T. Aoto

Appearance of interatomic Coulombic decay in Ar, Kr, and Xe homonuclear dimers

Interatomic Coulombic decay (ICD) is observed in the rare gas homonuclear dimers Ar2, Kr2, and Xe2 with photoion spectroscopy techniques. Inner valence ionization of the outer ns shell of these systems is known to create a metastable state that dissociates to form a ground state ion and a neutral excited fragment. Inner valence ionization to form ns satellite states leads to similar dissociations, but the neutral fragment gets all the more excited as the internal energy of the ns satellite state increases. When enough excitation energy is transferred to reach the ionization potential, ICD occurs. ICD threshold is observed to coincide with the position of the A+A+ ground state in the Franck-Condon region.

Photoemission of threshold electrons in the vicinity of the xenon 4d hole: dynamics of Auger decay

Threshold electrons observed in the vicinity of the Xe 4d inner shell hole are shown to have three possible origins: double photoionization of valence electrons, release of 4d photoelectrons or Auger processes with the emission of two electrons. Coincidence spectra were taken between these threshold electrons and the associated fast ones. On the 4d→np resonances, they show that ejection of two Auger electrons is due to a two-step process and that simultaneous removal of two electrons is limited at most to the formation of excited Xe2+* states. Above the 4d threshold, coincidence lineshapes are distorted by post-collision interaction and reflect the decay dynamics of the 4d hole. An analysis within the eikonal approach reveals that the dominant process is decay of Xe+ (4d-1) to Xe3+ through cascade emission of a threshold Auger electron followed by a fast Auger electron. A more refined analysis shows that the intermediate Xe2+* states must be short-lived (widths of more than 400 meV) and that double Auger decay i.e. simultaneous ejection of two electrons, can contribute as well.

Electron correlation in Xe 4d Auger decay studied by slow photoelectron-Auger electron coincidence spectroscopy

Two different experimental methods, namely threshold electron–Auger electron coincidences and slow photoelectron–Auger electron coincidences are applied to investigate the Xe 4d Auger decay in the near-threshold region and reveal the essential role of electron correlation. The coincidences allow us to select the different channels for the 4d hole Auger decay which lead to different final states of the Xe2+ ion: 5s−2(1S0), 5s−15p−1(1P1), 5p−2(1S0), 5p−2(1D2), 5p−2(3P0,1) and 5p−2(3P2). Measurements of the threshold electrons with the first method reveal strong PCI distortion of electron spectra in all channels. Comparison with calculations carried out in the framework of the quantum-mechanical PCI model allows us to clarify the dynamics of threshold electron production. In the 5p−2(1S0) channel, the main contribution comes from the PCI retardation of slow photoelectrons. In the 5p−2(1D2) and 5p−2(3P) final state channels, additional processes of PCI recapture followed by valence multiplet decays play a role at and below the N4 and N5 thresholds. The slow photoelectron spectra measured by the second method reveal also a strong PCI distortion. Analysis within the framework of the eikonal model shows the influence of the Auger electron on the PCI distorted line shapes.

Auger decay of Ne 1s photoionization satellites studied by a multi-electron coincidence method

The Auger decay of Ne 1s photoionization satellite states is studied with a magnetic-bottle multi-electron coincidence method. The energy correlations among the multi-electrons associated with the double Auger decay from the Ne+ satellite states are extracted from the accumulated coincidence dataset. It is concluded that the Ne2+ states populated through the initial Auger decays from the Ne+ satellite states can be excited states that lie above the Ne3+ threshold. Their subsequent decay produces slow electrons in the 0–25 eV kinetic energy range. Possible assignments of the intermediate Ne2+ and the final Ne3+ states are given.

Single, double, and triple Auger decay of the Xe 4p core-hole states

Auger decay of Xe{sup +} states arising from 4p ionization has been studied with a very efficient multielectron coincidence method. Coster-Kronig decay from Xe{sup +} 4p{sup -1} and the subsequent decay into Xe{sup 3+} states with three valence holes are identified. Formation of Xe{sup 4+} is also observed as quadruple coincidences between a 4p photoelectron and three Auger electrons. The relative probabilities of individual multi-ionization processes are determined from the coincidence yields.

Multiple direct and sequential Auger effect in the rare gases

The use of a magnetic bottle spectrometer with synchrotron radiation allows multi dimensional electron spectroscopy to be performed by detecting in coincidence all electrons (2, 3, 4) ejected in multiple ionization events. Multiple Auger effect following inner‐shell ionization can be investigated in this way. Application of the technique to rare gases (Xe 4d and Kr 3d) double Auger decay reveals all the energy pathways involved. The dominant decay path proceeds by Auger cascade through autoionizing states of the doubly charged ion. Processes where 3 electrons are involved are also observed as direct double Auger and as involving precursor Rydberg series.