Post-collision Interaction Effects Research Articles

Experimental observation of post-collision interaction and interference effects in resonant double photoionization processes.

The combined effects of post-collision interaction in the final state and interference due to the indistinguishability of the two electrons have been studied in a selected case of resonant double photoionization of neon. In our coincidence experiments, the photo- and Auger-electron pair was measured when the two electrons have nearly equal energy and are ejected at small mutual angle. The obtained energy distributions exhibit a strong interplay of post-collision interaction and interference effects, in agreement with the theoretical prediction of Sheinerman and Schmidt [J. Phys. B 30, 1677 (1997)] made on beryllium.

Multi-step post-collision interaction effects in K-shell photoionization of Kr

Yield spectra of multiply charged ions produced from cascade decays following K-shell photoionization of Kr have been measured in coincidence with threshold photoelectrons having near-zero kinetic energies (<0.05 eV). These observed yield spectra exhibit characteristic profiles of multi-step post-collision interaction effects induced by the cascades. Peak shifts of the profiles do not increase gradually along with the increase of the charged states, but bring about a step-like structure as a function of charge state. The step-like structure is due to the participation of radiative decays in the cascades following the K-shell photoionization.

Interference, post-collision interaction and exchange effects in angular distribution patterns of resonant photo double-ionization in neon for equal electron energies

Angle-resolved patterns of the fivefold-differential cross section in neon are calculated ab initio for the case of resonant photo double-ionization under the condition of equal energies of the ejected electrons. The patterns reveal the important role of direct and indirect processes as well as post-collision interaction (PCI), interference and exchange effects. Comparison with available experimental data shows that some of these effects are readily confirmed, but the angular range with dramatic PCI effects still awaits experimental verification.

Postcollision Interaction in the Coincident Emission of Photoelectrons and Auger Electrons at Small Relative Angles

Postcollision interaction effects have been investigated for sequential two-electron emission in xenon for particular kinematical conditions, small relative angles between the emitted electrons, and selected energies ${\ensuremath{\epsilon}}_{a}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}{\ensuremath{\epsilon}}_{b}$ which are halfway in between the nominal values of the ``photo''-electron and the ``Auger'' electron. The experimental results are in good agreement with the theoretical prediction. Our successful technique to measure two coincident electrons of nearly the same energy and the same directions opens a plethora of possibilities to probe for different cases the Coulomb interaction of three charged particles and the impact of the intermediate resonance state.

Post-collision interaction effects induced by Auger cascades following Xe L1-shell photoionization

Abstract Yield spectra of multiply charged ions have been measured by threshold electron-ion coincidence spectroscopy near the Xe L 1 -shell photoionization region. These spectra exhibit asymmetric and broadened profiles of post-collision interaction effects induced by multi-step Auger decays in Auger cascades following Xe L 1 -shell photoionization. The peak shifts of the profiles increase gradually along with the increase of the charge states, but the peak widths are almost independent of the charge states. The spectral profiles are reproduced by a simple model based on the assumption that double Auger (Auger shake-off) decays reduce the effective number of steps in the Auger cascades.

Observation of post-collision interaction (PCI) in HBr using two-dimensional photoelectron spectroscopy

The creation and subsequent Auger decay of the 3d-1 core-hole state in HBr is studied using two-dimensional photoelectron spectroscopy. In this technique the yield of electrons is collected as a function of both photon and electron kinetic energy. The effects of post-collision interaction (PCI) observed in the energy shift of the HBr+(3d-1) photoelectron lines. The energy shifts are compared with previous experimental results on Kr. This comparison suggests that the molecular nature of HBr has little effect on the PCI energy exchange mechanism.

Post-collision interaction effects of threshold photoelectrons in Kr -shell photoionization

A threshold photoelectron spectrum and its component spectra of Auger cascades following Kr -shell photoionization have been measured by a threshold photoelectron-ion coincidence technique. These spectra exhibit characteristic profiles of post-collision interaction effects induced by the Auger cascades. Peak shifts of the profiles increase gradually according to the times of the interactions, but peak widths of the profiles are almost independent of the times of interactions. These characteristics are analysed in terms of a simple model based on the assumption that double-Auger (Auger shake-off) decays reduce the effective number of steps of the Auger cascades.

