Post-collision Interaction Effects Research Articles

Interplay of post-collision interaction and photoelectron recapture in the near threshold inner shell ionization of rare gases

The Ar 2p−11/2 nℓ, Kr 3d1/2−1 nℓ and Xe 4d−11/2 nℓ resonances have been photoexcited and the angular distributions of the low energy electrons ejected in the autoionistaion to the lower spin orbit component of the respective ions have been measured. The observed lineshapes can be described by a combination of the post-collision interaction effects and the electron recapture probability in the very near threshold region.

Site-dependent Si KL23L23 resonant Auger electron spectra following inner-shell excitation of Cl3SiSi(CH3)3

Spectator resonant Auger electron spectra with the Si 1s photoexcitation of Cl3SiSi(CH3)3 have been measured using an electron spectroscopic technique combined with undulator radiation. The transition with the highest intensity in the total ion yield (TIY) spectrum, coming from excitation of a Si 1s electron on the Cl-side into a vacant valence orbital, generates the resonant Auger decay in which the excited electron remains in this valence orbital. Photoexcitation of 1s electrons into some Rydberg orbitals induces Auger shake-down transitions, because higher-lying Rydberg orbitals in the two Si atoms closely positioned hold spatially overlapping considerably. A broad TIY peak slightly above the 1s ionization thresholds appreciably yields resonant Auger decays in which a slow photoelectron is re-captured into a higher-lying Rydberg orbital. The normal Auger peak shape at this photon energy is distorted due to a post-collision interaction effect. These findings provide a clear understanding on properties of the excited orbitals which are ambiguous in the measurement of the TIY only.

A variety of characteristic behaviour of resonant KL23L23 Auger decays following Si K-shell photoexcitation of SiCl4

Spectator resonant Auger electron spectra with the Si 1s photoexcitation of SiCl4 have been measured using an electron spectroscopic technique combined with undulator radiation. The transition with the highest intensity in the total ion yield (TIY) spectrum, coming from excitation of a 1s electron into the 9t2 valence orbital, generates the resonant Auger decay in which the excited electron remains in the 9t2 orbital. A TIY peak positioned slightly above the 1s ionization threshold induces Auger decay in which the slow photoelectron is re-captured into a higher lying Rydberg orbital or the normal Auger peak shape is distorted due to a post-collision interaction effect. Another structure above the threshold, originating from a doubly excited state, yields the normal Auger peak with the distortion of peak shape and a resonant Auger peak with a higher kinetic energy. These findings provide a clear understanding of the properties of the excited orbitals which were ambiguous previously.

The post-collision interaction effect in autoionizing spectra of alkali atoms excited by electron impact

Ejected-electron energy spectra resulting from decay of the (4p55s2)2P3/2, 1/2 autoionizing states in rubidium atoms have been studied at electron impact energies up to 5 eV above the excitation thresholds. The energy shift of corresponding lines due to the post-collision interaction (PCI) involving the outgoing electrons and the residual ion have been observed for the first time. For both states the energy dependence of the shift possesses a smooth asymptotic behaviour which is satisfactorily described by the classical model of the PCI effect. Similar behaviour of the line shift was reported earlier also for the (5p56s2)2P3/2 autoionizing state in heavier Cs atoms. Both results contrast with the clear oscillatory-like character of the line shift for the (np5[n + 1]s2)2P3/2, 1/2 autoionizing states in the lighter Na (n = 2) and K (n = 3) atoms. The comparative analysis of the data on the PCI effect and excitation cross-sections of the (np5[n + 1]s2)2P3/2, 1/2 autoionizing states in Na (n = 2), K (n = 3), Rb (n = 4) and Cs (n = 5) atoms has shown that the high polarizability of the resonantly excited target and partial structure of resonances are the main factors which determine the dynamics of the PCI shift of lines in ejected-electron spectra of alkali atoms.

