Photoelectron Recapture Research Articles

Conjugate photoelectron recapture peaks in the high resolution Auger electron spectra following near-threshold Ar 2p photoionization

We have observed formation of Rydberg series superimposed on the Ar (L3M2,3M2,3) Auger electron spectral profile with photoexcitation just above the 2p−13/2 photoionization threshold. This is due to the recapture of photoelectrons induced by post-collision interaction between the Auger electron and photoelectron: Ar+ 2p−13/2 + e− → Ar+ 3p−2ml + e−. The intensity distribution of the angular momentum state resolved Rydberg peaks revealed prominent conjugate peaks due to angular momentum exchange between photoelectrons and Auger electrons in the process. In addition, we measured the 2p−13/2nl → 3p−2ml + e− resonant Auger spectra with high resolution below the 2p−13/2 threshold. Our results showed that conjugate shake-up/down 2p−13/2nd → 3p−2mp + e− processes are stronger than the direct 2p−13/2nd → 3p−2ms, (m−1)d + e− processes in the spectator Auger processes, implying a strong angular momentum exchange effect between photoelectron and Auger electron.

Post-collision interaction in sequential double-ionization of helium

SynopsisWe investigate the post-collision interaction between a slow electron released by a pump pulse from the neutral helium atom and a fast electron released by a time-delayed probe pulse from the created ion. To this end, we numerically solve the time-dependent Schrödinger equation in the basis of the essential eigenstates of the unperturbed Hamiltonian. We demonstrate that the energy exchange between the slow and the fast electrons and the recapture of slow photoelectrons can be controlled by the properties of the pulse-sequence.

Dominance of angular momentum exchange in the PCI recapture of photoelectrons revealed by high resolution Auger electron measurements of Kr

An unexpectedly prominent angular momentum exchange effect in the double photoionization continuum was identified through the photoelectron recapture process induced by post-collision interaction (PCI). The Kr (M5N2,3N2,3) Auger electron spectrum originating from 3d−15/2 photoionization was measured with the photon energy tuned very close above the ionization threshold. As the photon energy approached the 3d−15/2 photoionization threshold, Rydberg series structures including several angular momentum components were formed within the Auger peak profile by the recapture of the photoelectrons into high-lying final ion orbitals. Our spectrum with line-width much narrower than the lifetime width of the corresponding core excited state allowed us to resolve detailed structures due to the orbital angular momentum states very clearly. Unexpectedly, conjugate peaks originating from the exchange of angular momentum between the photoelectron and the Auger electron through PCI were found to dominate the spectrum. Comparison with valence photoelectron spectrum of Kr via direct photoionization confirms the new assignment. Conjugate peaks were also found to be in accord with the quantum defect values obtained for the high Rydberg series Kr+4p−2(1S0, 1D2)ml. Extension of the new assignments to photon energies below threshold may call for the revision of previous interpretations of resonance Auger processes.

Controlling Dynamics of Postcollision Interaction.

A general scheme to get insight and to control postcollision interaction (PCI) by means of sequential double ionization with two high-frequency pulses is discussed. In particular, we propose to consider PCI of a slow photoelectron released by the pump pulse from a neutral atom with a fast photoelectron released by the time-delayed probe pulse from the created ion. This scheme is exemplified by the abinitio calculations performed for the prototypical helium atom. In order to visualize PCI effects in real time and real space, the corresponding time-dependent Schrödinger equation is solved by propagating two-electron wave packets in terms of essential stationary eigenstates of the unperturbed Hamiltonian. It is demonstrated that the exchange of energy between the slow and fast photoelectron wave packets in continuum, as well as the recapture of threshold photoelectrons owing to the PCI, can be controlled by the properties of the ionizing pulses and the time delay between them.

