Double Auger Processes Research Articles

Triple ionization of HCl via states with a 2p core hole.

The triple ionization of HCl by double Auger decay and related processes has been studied using a multiparticle coincidence technique combined with synchrotron radiation. Four contributing processes are identified; direct double Auger, two indirect double Auger decay pathways, and single Auger decay from core-valence doubly ionized intermediate states. One indirect Auger process involves autoionization from superexcited states of Cl+. Double Auger decay from HCl+ (2p-1, 2PJ), which makes up 11% ± 2% of total Auger decay, is estimated to be 40% direct, 15% indirect via atomic Cl+* and 45% indirect via molecular intermediate doubly ionized states. The vertical triple ionization energy of HCl is determined as 73.8 ± 0.5 eV. Molecular field effects are found to affect the direct double Auger process as well as normal single Auger decay. A comparison between spectra of the HCl and DCl isotopomers indicates that electronic decay is faster in all the processes than molecular dissociation.

Multiple Auger decay probabilities of neon from the 1s -core-hole state

The multiple Auger decays of the $\mathrm{Ne}\phantom{\rule{0.28em}{0ex}}1{s}^{\ensuremath{-}1}$ state including double and triple Auger processes are investigated within the framework of perturbation theory. The contributions of the cascade and direct processes are determined for the double Auger decay. In the cascade processes, the choice of the orbital sets and configuration interaction can strongly affect the partial probabilities for the specific configurations of $\mathrm{N}{\mathrm{e}}^{3+}$. The multistep approaches, i.e., the knockout and shakeoff mechanisms, are implemented to deal with the direct double Auger processes for which the total and partial probabilities corresponding to specific configurations of $\mathrm{N}{\mathrm{e}}^{3+}$ are calculated and reveal that the knockout is dominant. Finally, the probabilities of the triple Auger decays that are decomposed into a double Auger process and a subsequent emission of a single electron are obtained using the cascade and knockout mechanisms. The calculated probabilities agree reasonably well with the available experimental data.

Single, double, and triple Auger decays from 1s shake-up states of the oxygen molecule.

The single, double, and triple Auger decays from the 1s shake-up states of O2 have been studied using a multi-electron coincidence method. Efficient populations of two-hole final states are observed in single Auger decays of the π-π* shake-up states, which is understood as a characteristic property of the Auger transitions from shake-up states of an open-shell molecule. The O23+ populations formed by double Auger decays show similar profiles for both the O1s-1 and shake-up states, which is due to the contributions from cascade double Auger processes. While the cascade contributions to the double Auger decays increase with the initial shake-up energy, the probability of direct double Auger processes remains unchanged between the O1s-1 and shake-up states, which implies a weak influence of the excited electron on the double Auger emission that originates from the electron correlation effect.

Auger decay paths of mercury5pand4fvacancies revealed by multielectron spectroscopy

Single and double Auger processes following ionization of $4f$ and $5p$ inner shells have been studied using multielectron coincidence spectroscopy. Coincidence technique enables us to resolve state by state all single and double Auger paths with a resolution better than the lifetime broadening. Drastic step-to-step decay lifetime changes are observed and reported as Coster-Kronig transition takes place either in the first $(5p)$ or in the second $(4f)$ step of the Auger cascade. Relativistic ab initio theory has been used to predict and interpret the experimental observations.

Complete Auger decay pathways of Kr 3d−1 hole levels including direct double processes

The detailed level-to-level single and double Auger decay rates of Kr 3d−1 hole states are investigated in the framework of the first and second perturbation theory implemented by distorted wave approximation with the balanced large-scale configuration interaction of the successive ions being taken into account. The branching ratios of cascade and direct double Auger decay to the total probability are predicted to be 16.5% and 16.1%, respectively, for Kr and 16.8% and 17.2% for Kr , resulting in total double branching ratios of 32.6% and 34.0% for the two levels of Kr 3d−1 hole. A comparison is made with the available experimental results on the branching ratio into triply charged ions and good agreement was found with the most recent published work. This work represents the first theoretical study to correctly explain the experimental measurements. The complete pathways were depicted by including the direct double Auger decay process. Compared with the Auger decay of Ar 2p−1, the fraction of the cascade double of Kr 3d−1 is dramatically enhanced yet that of direct double Auger processes is only slightly increased.

