Double Core Hole Research Articles

KL double core hole pre-edge states of HCl.

The formation of double core hole pre-edge states of the form 1s-12p-1(1,3P)σ*,nl for HCl, located on the binding energy scale as deep as 3 keV, has been investigated by means of a high resolution single channel electron spectroscopy technique recently developed for the hard X-ray region. A detailed spectroscopic assignment is performed based on ab initio quantum chemical calculations and by using a sophisticated fit model comprising regular Rydberg series. Quantum defects for the different Rydberg series are extracted and the energies for the associated double core hole ionization continua are extrapolated. Dynamical information such as the lifetime width of these double-core-hole pre-edge states and the slope of the related dissociative potential energy curves are also obtained. In addition, 1s-12p-1V-1nlλn'l'λ' double shake-up transitions and double core hole states of the form 1s-12s-1(1,3S)σ*,4s are observed.

Cationic double K-hole pre-edge states of CS2 and SF6

Recent advances in X-ray instrumentation have made it possible to measure the spectra of an essentially unexplored class of electronic states associated with double inner-shell vacancies. Using the technique of single electron spectroscopy, spectra of states in CS2 and SF6 with a double hole in the K-shell and one electron exited to a normally unoccupied orbital have been obtained. The spectra are interpreted with the aid of a high-level theoretical model giving excellent agreement with the experiment. The results shed new light on the important distinction between direct and conjugate shake-up in a molecular context. In particular, systematic similarities and differences between pre-edge states near single core holes investigated in X-ray absorption spectra and the corresponding states near double core holes studied here are brought out.

Interaction of molecular nitrogen with free-electron-laser radiation.

We compute molecular continuum orbitals in the single center expansion scheme. We then employ these orbitals to obtain molecular Auger rates and single-photon ionization cross sections to study the interaction of N2 with Free-Electron-Laser (FEL) pulses. The nuclei are kept fixed. We formulate rate equations for the energetically allowed molecular and atomic transitions and we account for dissociation through additional terms in the rate equations. Solving these equations for different parameters of the FEL pulse, allows us to identify the most efficient parameters of the FEL pulse for obtaining the highest contribution of double core hole states (DCH) in the final atomic ion fragments. Finally we identify the contribution of DCH states in the electron spectra and show that the DCH state contribution is more easily identified in the photo-ionization rather than the Auger transitions.

Auger decay rates of core hole states using equation of motion coupled cluster method

The recent development of Linac coherent light source high intense X-ray laser makes it possible to create double core ionization in the molecule. The generation of double core hole state and its decay is identified by Auger spectroscopy. The decay of this double core hole (DCH) states can be used as a powerful spectroscopic tool in chemical analysis. In the present work, we have implemented a promising approach, known as CAP-EOMCC method, which is a combination of complex absorbing potential (CAP) and equation-of-motion coupled cluster (EOMCC) approach to calculate the lifetime of single and double core hole states. We have applied this method to calculate the lifetime of the single core hole (K-LL) and double core hole (KK-KLL) states of CH4, NH3 and HF molecules. The predicted lifetime is found to be extremely short.

Calculation of Ar photoelectron satellites in the hard-x-ray region

The intensities of photoelectron satellite lines, corresponding to double core hole (DCH) states of Ar 1s ionization by hard X-rays are calculated using the many-body-perturbation theory. Calculations support the interpretation of the most intense lines as the shake-up excitations 2p 4p. It is demonstrated that the intensities of the spectrum lines corresponding to 4s (and 3d) excited states in DCH field can be explained only taking into account the direct knock-up process 2p 3d along with shake-up process 1s 4s that accompanies 2p photoionization.