Photoelectron recapture and reemission process associated with double Auger decay in Ar

Multielectron coincidence spectroscopy has been performed for Ar at a photon energy of only 0.2 eV above the $2{p}_{1/2}$ threshold. It is revealed that a postcollision interaction induced by double Auger decay leads to photoelectron recapture, followed by reemission of the captured electron, where the recapture of the slow photoelectron forms the $\mathrm{A}{\mathrm{r}}^{2+}$ Rydberg-excited states which subsequently undergo autoionization. The energy correlation of the emitted electrons discloses that both direct and cascade paths in the double Auger decay contribute to the photoelectron recapture.

PCI effects and the gradual formation of Rydberg series due to photoelectron recapture, in the Auger satellite lines upon Xe 4d−15/2 photoionization

The Xe (N5O2,3O2,3) Auger electron spectra originating from 4d−15/2 inner-shell photoionization were measured, with photon energy tuned close to the ionization threshold. As the photon energy approaches the threshold from above the 4d−15/2 photoionization threshold, Rydberg series structures are formed within the Auger electron peak by the recapture of the photoelectron into high-lying ion orbitals. They emerge in the tail on the higher energy side of the post–collision interaction (PCI) profile of the Auger electron. Discrete Rydberg peaks replace the continuous PCI tail and gradually form a series with intensity distribution emulating the intensity profile of the continuous tail. Structures due to the Xe+5p4(1S0, 1D2, 3P2,1,0) ml series were observed and assigned.

Metastable fragment production at the C 1s and O 1s edges of the CO2 molecule

We have studied the dissociation of the CO2 molecule into neutral fragments by observing coincidences between neutral metastable fragments and photoions at photon energies between 250 eV and 600 eV. We attribute the metastable signal to oxygen atoms in the 2s22p3(4S)3s(5S) state and to fragments in high Rydberg states. Neutral metastable fragments are produced by the dissociation of valence-excited states of that are populated by direct photoionization or, more efficiently, by core excitation and ionization followed by Auger decay. Metastable fragments were detected at all C 1s and O 1s single excitations, and their intensity was enhanced particularly just above the O 1s and C 1s ionization thresholds, where the recapture of slow photoelectrons is possible. They were also abundantly created at core-valence double excitations, at the thresholds of O 1s and C 1s in shake-up satellites, and to a lesser extent, also in regions of the photoionization continua where there are no resonances.

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.

Electron reemission processes following photoelectron recapture due to post-collision interaction in inner-shell photoionization of water molecules

Electron reemission following photoelectron recapture due to post-collision interaction has been studied at 0.7 eV the inner-shell photoionization threshold of water molecules, using a multi-electron coincidence method. Electron reemissions after single Auger decay occur from O and OH fragments which are produced by the dissociations of high-n Rydberg H2O(+) states populated through photoelectron recapture. In addition, electron reemissions after double Auger decay are identified in triple coincidence events, where autoionization lines from O and O(+) fragments are observed.

Enhanced production of anionic and excited neutral fragments of gaseous HCCl3 near the Cl 2p1/2,3/2 ionization threshold

Following photoexcitation of Cl 2p electrons to various resonances, the fragmentation dynamics of anionic and excited neutral fragments of gaseous HCCl3 were investigated on combining measurements of photon-induced anionic dissociation, X-ray absorption and UV/visible dispersed fluorescence. The Cl 2p core-to-Rydberg excitations near the ionization threshold produce a notable enhancement of excited neutral fragments (C* and CH*), which is explained in terms of the contribution from a shake-modified resonant Auger decay and post-collision interaction. The anion yields significantly reinforced just above Cl 2p ionization threshold of gaseous HCCl3 are predominantly mediated by photoelectron recapture through post-collision interaction. The present work demonstrates that the detection of excited neutral fragments and anions offer specific insight into the fragmentation dynamics of high-lying molecular-ion states of core-excited molecules.

