Fano-ADC(2,2) method for electronic decay rates.

Fano-ADC is a family of ab initio methods for the prediction of electronic decay widths in excited, singly and doubly ionized systems. It has been particularly successful in elucidating the geometry dependence of the inter-atomic decay widths in clusters and facilitated the prediction of new electronic decay phenomena. However, the available Fano-ADC schemes are limited to the second-order treatment of the initial state and the first-order treatment of the final states of the decay. This confines the applicability of the Fano-ADC approach to first-order decay processes, e.g., normal but not double Auger decay (DAD), and compromises the numerical accuracy of the schemes through the unbalanced treatment of electronic correlation. Here, we introduce the ADC(2,2) approximation for singly ionized states, which describes both initial and final states of the decay up to second order. We use the new scheme to construct the Fano-ADC(2,2) approximation for the decay widths and show that it provides superior accuracy for the decay widths of a series of processes. Moreover, the Fano-ADC(2,2) method provides access to second-order decay processes, such as DAD, which are qualitatively beyond the reach of the previously available Fano-ADC implementations.

Molecular Auger Interferometry.

We introduce and present a theory of interferometric measurement of a normal Auger decay lifetime in molecules. Molecular Auger interferometry is based on the coherent phase control of Auger dynamics in a two-color (ω/2ω) laser field. We show that, in contrast to atoms, in oriented molecules of certain point groups the relative ω/2ω phase modulates the total ionization yield. A simple analytical formula is derived for the extraction of the lifetimes of Auger-active states from a molecular Auger interferogram, circumventing the need in either high-resolution or attosecond spectroscopy. We demonstrate the principle of the interferometric Auger lifetime measurement using inner-valence decay in CH_{3}F.

The effect of the partner atom on the spectra of interatomic Coulombic decay triggered by resonant Auger processes.

The resonant-Auger - interatomic Coulombic decay (ICD) cascade was recently suggested as an efficient means of controlling the course of the ICD process. Recent theoretical and experimental works show that control over the energies of the emitted ICD electrons can be achieved either by varying the photon energy to produce different initial core excitations or by changing the neighboring species. This work presents a theoretical investigation on the role of the rare-gas neighbor and clarifies how the latter influences the ICD process. For this purpose, we compare fully ab initio computed ICD-electron and kinetic energy release spectra following the 2p(3/2) → 4s, 2p(1/2) → 4s and 2p(3/2) → 3d of Ar in ArKr and Ar2. We demonstrate that the presence of the chemically "softer" partner atom results in an increase in the energies of the emitted ICD electrons, and also in the appearance of additional ICD-active states. The latter leads to a threefold increase in the ICD yield for the case of the 2p(3/2, 1/2) → 4s parent core excitations.

Interatomic Coulombic decay following resonant core excitation of Ar in argon dimer.

A scheme utilizing excitation of core electrons followed by the resonant-Auger - interatomic Coulombic decay (RA-ICD) cascade was recently proposed as a means of controlling the generation site and energies of slow ICD electrons. This control mechanism was verified in a series of experiments in rare gas dimers. In this article, we present fully ab initio computed ICD electron and kinetic energy release spectra produced following 2p(3/2) → 4s, 2p(1/2) → 4s, and 2p(3/2) → 3d core excitations of Ar in Ar2. We demonstrate that the manifold of ICD states populated in the resonant Auger process comprises two groups. One consists of lower energy ionization satellites characterized by fast interatomic decay, while the other consists of slow decaying higher energy ionization satellites. We show that accurate description of nuclear dynamics in the latter ICD states is crucial for obtaining theoretical electron and kinetic energy release spectra in good agreement with the experiment.

Efficient pathway to neutralization of multiply charged ions produced in Auger processes.

After core ionization of an atom or molecule by an x-ray photon, multiply charged ions are produced in the Auger decay process. These ions tend to neutralize their charge when embedded in an environment. We demonstrate that, depending on the atom or molecule and its neighbors, electron transfer mediated decay (ETMD) provides a particularly efficient neutralization pathway for the majority of the ions produced by Auger decay. The mechanism is rather general. As a showcase example, we conducted an ab initio study of the NeKr2 cluster after core ionization of the Ne atom. This example has been chosen because it is amenable to both ab initio calculations and coincidence experiments. We find that even for frozen nuclei, the neutralization rate can be as fast as 0.130 ps(-1). We also show that nuclear dynamics may increase the rate by about an order of magnitude. The generality of the mechanism makes this neutralization pathway important in weakly bonded environments.