inner-shell ionization of argon at 500 eV primary electron energy are studied by the (e, 2e) coincidence technique. Emitted Auger electrons are detected in coincidence with the ionizing scattered electrons and the energy of the slow PCI inducer ejected electron is calculated from energy conservation. In particular, the strongly asymmetric energy sharing, and by this means the effect of the very low energy (i.e. 0–5 eV) ejected electrons, is measured and compared with calculations. The opposite case (equal energy sharing) is also studied.

Polarization effect and post-collisional interaction in (e, 2e) reaction process for He and Ar in coplanar asymmetric geometry

The (e, 2e) triple differential cross sections for He(1s2)，Ar(3p6)and Ar(2p6)have been calculated using the modified distorted wave Born approximation (DWBA) in coplanar asymmetric geometry. The kinematics employs large energy transfer and is close to minimum momentum transfer. The theoretical results have been compared with those of the experiment, the Brauner, Briggs and Klar (BBK) method and the standard DWBA calculation. It is shown that the polarization and post-collisional interaction effects are very important in coplanar asymmetric (e, 2e)reaction with special kinematics for He(1s2), Ar(3p6) and Ar(2p6).

Simultaneous ionization and excitation of helium by electron impact

We present numerical results for He (1s2) (e,2e) He+ reaction process for transitions to the n=1, 2 and 3 states of He+ for noncoplanar symmetric geometry at incident energies of 1000 and 1600eV. The calculations are performed using the plane wave impulse approximation (PWIA) and the 3C method (also called the Brauner, Briggs and Klar (BBK) model) that includes post collision interaction and multiple scattering effects. In both the methods we have used the highly correlated configuration interaction wave function for the ground state of helium. A comparison of the present theoretical cross sections with the recent measured data of Ren et al. [X.G. Ren, C.G. Ning, J.K. Deng, G.L. Su, S.F. Zhang, Y.R. Huang, G.Q. Li, Phys. Rev. A 72 (2005) 042718] shows reasonably good agreement.

Electron impact single ionization of helium with large energy transfer

We report triply differential cross section (TDCS) for electron impact single ionization of helium at the incident energy of 729.6eV for a variety of scattering angles and ejection energies. The calculations are based on two different methods, namely, the Glauber approximation (GA) and the 3C approximation, also called the BBK method. In the GA the effects of post collision interaction have been taken into account by a classical method, whereas the BBK method has been studied with and without exchange. A comparison is made of the present TDCS with the results of other calculations and the experimental data from two different laboratories. For large energy transfer we find a significant contribution from the exchange effect. Both the GA–PCI and the BBK methods show some shifts in the position of binary maxima which are in reasonably good agreement with experiment.

Study of postcollision interaction upon electron-impact excitation of the magnesium atom

The excitation functions of the spectral lines of the MgI principal series in the VUV region (for the principal quantum numbers n = 4–7) are studied in experiments on collisions of electrons with magnesium atoms in the range of electron energies from the excitation thresholds to ∼25 eV. The scatter in the electron energy was ∼0.7 eV. Above the ionization potential, some singularities attributed to autodetachment and autoionization states were found in excitation functions. Shifts to higher energies with increasing n, which take place for the most significant singularity (maximum) relative to the autoionization state 3p4s 1 P 1 o , are explained by the important role played by this state in the population of the initial levels through the mechanism of post-collision interaction. The theoretical analysis of the experimental results based on the classical model of the postcollision interaction estimates the width of the autoionization state as ∼0.45 eV, which is close to the data of other researchers.

Localized versus delocalized excitations just above the 3d threshold in krypton clusters studied by Auger electron spectroscopy

We present Auger spectroscopy studies of large krypton clusters excited by soft x-ray photons with energies on and just above the 3d(52) ionization threshold. The deexcitation spectra contain new features as compared to the spectra measured both below and far above threshold. Possible origins of these extra features, which stay at constant kinetic energies, are discussed: (1) normal Auger process with a postcollision interaction induced energy shift, (2) recapture of photoelectrons into high Rydberg orbitals after Auger decay, and (3) excitation into the conduction band (or "internal" ionization) followed by Auger decay. The first two schemes are ruled out, hence internal ionization remains the most probable explanation.

(e, 2e) Study of Ca in Coplanar Symmetric Geometry at low impact energies using Distorted Wave Born Approximation

The triple differential cross section of Ca (4s2) in coplanar symmetric geometry at low impact energies is calculated using a spin averaged static exchange potential and a modified semi-classical exchange potential in Distorted Wave Born Approximation. The calculations have been carried out at incident energies of 10.11, 12.61, 19.61, 24.61, 34.61, 44.61, 54.61 and 64.61 eV for Ca, at which experimental data were available for comparison. The effect of post collision interaction has also been studied using Gamow factor introduced by Whelan et al. (1994). The present calculations have been compared with the recent (e, 2e) experimental studies of Murray (2005) and the earlier theoretical work of Chauhan et al. (2005). The present results are found to be in better agreement with experimental data as compared to the earlier study.

Coplanar symmetric and asymmetric electron impact ionization studies from the 1b1 state of H2O at low to intermediate impact energies

(e, 2e) ionization differential cross sections are presented for incident electron energies ranging from 15 eV to 95 eV above the ionization threshold of the 1b1 molecular state of H2O. Experimental results and theoretical analysis were derived for three energies in a coplanar symmetric geometry, and for three energies in an asymmetric geometry. The experimental data show a wide variation in the cross section over this range of energies, whereas the theoretical analysis carried out using a sophisticated molecular DWBA model, which includes the final state post collision interaction (PCI), shows best agreement at lower energies. The experimental techniques used to collect the data are described here as well as an improved theoretical approach using elastic scattering cross sections to evaluate the accuracy of the distorted waves utilized in the calculation of the ionization cross sections.

Double photoionization near the inner-atomic shell threshold: dynamics of two-electron emission

The double-electron ejection process after photon absorption by an atom is considered near the inner-atomic shell threshold. The aim of our work is to clarify the dynamics of double-electron emission. For this purpose, it is of primary importance to investigate processes with well-defined final ionic states. The focus of our study in this case is resonant reactions with a low energy for one electron, whereas the energy of the other one is close to the energy of the Auger electron. In this case, important contributions to the cross section of three different processes have to be considered: (1) photoionization of the inner shell followed by Auger decay of the inner vacancy influenced by post-collision interaction (PCI); (2) the PCI capture of the slow photoelectron into a discrete state followed by valence multiplet decay, and (3) the double photoionization of the outer shell. A quantum-mechanical approach has been developed to take into account the contribution to the cross section of amplitudes of all three processes. This model is applied to the case of double photoionization of the Kr atom near the 3d-shell threshold. Our calculation predicts an interference effect in the channel of the 1D final state, whereas in the 3P final state channel the role of interference is negligible.

CDW theoretical description of the single ionization of argon by positron and electron impact

In this work, we consider the single ionization of Ar by 200eV positron and electron impact. The quantum mechanical continuum distorted wave (CDW) model is used in the one-active electron approximation. Parametrizations of Hartree–Fock potentials by Garvey et al. are used to represent the interaction of the light particles with the Ar+ ion. The post collisional interaction (PCI) between the emitted electron and the receding projectile is naturally considered by the present model. Comparison with recent experimental data for very asymmetric geometries is made. The role of the PCI is studied by analyzing the differences in the angular distributions for positron and electron impact predicted by the CDW model and the first Born approximation.