Core-level positive-ion and negative-ion fragmentation of gaseous and condensed HCCl3 using synchrotron radiation

We investigated the dissociation dynamics of positive-ion and negative-ion fragments of gaseous and condensed HCCl(3) following photoexcitation of Cl 2p electrons to various resonances. Based on ab initio calculations at levels HF/cc-pVTZ and QCISD/6-311G∗, the first doublet structures in Cl L-edge x-ray absorption spectrum of HCCl(3) are assigned to transitions from the Cl (2P(3/2),(1/2)) initial states to the 10a(1)(∗) orbitals. The Cl 2p → 10a(1)(∗) excitation of HCCl(3) induces a significant enhancement of the Cl(+) desorption yield in the condensed phase and a small increase in the HCCl(+) yield in the gaseous phase. Based on the resonant photoemission of condensed HCCl(3), excitations of Cl 2p electrons to valence orbitals decay predominantly via spectator Auger transitions. The kinetic energy distributions of Cl(+) ion via the Cl 2p → 10a(1)(∗) excitation are shifted to higher energy ∼0.2 eV and ∼0.1 eV relative to those via the Cl 2p → 10e(∗) excitation and Cl 2p → shape resonance excitation, respectively. The enhancement of the yields of ionic fragments at specific core-excited resonance states is assisted by a strongly repulsive surface that is directly related to the spectator electrons localized in the antibonding orbitals. The Cl(-) anion is significantly reinforced in the vicinity of Cl 2p ionization threshold of gaseous HCCl(3), mediated by photoelectron recapture through post-collision interaction.

Core-level anionic photofragmentation of gaseousCCl4and solid-state analogs

The dissociation dynamics of anionic and excited neutral fragments of gaseous ${\text{CCl}}_{4}$ and ${\text{CCl}}_{4}$ adsorbed on Si(100) $\ensuremath{\sim}90\text{ }\text{K}$ following $\text{Cl}\text{ }2p$ core-level excitations were investigated on combining measurements of photon-induced anionic dissociation, x-ray absorption, and uv-visible dispersed fluorescence. The transitions of core electrons to high Rydberg states or doubly excited states near $\text{Cl}\text{ }2p$ ionization thresholds of gaseous ${\text{CCl}}_{4}$ remarkably enhance the production of excited neutral fragments (${\text{C}}^{\ensuremath{\ast}}$ and ${\text{CCl}}^{\ensuremath{\ast}}$); this enhancement is attributed to the contribution from the shake-modified resonant Auger decay and/or postcollision interaction (PCI). The ${\text{Cl}}^{\ensuremath{-}}$ anion is significantly reinforced in the vicinity of the $\text{Cl}\text{ }2{p}_{1/2,3/2}$ ionization threshold of gaseous ${\text{CCl}}_{4}$, originating from PCI-mediated photoelectron recapture. The $\text{Cl}\text{ }2p\ensuremath{\rightarrow}7{a}_{1}^{\ensuremath{\ast}}$ excitation for ${\text{CCl}}_{4}/\text{Si}(100)$ at $\ensuremath{\sim}90\text{ }\text{K}$ enhances the ${\text{Cl}}^{\ensuremath{-}}$ desorption yield at a submonolayer level. This resonant enhancement of ${\text{Cl}}^{\ensuremath{-}}$ yield at the $7{a}_{1}^{\ensuremath{\ast}}$ resonance in the $\text{Cl}\text{ }2p$ edge at a submonolayer level occurs through the formation of high-lying molecular-ion states of ${\text{CCl}}_{4}$ adsorbed on a Si surface.

Neighbor-induced photoelectron recapture in argon clusters: A photon-energy-dependent study of Auger spectra

A photon-energy-dependent experimental study of the L2,3M2,3M2,3 Auger transitions in argon clusters is presented, and features without atomic counterparts are observed in the spectra up to photon ...

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.

