Formation and relaxation of K-2 and K-2V double-core-hole states in n-butane.

Using a magnetic bottle multi-electron time-of-flight spectrometer in combination with synchrotron radiation, double-core-hole pre-edge and continuum states involving the K-shell of the carbon atoms in n-butane (n-C4H10) have been identified, where the ejected core electron(s) and the emitted Auger electrons from the decay of such states have been detected in coincidence. An assignment of the main observed spectral features is based on the results of multi-configurational self-consistent field (MCSCF) calculations for the excitation energies and static exchange (STEX) calculations for energies and intensities. MCSCF results have been analyzed in terms of static and dynamic electron relaxation as well as electron correlation contributions to double-core-hole state ionization potentials. The analysis of applicability of the STEX method, which implements the one-particle picture toward the complete basis set limit, is motivated by the fact that it scales well toward large species. We find that combining the MCSCF and STEX techniques is a viable approach to analyze double-core-hole spectra.

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. The fractions of triple Auger decay are compared with triple Auger fractions for carbon atoms and some noble gas atoms, and are found to follow a linear trend correlated to the number of valence electrons on the atom with the initial core vacancy and on its closest neighbours. This linear trend for the percentage of triple Auger decay is represented by a predictive equation TA = 0.13·Nve - 0.5.

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.

Dissociations of water ions after valence and inner-valence ionization.

Energy selected and mass-resolved electron-ion coincidence spectra of heavy water have been recorded for ionization energies from 18 to 35 eV. Dissociation from the B2B2 state produces both O+ and D2 + at energies near their thermodynamic thresholds in addition to the known products D+ and OD+. The relative yields of O+, OD+, and D+ in the B2B2 state breakdown diagram are modulated by the vibrational structure of the B-state population, implying incomplete energy equilibration before fragmentation. Decay from the C-state produces OD+ in addition to the known O+ and D+. The fragment kinetic energies suggest that O+ and D+ from the C state are the products of full atomization of the molecule.

Relative extent of double and single Auger decay in molecules containing C, N and O atoms.

We show that the proportion of double Auger decay following creation of single 1s core holes in molecules containing C, N and O atoms is greater than usually assumed, amounting to about 10% of single Auger decay in many cases. It varies from molecule to molecule, where the size of the molecule has a positive correlation to the amount of double Auger decay. In neon, examined as a related benchmark, the proportion of double Auger decay is similar to that in methane, and is in the order of 5%.

An experimental and theoretical study of core-valence double ionisation of acetaldehyde (ethanal).

Core-valence double ionisation spectra of acetaldehyde (ethanal) are presented at photon energies above the carbon and oxygen 1s ionisation edges, measured by a versatile multi-electron coincidence spectroscopy technique. We use this molecule as a testbed for analyzing core-valence spectra by means of quantum chemical calculations of transition energies. These theoretical approaches range from two simple models, one based on orbital energies corrected by core valence interaction and one based on the equivalent core approximation, to a systematic series of quantum chemical electronic structure methods of increasing sophistication. The two simple models are found to provide a fast orbital interpretation of the spectra, in particular in the low energy parts, while the coverage of the full spectrum is best fulfilled by correlated models. CASPT2 is the most sophisticated model applied, but considering precision as well as computational costs, the single and double excitation configuration interaction model seems to provide the best option to analyze core-valence double hole spectra.

Dissociation of multiply charged ICN by Coulomb explosion.

The fragmentations of iodine cyanide ions created with 2 to 8 positive charges by photoionization from inner shells with binding energies from 59 eV (I 4d) to ca. 900 eV (I 3p) have been examined by multi-electron and multi-ion coincidence spectroscopy with velocity map imaging ion capability. The charge distributions produced by hole formation in each shell are characterised and systematic effects of the number of charges and of initial charge localisation are found.

Carbon dioxide ion dissociations after inner shell excitation and ionization: the origin of site-specific effects.

Multi-coincidence experiments with detection of both electrons and ions from decay of core-excited and core-ionized states of CO2 confirm that O2(+) is formed specifically in Auger decay from the C1s-π* and O1s-π* resonances. Molecular rearrangement occurs by bending in the resonant states, and O2(+) is produced by both single and double Auger decay. It is suggested that electron capture by C(+) after partial dissociation in the doubly ionized core of excited CO2(+), formed by shake-up in spectator resonant Auger decay, accounts for high kinetic energy and high internal energy in some C + O2(+) fragments.