High resolution study of the inner-shell 3p–3d and 3p–5s resonance regions in calcium atoms

The 3p54s23d1P1 and 3p5(2P3/2)4s25s2[3/2]1 resonance regions of Ca atoms have been studied experimentally with the aid of synchrotron radiation excited electron spectroscopy. The strong configuration interaction of the 3p54s23d, 3p54s25s and several other nearly degenerate configurations leads to complex structures observed in the photoelectron yield spectrum. Multiconfiguration Dirac–Fock (MCDF) calculations have been performed for these 3p-excited resonances of neutral calcium and compared to experimental results. The excitation and subsequent autoionization of these resonances lead to the final ionic 3p6nl states from which new high-lying members of the Rydberg p-series have been resolved experimentally. These new levels agree reasonably well with calculations as well as with the extrapolation of the Rydberg formula. The post collision interaction (PCI) effect has been observed in the transition Ca+ 3p53d4s2P1/2,3/2→ Ca2+ 3p61S0+eAuger− and explained by means of the rather long core-hole lifetime, while the decay of Ca+ 3p53d4s4P1/2,3/2,5/2 states is not affected by PCI.

Role of cascade double Auger decay in the PCI distortion of the Ar 2s photoelectron line

We have investigated both experimentally and theoretically the post collision interaction effect on Argon 2s photoelectrons in the case of cascade double Auger decay. New features of this effect are revealed.

Influence of double Auger decay on low-energy 3dphotoelectrons of krypton

Effects of postcollision interaction (PCI) observable in low-energy 3$d$ photoelectron spectra of Kr, which are associated with double Auger decay of the created inner-shell vacancy, are investigated by a combined experimental and theoretical approach. Measurements are based on an efficient multielectron coincidence method. Calculations have been carried out in the framework of a semiclassical approach. Our investigation reveals strong PCI distortion of the photoelectron line shapes, which depends on the kinematics of the process and the characteristics of the double Auger decay.

Low-energy electron-impact ionization of argon: Three-dimensional cross section

Low-energy (${E}_{0}$ $=$ 70.8 eV) electron-impact single ionization of a 3$p$ electron in argon has been studied experimentally and theoretically. Our measurements are performed using the so-called reaction microscope technique, which can cover nearly a full 4\ensuremath{\pi} solid angle for the emission of a secondary electron with energy below 15 eV and projectile scattering angles ranging from \ensuremath{-}8\ifmmode^\circ\else\textdegree\fi{} to \ensuremath{-}30\ifmmode^\circ\else\textdegree\fi{}. The measured cross sections are internormalized across all scattering angles and ejected energies. Several theoretical models were employed to predict the triple-differential cross sections (TDCSs). They include a standard distorted-wave Born approximation (DWBA), a modified version to account for the effects of postcollision interaction (DWBA-PCI), a hybrid second-order distorted-wave plus $R$-matrix (DWB2-RM) method, and the recently developed $B$-spline $R$-matrix with pseudostates (BSR) approach. The relative angular dependence of the BSR cross sections is generally found to be in reasonable agreement with experiment, and the importance of the PCI effect is clearly visible in this low-energy electron-impact ionization process. However, there remain significant differences in the magnitude of the calculated and the measured TDCSs.

Effects of target polarization and postcollision interaction on the electron-impact single ionization of Ne(2p), Ar(3p), and Na(3s) atoms

We report the perpendicular plane ionization results of ($e$, 2$e$) triple-differential cross sections for the Ne(2$p$) and Ar(3$p$) atoms at the incident electron energies ranging from 5 to 50 eV above ionization potential for neon atoms and from 2 to 50 eV above ionization potential for the argon atoms. The present investigation has been done in the modified distorted wave Born approximation and it has been observed that postcollision interaction and polarization of target are important in the perpendicular plane ionization of atoms. We also present the results of triple-differential cross section for the doubly symmetric coplanar ionization of Na(3$s$) atoms at the incident electron energy ranging from 6 to 60 eV above ionization potential. Thus we are able to see the effects of target polarization and postcollision interaction in coplanar as well as the perpendicular plane geometrical conditions. The results of our calculations for the Ne(2$p$) and Ar(3$p$) have been compared with the very recent measurements of Nixon et al. [K. L. Nixon, A. J. Murray, and C. Kaiser, J. Phys. B 43, 085202 (2010)] and the results of coplanar ionization of Na(3$s$) have been compared with the experimental data of Murray [A. J. Murray, Phys. Rev. A 72, 062711 (2005)]. It is observed that there are certain discrepancies between theoretical and experimental results which indicate that further theoretical efforts are required to understand the cross section trends of neon, argon, and sodium atoms. The cross section in the perpendicular plane ionization exhibits complex variations as a function of incident energy and target.