Rate equations for nitrogen molecules in ultrashort and intense x-ray pulses

We study theoretically the quantum dynamics of nitrogen molecules (N2) exposed to intense and ultrafast x-rays at a wavelength of ( photon energy) from the Linac Coherent Light Source (LCLS) free electron laser. Molecular rate equations are derived to describe the intertwined photoionization, decay, and dissociation processes occurring for N2. This model complements our earlier phenomenological approaches, the single-atom, symmetric-sharing, and fragmentation-matrix models of 2012 (J. Chem. Phys. 136 214310). Our rate-equations are used to obtain the effective pulse energy at the sample and the time scale for the dissociation of the metastable dication . This leads to a very good agreement between the theoretically and experimentally determined ion yields and, consequently, the average charge states. The effective pulse energy is found to decrease with shortening pulse duration. This variation together with a change in the molecular fragmentation pattern and frustrated absorption—an effect that reduces absorption of x-rays due to (double) core hole formation—are the causes for the drop of the average charge state with shortening LCLS pulse duration discovered previously.

Molecular dynamics induced by short and intense x-ray pulses from the LCLS

The past six years have led to a wealth of experimental and theoretical data revealing the nature of the interaction between gas-phase molecules and short and intense x-ray pulses, from the Linac coherent light source free electron laser (FEL). We present here a few highlights that describe some of the first photoabsorption measurements of gas-phase molecules. In particular, we report on a three decades long prediction of single-site double core holes (ss-DCH) and two-site double core holes (ts-DCH) in diatomic and triatomic molecules. We also describe recent measurements that validate a simple theory regarding femtosecond intense x-ray induced fragmentation dynamics of C60 as well as photoabsorption measurements of encapsulated fullerenes, Ho3N@C80. The latter investigation opens the way for even more complex molecular studies with FELs. In all of the described highlights, working in close collaboration with theorists enabled the interpretation of, or predicted our measurements, and in some cases our experiments guided the modeling. We conclude this article by describing the potential of new instrumentation for chemical and biological sciences especially in light of new or improved FELs.

Ultrafast Molecular Three-Electron Auger Decay

Three-electron Auger decay is an exotic and elusive process, in which two outer-shell electrons simultaneously refill an inner-shell double vacancy with emission of a single Auger electron. Such transitions are forbidden by the many-electron selection rules, normally making their decay lifetimes orders of magnitude longer than the few-femtosecond lifetimes of normal (two-electron) Auger decay. Here we present theoretical predictions and direct experimental evidence for a few-femtosecond three-electron Auger decay of a double inner-valence-hole state in CH_{3}F. Our analysis shows that in contrast to double core holes, double inner-valence vacancies in molecules can decay exclusively by this ultrafast three-electron Auger process, and we predict that this phenomenon occurs widely.

Single- and double-core-hole ion emission spectroscopy of transient neon plasmas produced by ultraintense x-ray laser pulses

Single-core-hole (SCH) and double-core-hole (DCH) spectroscopy is investigated systematically for neon gas in the interaction with ultraintense x-ray pulses with photon energy from 937 eV to 2000 eV. A time-dependent rate equation, implemented in detailed level accounting approximation, is utilized to study the dynamical evolution of the level population and emission properties of the laser-produced highly transient plasmas. The plasma-density effects on level populations and charge-state distribution are demonstrated with an x-ray photon energy of 2000 eV. It is shown that atomic number density of relevant experiment is about 1 × 1018 cm−3, which is comparable to a recent experiment. At this density, we systematically investigate the emissivity of the transient neon plasmas. For laser photon energy in the range 937–1360 eV, resonant absorptions (RA) of ( 2) transitions play important roles in time evolution of the population and DCH emission spectroscopy. The RA effects are illustrated in detail for an x-ray pulse of 944 eV photon energy, which creates the RA from the SCH states (, 1s2s2p5, and 1s2p6) of Ne3+. After averaging over the space and time distribution of x-ray pulse, DCH emission spectroscopy is studied at x-ray photon energies of 937, 944, 955, 968, 980, and 990 eV, where there exist resonances from SCH states of Ne2+-Ne7+. The processes with producing DCH states are discussed. For x-ray photon energy larger than 1360 eV, no RA exist and transient plasmas show different features in the DCH spectroscopy.

