Separation of Two-Electron Photoexcited Atomic Processes near the Inner-Shell Threshold

Abstract

By means of a high resolution resonant inelastic x-ray scattering spectroscopy, we have for the first time separated spectral features pertaining to different two-electron atomic processes in the vicinity of an innershell threshold. Contributions of shakeoff, shakeup, and resonant 1s3p double excitations were extracted from the Ar KM-M 2;3 M x-ray satellite line intensity measured as a function of photon energy from ½1s3p double excitation threshold to saturation. The isolated ½1s3pnln 0 l 0 excitation spectrum is critically compared to the outcome of the multiconfiguration Dirac-Fock model with relaxation.According to the nature of photon-electron interaction a photon is absorbed only by a single electron. On the other hand, photoionization of an inner-shell electron is sometimes accompanied by removal of another electron (shakeoff) or its excitation into a higher empty atomic level (shakeup), and below the double photoionization threshold, doubly excited atomic states can be created. These multielectron excitations are straightforward manifestations of the breakdown of the independent electron picture as they appear due to the electron-electron interactions. Despite the persistent and intensive research the mechanisms of electron-electron correlations have not been fully elucidated yet. While the well-known shake mechanism (governed by monopole transitions between relaxed states) is commonly taken as a synonym for the double photoionization or excitation process, the presence of dynamical effects in the near-threshold region makes the interpretation of double photoexcitation spectra much more complicated.The main experimental problem hindering the study of near-threshold multielectron excitations is the overlap of the shakeoff, shakeup, and double excitation spectral contributions. Strong dependence of the shakeoff to shakeup probability ratio on the atomic number and atomic shell was employed to bypass this difficulty in some special cases. While the shakeup mechanism fully dominates the electron excitation following 1s photoionization of Be [1], the shakeoff prevails in the 1s2p double photoionization of mid-Z atoms [2]. It was found that a time-dependent perturbation theory [3] describes well the double photoionization of outer-shell electrons in low-Z atoms; for inner shells and higher Z atoms significant deviations were found [4,5], indicating that other mechanisms besides the shakeoff are at work. In the latest work a semiclassical knockout effect was suggested to dominate the near-threshold double 1s photoionization in 3d transition metals [6].One of the most interesting multielectron features is double excitations exhibiting discrete resonances in the preedge region. These processes are extremely weak when both electrons are excited from core levels, but they become more probable if a valence electron takes part in the excitation. In such cases structural solid-state effects obscure spectral details related to multiple atomic excitations, limiting such studies to gaseous atomic targets. This is the mai...

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