Single and multiple photoionisation of H2S by 40–250 eV photons

Multi-electron coincidence measurements on photoionisation of H(2)S have been carried out at photon energies from 40 to 250 eV. They quantify molecular field effects on the Auger process in detail and are in good agreement with the existing theory. Spectra of core-valence double ionisation of H(2)S are presented and partially analysed. Auger decays from the core-valence states produce triply charged product spectra with unexplained and surprising intensity distributions. Triple ionisation by the double Auger process from 2p hole states shows little effect of the molecular field splitting, but includes a substantial contribution from cascade processes, some involving dissociation in intermediate states. The onset of triple ionisation at the molecular geometry is determined as 61 ± 0.5 eV.

PCI effects in argon 2p double Auger decay probed by multielectron coincidence methods

Electron correlations in three electron emission associated with inner shell photoionization are investigated for the first time through a combined theoretical and experimental approach. Namely, different kinds of post-collisional interactions (PCI) occurring in the decay of argon 2p holes by emission of two Auger electrons are isolated experimentally with a powerful coincidence technique and are modelled with an eikonal approach. It is shown that PCI distortion of the photoelectron line shape is stronger than in the single Auger paths and depends critically on the process: in direct double Auger decays where two Auger electrons are emitted simultaneously, direct interaction of the 2p photoelectron with the two Auger electrons is demonstrated; in cascade double Auger processes, the PCI distortion depends only slightly on the last electron emitted in the cascade. This perturbation depends on and reveals the lifetime of the intermediate Ar2 + state.

Multi-coincidence in cascade Auger decay processes

Two-electron emission following inner shell ionization of rare-gas atoms by synchrotron radiation has been investigated experimentally with a magnetic bottle electron time of flight spectrometer. The detection in coincidence of a photoelectron with all Auger electrons allows a complete understanding of inner shell hole decay. This method has been applied to the study of single and double Auger decays following Xe 4d, Ar 2p and Kr 3d inner shell ionization. The double Auger decay scheme reveals direct double Auger process but is dominated by cascade processes involving inner-valence electrons in a first step followed by autoionization. Post collision interaction (PCI) associated to distorted Auger spectra and double photoionization paths in the vicinity of Xe 4d threshold, is also discussed.

Mechanisms of Spontaneous Two-Electron Emission from Core-Excited States of Molecular CO

We demonstrate that the observation of slow electrons emitted in the decay of molecular core-excited states can be a sensitive probe of the double Auger processes, and that in combination with electron-electron coincidence spectroscopy, it can provide clear insight into the mechanisms involved. The present study identifies all cascade Auger paths from the C1s-to-Rydberg states in CO to final states of CO2+. One pathway includes the first directly identified case of molecular level-to-level autoionization of a cation and shows remarkable selectivity for a specific final state.

Cascade decays and final charge-state distribution of hollow argon ions

On the basis of a previous model of radiative-Auger-cascade for calculation of final-charge-state distributions of hollow ions, a new model of radiative-Auger-double Auger-cascade has been constructed by including double Auger process, and was applied to study the final-charge-state distribution in the production of Ar+(1s-1) (argon ion with an initial 1s hole) and Ar+(2s-1) (argon ion with an initial 2s hole). Comparing with the previous calculations and experiments, the present results show excellent agreement.

Auger cascades versus direct double Auger: relaxation processes following photoionization of the Kr 3d and Xe 4d, 3d inner shells

The relaxation processes after non-resonant inner-shell photoionization are studied experimentally using electron–electron time-of-flight coincidence spectroscopy. Results for krypton 3d and xenon 4d as well as 3d photoionization are presented. The experimental data make it possible to disentangle sequential from simultaneous processes using the different electron emission characteristics as the differentiating property. For the population of final states having charges higher than 2, the measurements show a strong preference for sequential Auger cascade decay. Clear evidence for direct double Auger processes is found in the case of Xe 3d photoionization only.

Electron–electron coincidence study of double Auger processes in atoms

The double Auger process after Ar 2p and Ne 1s inner-shell photoionization is investigated by means of angle resolved time-of-flight electron–electron coincidence spectroscopy. This method allows to disentangle direct double Auger from cascade Auger processes. Information about the energy sharing as well as the angular correlation of the two emitted electrons is obtained. Circular dichroism in the double Auger emission produced by circularly polarized light is discussed.

Angular correlation patterns in double Auger decay

The angular distribution of two emitted electrons in the double Auger process in atoms has been theoretically investigated. In particular, the double Auger decay of a 1s vacancy in Ne and a 2p vacancy in Ar is considered. In analogy with double photoionization, we present a convenient parametrization of the angular correlation patterns for all possible angular momenta of the emitted electron pair. As a limiting case we investigate the angular correlations within the shake-off mechanism of the double Auger process. We also consider the influence of initial orientation of the vacancy on the electron angular distributions (circular dichroism).