Study of double core hole excitations in molecules by X-ray double-quantum-coherence signals: a multi-configuration simulation.

The multi-configurational self-consistent field method is employed to simulate the two-dimensional all-X-ray double-quantum-coherence (XDQC) spectroscopy, a four-wave mixing signal that provides direct signatures of double core hole (DCH) states. The valence electronic structure is probed by capturing the correlation between the single (SCH) and double core hole states. The state-averaged restricted-active-space self-consistent field (SA-RASSCF) approach is used which can treat the valence, SCH, and DCH states at the same theoretical level, and applies to all types of DCHs (located on one or two atoms, K-edge or L-edge), with both accuracy and efficiency. Orbital relaxation introduced by the core hole(s) and the static electron correlation is properly accounted for. The XDQC process can take place via different intermediate DCH state channels by tuning the pulse frequencies. We simulate the XDQC signals for the three isomers of aminophenol at 8 pulse frequency configurations, covering all DCH pathways involving the N1s and O1s core hole (N1sN1s, O1sO1s and N1sO1s), which reveal different patterns of valence excitations.

比較的大きな分子に対する内殻二重正孔分光

The ionization energies of single, double, and triple core-hole (SCH, DCH, and TCH) states were calculated for three isomers of aminophenol with the framework of density functional theory. Our calculated single core-hole ionization potentials were consistent with experiment and the previous theoretical values. Core-ionization energies via complex processes were also estimated. The difference of relaxation energy of two-site DCH (tsDCH) states can be visualized using Wagner plots. We found that the electron density of π orbitals on benzene ring are relaxed to core-vacancy sites at N,O tsDCH state.

Disentangling formation of multiple-core holes in aminophenol molecules exposed to bright X-FEL radiation

Competing multi-photon ionization processes, some leading to the formation of double core hole states, have been examined in 4-aminophenol. The experiments used the linac coherent light source (LCLS) x-ray free electron laser, in combination with a time-of-flight magnetic bottle electron spectrometer and the correlation analysis method of covariance mapping. The results imply that 4-aminophenol molecules exposed to the focused x-ray pulses of the LCLS sequentially absorb more than two x-ray photons, resulting in the formation of multiple core holes as well as in the sequential removal of photoelectrons and Auger electrons (so-called PAPA sequences).

Double core-hole states in SiX4 (X = F, Cl, Br, and CH3) molecules derived by photoelectron and KLL Auger spectroscopy

SynopsisIn recent years double core-hole states are intensively studied since their chemical shifts provide detailed information about initial-state and relaxation effects in a molecule. We derived the Si 1s−1, 2s−1, and 2p−1 binding energies as well as the Si 2s−2, 2s−1, 2p−1, and 2p−2 double-core hole binding energies of different SiX4 systems in order to derive the chemical shifts. Based on these results we created Wagner plots, which give insight in the initial state and the relaxation effects in the different molecules.

Electronic and nuclear dynamics of doubly core-ionized and excited states: Vibrationally resolved photoelectron and Auger spectra of small molecules

SynopsisIn this work we want to assess the influence of nuclear dynamics on the features of the double corehole photoelectron and Auger spectra of small isolated molecules by performing wave packet propagation on precomputed ab initio potential energy surfaces.

