Double Auger decay in atoms: Probability and angular distribution

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.

Relative extent of triple Auger decay in CO and CO2

Systematic measurements on single and triple Auger decay in CO and CO₂ after the creation of a C 1s or a O 1s core vacancy show that the percentage of triple Auger decay is on the order of 10⁻² of the single Auger decay in these molecules.

Abundance of molecular triple ionization by double Auger decay

Systematic measurements of electron emission following formation of single 1s or 2p core holes in molecules with C, O, F, Si, S and Cl atoms show that overall triple ionization can make up as much as 20% of the decay. The proportion of triple ionization is observed to follow a linear trend correlated to the number of available valence electrons on the atom bearing the initial core hole and on closest neighbouring atoms, where the interatomic distance is assumed to play a large role. The amounts of triple ionization (double Auger decay) after 1s or 2p core hole formation follow the same linear trend, which indicates that the hole identity is not a crucial determining factor in the number of electrons emitted. The observed linear trend for the percentage of double Auger decay follows a predictive line equation of the form DA = 0.415 · N_ve + 5.46.

Observation of a four-electron Auger process in near-K-edge photoionization of singly charged carbon ions

Single, double, and triple ionization of C(1+) ions by single photons is investigated in the energy range of 286-326 eV, i.e., in the range from the lowest-energy K-vacancy resonances to well beyond the K-shell ionization threshold. Clear signatures of C(1+)(1s2s(2)2p(2) (2)D,(2)P) resonances are found in the triple-ionization channel. The only possible mechanism producing C(4+)(1s(2)) via these resonances is direct triple-Auger decay, i.e., a four-electron process with simultaneous emission of three electrons.

Femtosecond electronic response of atoms to ultra-intense X-rays

An era of exploring the interactions of high-intensity, hard X-rays with matter has begun with the start-up of a hard-X-ray free-electron laser, the Linac Coherent Light Source (LCLS). Understanding how electrons in matter respond to ultra-intense X-ray radiation is essential for all applications. Here we reveal the nature of the electronic response in a free atom to unprecedented high-intensity, short-wavelength, high-fluence radiation (respectively 10(18) W cm(-2), 1.5-0.6 nm, approximately 10(5) X-ray photons per A(2)). At this fluence, the neon target inevitably changes during the course of a single femtosecond-duration X-ray pulse-by sequentially ejecting electrons-to produce fully-stripped neon through absorption of six photons. Rapid photoejection of inner-shell electrons produces 'hollow' atoms and an intensity-induced X-ray transparency. Such transparency, due to the presence of inner-shell vacancies, can be induced in all atomic, molecular and condensed matter systems at high intensity. Quantitative comparison with theory allows us to extract LCLS fluence and pulse duration. Our successful modelling of X-ray/atom interactions using a straightforward rate equation approach augurs favourably for extension to complex systems.

Electron-Induced Oxygen Desorption from the TiO2(011)-2x1 Surface Leads to Self-Organized Vacancies

When low-energy electrons strike a titanium dioxide surface, they may cause the desorption of surface oxygen. Oxygen vacancies that result from irradiating a TiO2(011)-2x1 surface with electrons with an energy of 300 electron volts were analyzed by scanning tunneling microscopy. The cross section for desorbing oxygen from the pristine surface was found to be 9 (+/-6) x 10(-17) square centimeters, which means that the initial electronic excitation was converted into atomic motion with a probability near unity. Once an O vacancy had formed, the desorption cross sections for its nearest and next-nearest oxygen neighbors were reduced by factors of 100 and 10, respectively. This site-specific desorption probability resulted in one-dimensional arrays of oxygen vacancies.

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.

Shake-up and shake-off excitations with associated electron losses in X-ray studies of proteins

Photoionization of an atom by X-rays usually removes an inner shell electron from the atom, leaving behind a perturbed "hollow ion" whose relaxation may take different routes. In light elements, emission of an Auger electron is common. However, the energy and the total number of electrons released from the atom may be modulated by shake-up and shake-off effects. When the inner shell electron leaves, the outer shell electrons may find themselves in a state that is not an eigen-state of the atom in its surroundings. The resulting collective excitation is called shake-up. If this process also involves the release of low energy electrons from the outer shell, then the process is called shake-off. It is not clear how significant shake-up and shake-off contributions are to the overall ionization of biological materials like proteins. In particular, the interaction between the outgoing electron and the remaining system depends on the chemical environment of the atom, which can be studied by quantum chemical methods. Here we present calculations on model compounds to represent the most common chemical environments in proteins. The results show that the shake-up and shake-off processes affect approximately 20% of all emissions from nitrogen, 30% from carbon, 40% from oxygen, and 23% from sulfur. Triple and higher ionizations are rare for carbon, nitrogen, and oxygen, but are frequent for sulfur. The findings are relevant to the design of biological experiments at emerging X-ray free-electron lasers.