Probabilities of double Auger processes upon the decay of the K-vacancy in neon

Probabilities of double Auger processes upon the decay of the K-vacancy in the neon atom are calculated. The final-state correlations of core electrons and correlations of core electrons with Auger electrons are found to be the principal contributors to the double Auger processes. Calculated total probability of double Auger processes upon the decay of the K-vacancy in Ne is 5.39% against experimental 5.97%.

Energy and angular distributions of electrons emitted by direct double auger decay.

We have observed the direct L(2,3)MMM double Auger transition after photoionization of the 2p shell of argon by angle-resolved electron-electron coincidence spectroscopy. The process is responsible for about 20% of the observed Auger electron intensity. In contrast to the normal Auger lines, the spectra in double Auger decay show a continuous intensity distribution. The energy and angular distributions of the emitted electrons allow one to obtain information on the electron correlations giving rise to the double Auger process as well as the symmetry of the associated two-electron continuum state.

Decay of theAr2s−1and2p−1andKr3p−1and3d−1hole states studied by photoelectron-ion coincidence spectroscopy

The decay channels of the $\mathrm{Ar}{2s}^{\ensuremath{-}1}$ and ${2p}^{\ensuremath{-}1}$ and $\mathrm{Kr}{3p}^{\ensuremath{-}1}$ and ${3d}^{\ensuremath{-}1}$ electronic hole states have been investigated by means of photoelectron-photoion coincidence measurements following innershell ionization using synchrotron radiation. With the method of final ion-charge resolving electron spectroscopy it has become possible to disentangle different contributions to the electron spectrum and to determine the decay probabilities ${P(nl}^{\ensuremath{-}1}\ensuremath{\rightarrow}n+)$ of the above-mentioned hole states ${(nl}^{\ensuremath{-}1})$ to the final ionic charge states $n+.$ A high correlation with threefold or even fourfold charged ions has been found in all cases. Possible decay routes, via cascade or direct double Auger processes, are discussed on the basis of energy-level schemes calculated with the Hartree-Fock method. Special emphasis is laid on the examination of the $\mathrm{Kr}{3p}^{\ensuremath{-}1}$ decay process, where the two fine-structure components $(j=1/2,3/2)$ exhibit noticeably different decay probabilities to ${\mathrm{Kr}}^{3+}$ and ${\mathrm{Kr}}^{4+}$ final ionic charge states.

Simultaneous determination of radiative and nonradiative decay channels in the neonKshell

The photoelectron-photoion coincidence method is shown to be very successful for the quantitative investigation of the ratio of radiative to nonradiative transitions and for the ratio of single to double Auger transitions. These decay probabilities are important parameters for analytical methods as well as for theoretical descriptions. The method is demonstrated for the neon $1s$ decay. Especially the fluorescence yield and the yield for the double Auger process have been determined both experimentally and theoretically.

Multi-charged rare gas ions measured with a coincidence technique

Multi-charged ions of rare gas toms have been measured in coincidence with a photoelectron from a core orbital using a time-of-flight mass spectrometer combined with an electron energy analyzer of the spherical mirror type. Probability ratios obtained for double Auger processes of Ar 2p, Kr 3d and Xe 4d hole states have confirmed that those estimated from previously observed ion yield spectra around core-ionization thresholds are correct. These ratios for spin split states in Ar 2p and Kr 3d holes were found to be almost the same, although those in Xe 4d hole states are different.

Double Auger processes in rare gas atoms

Combination of a spherical mirror electron energy analyzer and a time-of-flight ion spectrometer has efficiently enabled coincident measurements between energy-selected electrons and ions. Double Auger probabilities for rare gas atoms with a core hole were estimated through measurements of multi-charged ion yields coincident with a photoelectron from the core orbital. The double Auger probability for Xe with the 4d5/2 hole is lower than that for Xe with the 4d3/2 hole.

Multiple photoionization of Ne in the K-shell ionization region

Multiple photoionization of Ne in the K-shell ionization region (810-1340eV) has been studied using a time-of-flight mass spectrometer. The singly charged ion Ne+ appears only at the 1s → 3p excitation, except for production through direct L-shell ionization. The doubly charged ion Ne2+ shows the most intense yield above the 1s → 3p excitation, and K-shell excited Rydberg states mainly turn into Ne2+ through Auger shake processes. The partial photoionization cross sections of Ne3+ and Ne4+ have maxima at about 130eV and 230eV above the 1s ionization threshold, respectively. These maxima are attributed to shake-off processes at the initial photoabsorption step. Double Auger processes following the 1s photoionization are estimated to happen at about 6% relative to normal Auger processes.