Photoelectron recapture through post-collision interaction in N 2

Photoelectron spectroscopy has been applied to investigate electron emissions from excited atomic fragments in the vicinity of the K-shell ionization threshold of N 2. It is found that low kinetic energy electrons are emitted in the photon energy region both immediately below and above the K-shell ionization threshold. The electron emissions are attributable to autoionization from the excited fragments in low- n Rydberg states. The N 2 +* states leading to the formation of the excited fragments which autoionize with emitting slow electrons, are populated not only by spectator Auger decay from the core-excited states, but also by the recapture of slow photoelectrons into the Rydberg orbitals.

Investigation of photoelectron recapture in Ar using two-dimensional photoelectron spectroscopy

``Complete'' two-dimensional photoelectron spectra of Ar in the vicinity of the $2p$ ionization thresholds have been measured allowing several features in the spectra to be explained. Nine reemission series are observed and their initial and final states are determined based on the kinetic energies of these electrons. The photoelectron recapture yield above the ${L}_{2}$ threshold has been studied by measuring directly the reemitted electrons as a function of the photon energy. Our experimental results are found to be in reasonable agreement with our semiclassical calculations which in turn are in good agreement with the quantum-mechanical calculation of Tulkki et al. [Phys. Rev. A 41, 181 (1990)].

Efficient production of metastable fragments around the 1s ionization threshold in N2

Metastable formation has been investigated in the inner-shell region of N2. Enhanced N* formation has been observed around the 1s threshold, and is shown to result from dissociation of high N2+* Rydberg states. These N2+* states are populated by spectator Auger decay from core-excited states, as well as by the recapture of slow photoelectrons into the Rydberg orbitals. The present measurement demonstrates that the metastable observation is a new and sensitive tool to study the decay dynamics of core-excited states and the photoelectron recapture due to post-collision interaction.

A study of photoelectron recapture due to post-collision interaction in Ne at the 1s photoionization threshold

Electron spectra from radiationless decay of a Ne 1s vacancy are measured with excitation energies immediately at the 1s ionization threshold. Special attention is given to the excess energy region where Γ is the lifetime width of the 1s vacancy. Spectra are dominated by recapture of photoelectrons into Rydberg states of Ne+ above the threshold, and by a smooth transition into the spectator resonant Auger spectrum below the threshold. High resolution of the present experiment allowed us to clearly observe Rydberg states up to n = 8 and to determine the individual probabilities for populating of these states by photoelectron recapture. Results are compared with calculations based on the semiclassical theory of post-collision interaction extended to high Rydberg excitations of the primary photoexcited electron, as well as with a non-stationary quantum mechanical theory of near-threshold photoionization with subsequent Auger decay.

Investigation of valence inter-multiplet Auger transitions in Ne following 1s photoelectron recapture

We employ a novel technique in which highly excited Rydberg states of Ne+ 2p4np are populated via PCI-induced recapture of the near-threshold 1s photoelectron (De Fanis et al 2004 Phys. Rev. A 70 040702) to investigate valence inter-multiplet Auger transitions. The following series of the transitions have been observed: Ne+ 2p4(1D)np 2L → Ne2+ 2p4 3PJ, Ne+ 2p4(1S)np 2P →Ne2+ 2p4 3PJ and Ne+ 2p4(1S)np 2P →Ne2+ 2p4 1D. Their energy positions, quantum defects and the anisotropy parameters of the Auger electron emission have been determined. Experimental results are in good agreement with multi-configuration Dirac–Fock calculations carried out as a part of this study. The importance of interference effects for decays via naturally overlapping fine-structure components of the intermediate state is discussed.

Electron correlation during photoionization and relaxation of potassium and argon afterK-shell photoexcitation

We have measured the charge-state distributions of argon and potassium after ionization by photons with energies near the K-shell ionization threshold. Despite the similarity in core electron configurations, the two atoms show remarkable differences in the resulting distribution of ion charge states. The valence electron in potassium is rarely a spectator during core relaxation, and its presence enhances the loss of electrons excited into Rydberg levels or strongly reduces the recapture of slow photoelectrons during postcollision interaction.