Double core hole spectroscopy with synchrotron radiation

In the last five years, a strong experimental and theoretical activity has been dedicated to the study of inner-shell double photoionization. Almost simultaneously, between 2009 and 2011, different experiments were performed using two different light sources: well established third generation synchrotron light sources that exist for more than two decades and recently developed x-ray free electron lasers (X-FEL) that provide brightness of about 10 orders of magnitude higher. The high photon flux of X-FEL has made possible the successive absorption of many x-ray photons by inner-shell electrons in femtosecond timescale. The possibility to create multiple inner-shell vacancies with this new X-FEL light source has resuscitated “prophetic” 25 years old calculations by Cederbaum et al. [1,2] concerning double K-shell ionization that could be of great interest when conventional Electron Spectroscopy for Chemical Analysis (ESCA) finds some limitations. As a feedback, this has stimulated new theoretical developments. On the other hand, it was considered practically out of reach to observe such states with conventional third generation synchrotron light sources due to the very weak probability to eject two inner-shell electrons by a single photon when the dominant process, that could mask everything else, is single inner-shell ionization. However, due to the development of very efficient detection techniques, it was also possible to perform spectroscopy of double core-hole states with excellent accuracy using third generation synchrotron sources.In this paper we give an outlook on the basis of recently published results, of the complementarity of these light sources for double core-hole spectroscopy. Some new developments and results are also presented with a prospective of what could be done in the future with the technical breakthrough that will be necessary to improve our further understanding of chemical analysis.

Covariance mapping of two-photon double core hole states in C2H2 and C2H6 produced by an x-ray free electron laser

Few-photon ionization and relaxation processes in acetylene (C2H2) and ethane (C2H6) were investigated at the linac coherent light source x-ray free electron laser (FEL) at SLAC, Stanford using a highly efficient multi-particle correlation spectroscopy technique based on a magnetic bottle. The analysis method of covariance mapping has been applied and enhanced, allowing us to identify electron pairs associated with double core hole (DCH) production and competing multiple ionization processes including Auger decay sequences. The experimental technique and the analysis procedure are discussed in the light of earlier investigations of DCH studies carried out at the same FEL and at third generation synchrotron radiation sources. In particular, we demonstrate the capability of the covariance mapping technique to disentangle the formation of molecular DCH states which is barely feasible with conventional electron spectroscopy methods.

Chemical analysis: Double core-hole spectroscopy with free-electron lasers

Free-electron lasers with their femtosecond pulse duration, high pulse energy and tunable photon energy in a regime from XUV to hard-X-ray have opened up several research avenues in physics, chemistry and biology. We have exploited the attributes of these new lasers by exploring intriguing non-linear effects such as double core-hole formation via multiphoton absorption. We have carried out X-ray two-photon photoelectron spectroscopy for molecular chemical analysis using the Linac Coherent Light Source. Our experimental work aimed to probe the chemical environment of specific atomic sites in several molecules and validate the theory of two-photon photoelectron spectroscopy. This work demonstrates that our methodology will be very effective with the future intense, high repetition rate as well as laser-seeded or self-seeded photon facilities.

Single site double core level ionisation of OCS

Single site O1s, C1s and S2p double ionisation of the OCS molecule has been investigated using a magnetic bottle multi-electron coincidence time-of-flight spectrometer. Photon energies of 1300, 750 and 520eV, respectively, were used for the ionisation, and spectra were obtained from which the double core ionisation energies could be determined. The energies measured for 1s double ionisation are 1172eV (O1s−2) and 659eV (C1s−2). For the S2p double ionisation three dicationic states are expected, 3P, 1D and 1S. The ionisation energies obtained for these states are 373eV (3P), 380eV (1D) and 388eV (1S). The ratio between the double and single core ionisation energies are in all cases equal or close to 2.20. Auger spectra of OCS, associated with the O1s−2, C1s−2 and S2p−2 dicationic states, were also recorded incorporating both electrons emitted as a result of the filling of the two core vacancies. As for other small molecules, the spectra show an atomic-like character with Auger bands located in the range 480–560eV for oxygen, 235–295eV for carbon and 100–160eV for sulphur. The interpretation of the spectra is supported by CASSCF and CASCI calculations. The cross section ratio between double and single core hole creation was estimated as 3.7×10−4 for oxygen at 1300eV, 3.7×10−4 for carbon at 750eV and as 2.2×10−3 for sulphur at 520eV.

Mapping the decay of double core hole states of atoms and molecules

Covariance mapping is used to investigate electron emission of double core hole states and associated decay processes of atoms and molecules upon absorption of multiple X-ray photons provided by an FEL source.

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.