N1s and O1s double ionization of the NO and N2O molecules.

Single-site N1s and O1s double core ionisation of the NO and N2O molecules has been studied using a magnetic bottle many-electron coincidence time-of-flight spectrometer at photon energies of 1100 eV and 1300 eV. The double core hole energies obtained for NO are 904.8 eV (N1s(-2)) and 1179.4 eV (O1s(-2)). The corresponding energies obtained for N2O are 896.9 eV (terminal N1s(-2)), 906.5 eV (central N1s(-2)), and 1174.1 eV (O1s(-2)). The ratio between the double and single ionisation energies are in all cases close or equal to 2.20. Large chemical shifts are observed in some cases which suggest that reorganisation of the electrons upon the double ionization is significant. Δ-self-consistent field and complete active space self-consistent field (CASSCF) calculations were performed for both molecules and they are in good agreement with these results. Auger spectra of N2O, associated with the decay of the terminal and central N1s(-2) as well as with the O1s(-2) dicationic states, were extracted showing the two electrons emitted as a result of filling the double core holes. The spectra, which are interpreted using CASSCF and complete active space configuration interaction calculations, show atomic-like character. The cross section ratio between double and single core hole creation was estimated as 1.6 × 10(-3) for nitrogen at 1100 eV and as 1.3 × 10(-3) for oxygen at 1300 eV.

Dynamics of hollow atom formation in intense x-ray pulses probed by partial covariance mapping.

When exposed to ultraintense x-radiation sources such as free electron lasers (FELs) the innermost electronic shell can efficiently be emptied, creating a transient hollow atom or molecule. Understanding the femtosecond dynamics of such systems is fundamental to achieving atomic resolution in flash diffraction imaging of noncrystallized complex biological samples. We demonstrate the capacity of a correlation method called "partial covariance mapping" to probe the electron dynamics of neon atoms exposed to intense 8 fs pulses of 1062 eV photons. A complete picture of ionization processes competing in hollow atom formation and decay is visualized with unprecedented ease and the map reveals hitherto unobserved nonlinear sequences of photoionization and Auger events. The technique is particularly well suited to the high counting rate inherent in FEL experiments.

A local chemical environment effect in site-specific Auger spectra of ethyl trifluoroacetate.

We have investigated a local chemical environment effect on Auger spectra of ethyl trifluoroacetate (C(4)H(5)F(3)O(2)), using multi-electron coincidence spectroscopy and high-resolution electron spectroscopy. Site-specific KVV Auger spectra for each carbon atom, and for the fluorine and oxygen atoms are presented. The extent of hole localization in the final dicationic states was investigated with the help of theoretical calculations based on a two-hole population analysis. The Auger spectra have been simulated using a statistical approach. It is found that all Auger decays populate mainly localized dicationic states, with the two holes located either on the same fluorine atom or on adjacent fluorine atoms. While the decay of the F 1s hole populates exclusively the former states, the latter class of states is also populated by the decay of the C and O 1s holes.

Properties of hollow molecules probed by single-photon double ionization.

The formation of hollow molecules (with a completely empty K shell in one constituent atom) through single-photon core double ionization has been demonstrated using a sensitive magnetic bottle experimental technique combined with synchrotron radiation. Detailed properties are presented such as the spectroscopy, formation, and decay dynamics of the N(2)(2+) K(-2) main and satellite states and the strong chemical shifts of double K holes on an oxygen atom in CO, CO2, and O2 molecules.

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.

Triple ionization of CO2 by valence and inner shell photoionization.

Spectra of triply ionized CO(2) have been obtained from photoionization of the molecule using soft x-ray synchrotron light and an efficient multi-electron coincidence technique. Although all states of the CO(2) (+++) trication are unstable, the ionization energy for formation of molecular ions at a geometry similar to that of the neutral molecule is determined as 74 ± 0.5 eV.

State selective fragmentation of doubly ionized sulphur dioxide.

Using multi-electron-ion coincidence measurements combined with high level calculations, we show that double ionisation of SO2 at 40.81 eV can be state selective. It leads to high energy products, in good yield, via a newly identified mechanism, which is likely to apply widely to multiple ionisation by almost all impact processes.