Inner-shell excitation and ionization in carbon monoxide, near threshold, using a high-resolution electron spectrometer

The inner-shell excited states, (1s)-1(2pu)1,3u of carbon monoxide have been observed in electron impact excitation, as a function of incident energy via their decay, by electronic ejection, to an ionic state of the molecule. A shift in the energies of these ejected electrons has been observed and is explained in terms of a post-collision interaction effect. A detailed description of the apparatus is presented. The relative cross section for the formation of the CO+(B 2u) final ionic state, via the decay of these inner-shell excited states, is compared with predictions of a theoretical model.

Observation of angular correlation between subsequently emitted Auger electrons

In an exploratory feasibility study we have measured the angular correlation between Auger electrons that were emitted in a cascadelike decay process after resonant photoexcitation, using synchrotron radiation from the Brookhaven National Synchrotron Light Source. While the monochromator was tuned to the argon $1\stackrel{\ensuremath{\rightarrow}}{s}4p$ resonance (3203.5 eV) we recorded Ar $\mathrm{LMM}$ Auger electrons in coincidence with ${\mathrm{KL}}_{2,3}{L}_{2,3}{,KL}_{1}{L}_{2,3},$ and ${\mathrm{KL}}_{2,3}{M}_{1,2,3}$ Auger electrons. We found different nonisotropic angular correlations between distinct energy regions of the $\mathrm{LMM}$ group of Auger lines and the ${\mathrm{KL}}_{2,3}{L}_{2,3}$ Auger electrons, while for other kinetic energies the $\mathrm{LMM}$ Auger electrons exhibit isotropy. Because the $\mathrm{KLL}$ and $\mathrm{LMM}$ Auger energies are so different, we believe that the nonisotropic angular correlation observed is due to an alignment effect rather than a dynamical postcollision interaction effect.

The fragmentation of core ionized CF4 probed by threshold-photoelectron-photoion-photoion coincidence (TPEPIPICO) spectroscopy

The ionization and fragmentation of carbon tetrafluoride excited at the C and F 1s thresholds are studied using soft x-ray excitation, threshold electron spectroscopy and state-selective coincidence techniques. The threshold electron spectra reveal indirect low-energy production from pre-edge resonances, and the structure at the thresholds is attributable to post-collision interaction effects, with some evidence of underlying vibrational structure. The coincidence data allow determination of ion and ion pair yield ratios and deduction of the dissociation pathways and final states for the various channels.

Photofragmentation of third-row hydrides following photoexcitation at deep-core levels

The relaxation dynamics of HCl, DCl, H2S, and D2S following photoexcitation in the vicinities of the Cl and S K-shell thresholds ( ;2.8 keV for Cl, ;2.5 keV for S! were studied by means of ion time-of-flight mass spectroscopy. In all cases, the onset of pre-edge core-shell photoionization precedes the formation on resonance of a significant amount of neutral hydrogen as well as postcollision-interaction effects above threshold. Examination of the width of the H 1 peak in spectra taken with the analyzer parallel and perpendicular to the polarization vector of the incident light indicates that on resonance, the photofragmentation asymmetry parameter, b, is approximately two for HCl, and is clearly positive for H2S. @S1050-2947~98!08211-0# PACS number~s!: 33.80.Gj, 33.80.Eh Time-of-flight mass spectroscopy and coincidence measurements of atoms and molecules are relatively wellunderstood techniques @1‐3#. Coupled with the high bright

Density of the generalized oscillator strength of atomic hydrogen: A semiclassical approach

An expression of the density of the generalized oscillator strength (GOS) for electron impact ionization of atomic hydrogen using a classical trajectory to describe the state of the ejected electron is presented with the help of its linear-response representation in the framework of the first Born approximation (FBA). Based on the reproduction of the GOS density of the FBA, the target polarization and the postcollision interaction (PCI) effect are included within the expression. The target polarization expressed by the decreasing of the initial expectation momentum of the atomic electron decreases the GOS density for small energy transfer E and moves the Bethe ridge toward higher momentum transfer K for larger $E.$ The PCI effect should not be remarkable for the GOS density.

Substrate Mediated Suppression of Postcollision Interaction Effects

The line shapes and kinetic energies of argon ${2p}_{3/2}$ and ${2p}_{1/2}$ photoelectrons from monolayers adsorbed on a Ru(001) substrate were measured with an electron time-of-flight detector in the near-threshold region. Compared with gas phase results, the two main effects due to postcollision interaction, i.e., line broadening and redshift of the kinetic energy of threshold electrons, are strongly suppressed on the surface. We show that these changes are brought about by an effective, screening induced reduction of the energy loss encountered by the threshold electron upon transition from a singly to a doubly charged central argon atom at core-hole decay.

