Double Core-hole Spectroscopy Research Articles

Advanced Computation Method for Double Core Hole Spectra: Insight into the Nature of Intense Shake-up Satellites.

Double core hole spectroscopy is an ideal framework for investigating photoionization shake-up satellites. Their important intensity in a single site double core hole (ssDCH) spectrum allows the exploration of the subtle mix of relaxation and correlation effects associated with the inherent multielectronic character of the shake-up process. We present a high-accuracy computation method for single photon double core-shell photoelectron spectra that combines a selected configuration interaction procedure with the use of non-orthogonal molecular orbitals to obtain unbiased binding energy and intensity. This strategy leads to the oxygen ssDCH spectrum of the CO molecule that is in excellent agreement with the experimental result. Through a combined wave function and density analysis, we highlight that the most intense shake-up satellites are characterized by an electronic reorganization that opposes the core hole-induced relaxation.

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.

Probing chemical environment with molecular double core-hole electron spectroscopy

With the advent of X-ray free electron lasers and the development of elaborate multi-coincidence methods combined with conventional synchrotron radiation, the interest in double core-hole spectroscopy revived. To describe the measured data and provide the guideline for yet coming experiments, theoretical studies are required. In this paper we review previous theoretical works on double core-hole states and discuss new theoretical results for the XHm–YHn (X, Y=C, N, O, or F; m,n=0–3) molecules and their fluorine substituted compounds. We compute the single and double core-hole binding energies of these systems using different methods and show that the DFT results agree well with the results of other ab initio methods. In agreement with previous theoretical works, our study demonstrates that the changes in the ionization potential resulting from fluorine substitutions mainly arise due to changes in the ground state and not due to electron relaxation. Special emphasis is given to interatomic relaxation originating from creation of two core holes on different atomic sites. We demonstrate that there exists clear correlation between this quantity, the number of hydrogen atoms and the order of the bond between the heavy atoms in the systems studied.

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.

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.

K-shell double core-hole spectroscopy in molecules

A great deal of attention has been devoted in the last few years to photoionization processes in isolated molecules leading to the formation of double core-hole (DCH) states. There are two main experimental avenues to induce such processes, namely single-photon absorption followed by the simultaneous ejection of two core electrons, and x-ray-induced multiphoton processes leading to the production of DCH states via the sequential absorption of two soft x- ray photons on a time scale on the order of the molecular Auger lifetime (4–8 femtoseconds for light elements). The formation of molecular two-site (ts) DCH states, in particular, shows great potential as a powerful tool for chemical analysis. A compelling motivation for the study of ts-DCH states is their ability to probe the local chemical environment more sensitively than either single core-hole (SCH) or single-site (ss) DCH states. The enhanced sensitivity originates from the fact that the double ionization potential (DIP) of ts-DCH states is directly coupled to induced changes in the valence charge distribution at the two different atomic sites. Here a review of the recent literature is presented on both types of experiments, and on the related theoretical work.

Extracting chemical information of free molecules from K-shell double core-hole spectroscopy

Using density functional theory (DFT) methods, we have calculated ionization potential (IP) for K-shell single core hole (SCH) creation and double ionization potential (DIP) for K-shell double core hole (DCH) creation for XHmYHn (X, Y=C, N, O, F, m,n=0–3), NX2CXO (X=H or F) and C60. For these molecules, we estimated the relaxation energies (a measure of the electron density flow to the core-hole site) and the interatomic relaxation energies (a measure of the electron density flow to the two core-hole sites) from the calculated IPs and DIPs. For XHmYHn, we find that the interatomic relaxation energy for the DCH states having two holes at X and Y atoms decreases with the increase in the bond order between X and Y. For NX2CXO (X=H or F), we find that the substitution of the hydrogen atoms by the fluorine atoms affects the initial-state-bonding shifts but less influences the relaxation energy. For DCH states having two holes at two carbon atoms in C60, we find that the interatomic relaxation energy decreases with the increase in the hole–hole distance.

Double core–hole correlation satellite spectra of N2 and CO molecules

We have theoretically investigated the correlation satellite spectra associated with the single-site (ss) and two-site (ts) double core–hole (DCH) states. The calculated ssDCH satellite spectra of N2 (N 1s−2) and CO (O 1s−2) reproduced the observed experimental spectra satisfactorily and provided the first detailed assignments. DCH spectroscopy using X-ray two-photon photoelectron spectroscopy (XTPPS) was examined taking account of the single core–hole (SCH) and DCH satellite spectra. The results demonstrate that in XTPPS the ssDCH satellite spectrum is separated from other SCH/tsDCH spectra, while the tsDCH states and their satellite states may be overlapped with the SCH satellite spectrum.