Double core hole creation and subsequent Auger decay in NH3 and CH4 molecules.

Energies of the hollow molecules CH(4)(2+) and NH(3)(2+) with double vacancies in the 1s shells have been measured using an efficient coincidence technique combined with synchrotron radiation. The energies of these states have been determined accurately by high level electronic structure calculations and can be well understood on the basis of a simple theoretical model. Their major decay pathway, successive Auger emissions, leads first to a new form of triply charged ion with a core hole and two valence vacancies; experimental evidence for such a state is presented with its theoretical interpretation. Preedge 2-hole-1-particle (2h-1p) states at energies below the double core-hole states are located in the same experiments and their decay pathways are also identified.

Triple ionization spectra by coincidence measurements of double Auger decay: The case of OCS.

By combining multiple electron coincidence detection with ionization by synchrotron radiation, we have obtained resolved spectra of the OCS(3+) ion created through the double Auger effect. The form of the spectra depends critically on the identity of the atom bearing the initial hole. High and intermediate level electron structure calculations lead to an assignment of the resolved spectrum from ionization via the S 2p hole. From the analysis it appears that the double Auger effect from closed shell molecules favors formation of doublet states over quartet states. Molecular field effects in the double Auger effect are similar to those in the single Auger effect in linear molecules.

Core-valence double photoionization of the CS(2) molecule.

Double photoionization spectra of the CS(2) molecule have been recorded using the TOF-PEPECO technique in combination with synchrotron radiation at the photon energies hν=220, 230, 240, 243, and 362.7 eV. The spectra were recorded in the S 2p and C 1s inner-shell ionization regions and reflect dicationic states formed out of one inner-shell vacancy and one vacancy in the valence region. MCSCF calculations were performed to model the energies of the dicationic states. The spectra associated with a S 2p vacancy are well structured and have been interpreted in some detail by comparison to conventional S 2p and valence photoelectron spectra. The lowest inner-shell-valence dicationic state is observed at the vertical double ionization energy 188.45 eV and is associated with a (2p(3/2))(-1)(2π(g))(-1) double vacancy. The spectrum connected to the C 1s vacancy shows a distinct line at 310.8 eV, accompanied by additional broad features at higher double ionization energies. This line is associated with a (C 1s)(-1)(2π(g))(-1) double vacancy.

Spectra of the triply charged ion CS(2)(3+) and selectivity in molecular Auger effects.

Spectra of triply charged carbon disulphide have been obtained by measuring, in coincidence, all three electrons ejected in its formation by photoionization. Measurements of the CS(2)(3+) ion in coincidence with the three electrons identify the energy range where stable trications are formed. A sharp peak in this energy range is identified as the (2)Pi ground state at 53.1+/-0.1 eV, which is the lowest electronic state according to ab initio molecular orbital calculations. Triple ionization by the double Auger effect is provisionally divided, on the basis of the pattern of energy sharing between the two Auger electrons into contributions from direct and cascade Auger processes. The spectra from the direct double Auger effect via S 2p, S 2s, and C 1s hole states contain several resolved features and show selectivity based on the initial charge localization and on the identity of the initial state. Triple ionization spectra from single Auger decay of S 2p-based core-valence states CS(2)(2+) show retention of the valence holes in this Auger process. Related ion-electron coincidence measurements give the triple ionization yields and the breakdown patterns in triple photoionization at selected photon energies from 90 eV to above the inner shell edges.

Coulomb explosion of CD3I induced by single photon deep inner-shell ionisation.

L-shell ionisation and subsequent Coulomb explosion of fully deuterated methyl iodide, CD3I, irradiated with hard X-rays has been examined by a time-of-flight multi-ion coincidence technique. The core vacancies relax efficiently by Auger cascades, leading to charge states up to 16+. The dynamics of the Coulomb explosion process are investigated by calculating the ions' flight times numerically based on a geometric model of the experimental apparatus, for comparison with the experimental data. A parametric model of the explosion, previously introduced for multi-photon induced Coulomb explosion, is applied in numerical simulations, giving good agreement with the experimental results for medium charge states. Deviations for higher charges suggest the need to include nuclear motion in a putatively more complete model. Detection efficiency corrections from the simulations are used to determine the true distributions of molecular charge states produced by initial L1, L2 and L3 ionisation.