Postcollision-interaction effects in HCl following photofragmentation near the chlorineKedge

Ion time-of-flight mass spectroscopy was used to study the relaxation dynamics of HCl following photoexcitation in the vicinity of the Cl $K$ threshold $(\ensuremath{\approx}2.8\mathrm{keV}).$ Detailed observations of molecular fragmentation mediated by postcollision interaction between a photoelectron and an Auger electron are presented, evidenced by the recapture of Cl $K$ photoelectrons by either ${\mathrm{Cl}}^{n+}$ or ${\mathrm{H}}^{+}$ dissociation fragments.

Aspects of postcollision interactions near the ArLshell

In the present work we are interested in near-threshold photoionization experiments involving postcollision effects related to the Auger decay of a vacancy in the Ar $L$ shell. In particular, we have measured the photoelectron energy spectrum resulting from the above postcollision interaction effects and have observed electrons produced by the process of electron capture and reemission.

Post-collision interaction effects in the autoionization of atoms excited by ion impact

When an autoionizing state decays, the spectral line shape of the emitted electron is affected by the post-collision interaction (PCI) with other collision partners. This is the case in slow ion-atom collisions, where the autoionization line profile displays a strong enhancement in the forward direction. The aim of the present work is to describe the PCI effects that occur in outer-shell autoionization by ion impact. Different theoretical descriptions and experimental results are reviewed.

A study of autoionization phenomena in helium using (e, 2e) spectroscopy

Results are presented for the triple differential (e, 2e) cross sections in the region encompassing the helium n = 2 autoionization resonances at the respective incident energies 94.6, 96.6 and 99.6 eV. The scattered-electron angle in each case is and the range of ejected-electron angles are between and . The measured coincidence ejected-electron spectra are analysed in terms of the Shore - Balashov parametrization to obtain the direct (e, 2e) cross section and the resonance parameters and for the and resonances as a function of ejected-electron momentum. These derived parameters are compared against the results of a calculation where configuration-interaction expansions for the resonances and helium ground state, which employed hydrogenic and multiconfiguration Hartree - Fock orbitals, respectively, were used. Here the distorted-wave Born approximation was employed for the (e, 2e) cross section calculation. The calculated parameters agree quite well with the experimental results indicating that the theoretical description of the reaction mechanism is physically realistic. Further, both experiment and theory show strong correlations between the resonance amplitudes and the direct ionization amplitudes. Finally, we report and discuss our results of an experimental investigation into the post-collision interaction effects for the present kinematical conditions.

Interference and PCI in argon Auger (e, 2e) spectra

Angle-dependent interference is observed in electron-impact ionization (e, 2e) coincidence experiments on the argon and Auger transitions. Recapture of the slow ejected electron from the Auger process leads to interference with the satellite-state ionization processes. The post-collision interaction (PCI) and interference effects are quantified for a large range of excess energies.

Single-photon multiple ionization of neon in the K-edge region

The ion charge state yields resulting from the photoionization of neon gas have been measured with time-of-flight ion charge state spectroscopy for excitation energies in the K-edge region. The decay of the [1s]3p resonance excited state results in the formation ion charge states ${\mathrm{Ne}}^{+}$,${\mathrm{nNe}}^{2+}$,${\mathrm{nNe}}^{3+}$, and ${\mathrm{Ne}}^{4+}$, with a branching ratio of 0.65:0.32:0.030:0.002, whereas the final charge state for the decay of higher-order [1s]np(n\ensuremath{\geqslant}4) and [1s]\ensuremath{\varepsilon}p continuum excitations is mostly ${\mathrm{Ne}}^{2+}$. We have measured q-bar, the mean charge from photoionization, for excited states near the K edge as a function of photon energy. Postcollision interaction effects are shown to be quite small.

Observation of electron impact inner-shell excitation of argon by means of satellite Auger spectra

We have studied the inner-shell excited states of argon below the 2p thresholds for electron-impact excitation in the range of the incident energy . We carried this out through the measurement of the electrons emitted during the decay of the states. The corresponding satellite peaks in the spectra of the emitted electrons were separated from the normal (diagram) LMM Auger peaks by a special evaluating process. We observed the satellite peaks coming from the , and transitions at all applied incident energies. The intensity of the satellite peaks increased approaching the ionization threshold (). Nevertheless in the range - 1700 eV we could identify more satellites due to the lower continuous background and the smaller post-collision interaction effects.