Fivefold differential cross sections for the ionization of aligned hydrogen molecule by electron and positron impact

We report fivefold differential cross section (5DCS) for the ionization of aligned hydrogen molecule by electron and positron impact in coplanar geometry. The calculations have been performed for an incident energy of 200 eV and ejection energies of (3.5 ± 2.5) and (16 ± 4) eV. The present calculations are based on the eikonal approximation due to Glauber, and the BBK approximation. We have included the effect of post collision interaction (PCI) in the Glauber approximation classically. A comparison is made of the present calculations with the results of other theoretical methods and the recent experiment of Senftleben et al. [ 28]. The present theoretical models predict that the 5DCS is maximum when the intermolecular axis is aligned along the incident beam direction. The binary to recoil peak ratios predicted by the Glauber approximation with PCI (GA-PCI) are in reasonably good agreement with the experiment. The positions of the binary peaks predicted by the BBK approximation are also in good agreement with the experiment. The positron-impact ionization cross sections obtained in the BBK and GA-PCI methods are found to be higher than the electron-impact cross sections in the binary region while the converse is true for the recoil regime. In case of positron impact, the binary peaks predicted by both the GA-PCI and BBK models shifted away from the direction of momentum transfer, and showed a trend which is opposite to the case of electron impact ionization.

Post-collision-interaction distortion of low-energy photoelectron spectra associated with double Auger decay

Atomic inner-shell photoionization followed by double Auger decay is investigated. The focus of our study is the effect of post-collision interaction (PCI) on the photoelectron energy distribution. A semi-classical approach is employed to describe the PCI distortion of the photoelectron line shapes associated with both direct and cascade double Auger decays. This approach is shown to be valid at low photoelectron energies, whereas for large incident photon energies it reduces to the eikonal approximation. The theory is applied to the case of Ar $2p$ photoionization spectra and good agreement with available experimental data is achieved.

Triple differential cross section in (e, 2e) collisions for sodium in a coplanar symmetric geometry

The triple differential cross sections for (e, 2e) ionization of sodium in a coplanar symmetric geometry have been studied in distorted wave Born approximation with the modified semiclassical exchange potential. The present calculations improve the agreement between the theoretical and experimental results. The post-collision interaction and target polarization effects have also been studied. In addition, the variation trends of the shape and ratio for the forward and backward scattering peaks have been discussed. Besides, the small bump observed between these two peaks has been analyzed by the interaction between active electron and passive electrons.

Postcollision interaction upon electron excitation of 4sns 1 S 0, 3 S 1 and 4snd 1 D 2, 3 D J levels of zinc atom

The optical excitation functions (OEFs) for four series of spectral lines of the Zn atom at the transitions 4sn1s1S0 → 4s4p1P1∘ (n1 = 6–10), 4sn2s3S1 → 4s4p3P2∘ (n2 = 6−8), 4sn3d1D2 → 4s4p1P1∘ (n3 = 4−6), 4sn4d3D1, 2, 3 → 4s4p3P2∘ (n4 = 4−7) excited by a monoenergetic (0.04–0.08 eV) electron beam with energies from the excitation threshold to 18–19 eV are measured. In the energy dependences of the excitation cross sections, the effect of post-collision interaction of slow (scattered) and fast (emitted after the autoionization state decay) electrons is observed and described. This interaction leads to the capture of a slow electron into the listed discrete levels and the appearance of maxima in the excitation functions. The energies of the maxima are used to determine the energies and widths of single autoionization states or their groups by approximate formulas of the classical approach. The autoionization levels of the zinc atom responsible for the maxima observed are determined. Possible transfer of the orbital angular momentum between electrons is analyzed.

The role of projectile interactions in triply differential cross sections for excitation–ionization of helium

We report triply differential cross section (TDCS) for the simultaneous excitation–ionization of helium by electron impact for both coplanar and non-coplanar geometry. In the coplanar case, calculations have been performed for an incident energy of 500 eV and low ejection energies (3 and 10 eV), whereas in the noncoplanar case we have considered impact energies in the range 1240–4260 eV for a symmetric geometry. The present calculation is based on the eikonal approximation due to Glauber. We have incorporated the effect of post-collision interaction in the Glauber approximation. A comparison is made of the present calculations with the results of other theoretical methods and recent experiments. The Glauber results are in reasonably good agreement with the experiment for small scattering angles.

Inner-shell photodetachment fromRu−

Inner-shell photodetachment from ${\mathrm{Ru}}^{\ensuremath{-}}$ was studied near and above the $4p$ excitation region, 29-to-91-eV photon energy range, using a merged ion-photon-beam technique. The absolute photodetachment cross sections of ${\mathrm{Ru}}^{\ensuremath{-}}$ ($[\mathrm{Kr}]4{d}^{7}5{s}^{2}$) leading to ${\mathrm{Ru}}^{+}$, ${\mathrm{Ru}}^{2+}$, and ${\mathrm{Ru}}^{3+}$ ion production were measured. In the near-threshold region, a Wigner $s$-wave law, including estimated postcollision interaction effects, locates the $4{p}_{3/2}$ detachment threshold between 40.10 and 40.27 eV. Additionally, the ${\mathrm{Ru}}^{2+}$ product spectrum provides evidence for simultaneous two-electron photodetachment (likely to the ${\mathrm{Ru}}^{+}$ $4{p}^{5}4{d}^{6}5{s}^{2}$ state) located near 49 eV. Resonance effects are observed due to interference between transitions of the $4p$ electrons to the quasibound $4{p}^{5}4{d}^{8}5{s}^{2}$ states and the $4d\ensuremath{\rightarrow}\ensuremath{\varepsilon}f$ continuum. Despite the large number of possible terms resulting from the ${\mathrm{Ru}}^{\ensuremath{-}}$ $4d$ open shell, the cross section obtained from a 51-state $\mathit{LS}$-coupled $R$-matrix calculation agrees qualitatively well with the experimental data.

Triple differential cross section of potassium for doubly symmetric ionization

We report the results of our modified distorted wave Born approximation calculation of triple differential cross section in the coplanar symmetric single ionization of potassium atom. We have included correlation–polarization potential as a function of electron density parameter and also studied the effect of post-collision interaction. The present attempt significantly improves the agreement between theoretical and experimental results.

PCI effects in double Auger decays of Ar 2p inner shell vacancies

We have investigated experimentally the post-collision interaction effect on 2p photoelectrons of Argon in the case of double Auger decay. A theoretical model has been developed to understand this effect.

Triple differential cross section in [formula omitted] collisions for atomic potassium

In this Letter we report the results of our calculations using the Modified Semiclassical Exchange Approximation in the Distorted Wave Born Approximation to determine the ( e , 2 e ) cross section for Potassium. The results are compared with the experimental and theoretical studies. The effect of post collision interaction has also been studied.

Fully differential cross section for single ionization of helium by 1 KeV electrons in the eikonal approximation

We report new results for fully differential cross sections for the single ionization of helium by 1 KeV electron impact at the ejection energy of 10 eV. Investigations have been made for both the scattering and perpendicular planes. The present calculation is based on the eikonal approximation due to Glauber. Here we have also incorporated the effect of classical post collision interaction (PCI) in the Glauber approximation (GA). A comparison is made of the present calculation with the results of other theoretical methods and the recent experiment [M. Dürr, C. Dimopoulou, B. Najjari, A. Dorn, K. Bartschat, I. Bray, D.V. Fursa, Z. Chen, D.H. Madison, J. Ullrich, Phys. Rev. A 77 (2008) 032717]. The Glauber results are in good agreement with the experiment in the scattering plane, but strong discrepancies are observed in the plane perpendicular to it. The effect of PCI is not substantial in the present kinematics.

Time dependent aspects in the ionizations following atomic photoexcitations

The time dependent aspects of the probability amplitudes in atomic excitations and their subsequent decay processes are discussed. The spin-orbit precession in an atomic ion core gives us a good clock to measure the characteristics of the autoionizing states. The lowest 3s sub-valence shell excitation of potassium atoms is attributed to an optically forbidden quartet state resonance, which are realized by the spin-orbit migration of the residual ion cores in the course of indirect photoionization processes. There is a class of multiple-electron ionization processes which provide us with the post-collision interaction effects. An electron that slowly leaves the ion behaves as a clock in the atomic system. The role of these atomic clocks are studied by tracing the time dependent state vector of the total system with the aid of accurate relativistic calculations of atomic structures and ionization dynamics. The post-collision interaction effects in an artificial many step Auger cascade and in a multi-step Auger cascade following the argon 1s vacancy creation by photon impact are studied extensively.