Auger Shake-Up Assisted Electron Recapture.

The presence of doubly excited states (DESs) above the core-hole ionization threshold nontrivially modulates the x-ray absorption because the participator Auger decay couples DESs to the underlying low-energy core-hole continuum. We show that coupling also affects the high-energy continuum populated by the spectator Auger decay of DESs. For the K-L_{23}^{2} Auger decay of the 1s^{-1}3p^{-1}4s^{2}^{1}P state in argon, the competing nonresonant path is assigned to the recapture of the 1s photoelectron caused by emission of the fast electron from the shake-up K-L_{23}^{2} decay of the 1s^{-1} ion.

Isotope effects in dynamics of water isotopologues induced by core ionization at an x-ray free-electron laser.

Dynamical response of water exposed to x-rays is of utmost importance in a wealth of science areas. We exposed isolated water isotopologues to short x-ray pulses from a free-electron laser and detected momenta of all produced ions in coincidence. By combining experimental results and theoretical modeling, we identify significant structural dynamics with characteristic isotope effects in H2O2+, D2O2+, and HDO2+, such as asymmetric bond elongation and bond-angle opening, leading to two-body or three-body fragmentation on a timescale of a few femtoseconds. A method to disentangle the sequences of events taking place upon the consecutive absorption of two x-ray photons is described. The obtained deep look into structural properties and dynamics of dissociating water isotopologues provides essential insights into the underlying mechanisms.

Concerted and sequential three-body fragmentation of deep-core-ionized carbon disulfide.

Momentum vector correlation is a powerful tool to study molecular dissociation. We have studied the three-body fragmentation of carbon disulfide after sulfur 1s photoionization by means of momentum imaging techniques. Concerted and sequential pathways are disentangled in three-body fragmentation using adapted analysis strategies. In particular, we introduce various data visualization schemes that are proved to be particularly efficient to determine dissociation dynamics.

Simulation of Auger decay dynamics in the hard X-ray regime: HCl as a showcase.

Auger decay after photoexcitation or photoemission of an electron from a deep inner shell in the hard X-ray regime can be rather complex, implying a multitude of phenomena such as multiple-step cascades, post-collision interaction (PCI), and electronic state-lifetime interference. Furthermore, in a molecule nuclear motion can also be triggered. Here we discuss a comprehensive theoretical method which allows us to analyze in great detail Auger spectra measured around an inner-shell ionization threshold. HCl photoexcited or photoionized around the deep Cl 1s threshold is chosen as a showcase. Our method allows calculating Auger cross sections considering the nature of the ground, intermediate and final states (bound or dissociative), and the evolution of the relaxation process, including both electron and nuclear dynamics. In particular, we show that we can understand and reproduce a so-called experimental 2D-map, consisting of a series of resonant Auger spectra measured at different photon energies, therefore obtaining a detailed picture of all above-mentioned dynamical phenomena at once.

The O K-2V spectrum of CO: the influence of the second core-hole.

Using synchrotron radiation in the tender X-ray regime, a photoelectron spectrum showing the formation of single site double-core-hole pre-edge states, involving the K shell of the O atom in CO, has been recorded by means of high-resolution electron spectroscopy. The experimentally observed structures have been simulated, interpreted and assigned, employing state-of-the-art ab initio quantum chemical calculations, on the basis of a theoretical model, accounting for their so-called direct or conjugate character. Features appearing above the double ionization threshold have been reproduced by taking into account the strong mixing between multi-excited and continuum states. The shift of the σ* resonance below the double ionization threshold, in combination with the non-negligible contributions of multi-excited configurations in the final states reached, gives rise to a series of avoided crossings between the different potential energy curves.

Hard x-ray spectroscopy and dynamics of isolated atoms and molecules: a review.

We present here a review of the most significant recent achievements in the field of HAXPES (hard x-ray photoelectron spectroscopy) on isolated atoms and molecules, and related spectroscopies. The possibility of conducting hard x-ray photoexcitation and photoionization experiments under state-of-the art conditions in terms of photon and electron kinetic energy resolution has become available only in the last few years. HAXPES has then produced structural and dynamical information at the level of detail already reached in the VUV and soft-x-ray ranges. The much improved experimental conditions have allowed extending to the hard x-ray range some methods well established in soft x-ray spectroscopies. Investigations of electron and nuclear dynamics in the femtosecond (fs, 10-15 s) and even attosecond (as, 10-18 s) regime have become feasible. Complex relaxation phenomena following deep-core ionization can now be enlightened in great detail. Other phenomena like e.g. recoil-induced effects are much more important in fast photoelectron emission, which can be induced by hard x-rays. Furthermore, a new kind of ionic states with double core holes can be observed by x-ray single-photon absorption. Future perspectives are also discussed.

Double-core-hole states in CH3CN: Pre-edge structures and chemical-shift contributions.

Spectra reflecting the formation of single-site double-core-hole pre-edge states involving the N 1s and C 1s core levels of acetonitrile have been recorded by means of high-resolution single-channel photoelectron spectroscopy using hard X-ray excitation. The data are interpreted with the aid of ab initio quantum chemical calculations, which take into account the direct or conjugate nature of this type of electronic states. Furthermore, the photoelectron spectra of N 1s and C 1s singly core-ionized states have been measured. From these spectra, the chemical shift between the two C 1s-1 states is estimated. Finally, by utilizing C 1s single and double core-ionization potentials, initial and final state effects for the two inequivalent carbon atoms have been investigated.

Energy Transfer into Molecular Vibrations and Rotations by Recoil in Inner-Shell Photoemission.

A mixture of CF_{4} and CO gases is used to study photoelectron recoil effects extending into the tender x-ray region. In CF_{4}, the vibrational envelope of the C 1s photoelectron spectrum becomes fully dominated by the recoil-induced excitations, revealing vibrational modes hidden from Franck-Condon excitations. In CO, using CF_{4} as an accurate energy calibrant, we determine the partitioning of the recoil-induced internal excitation energy between rotational and vibrational excitation. The observed rotational recoil energy is 2.88(28) times larger than the observed vibrational recoil energy, well in excess of the ratio of 2 predicted by the basic recoil model. The experiment is, however, in good agreement with the value of 2.68 if energy transfer via Coriolis coupling is included.

Experimental setup for the study of resonant inelastic X-ray scattering of organometallic complexes in gas phase.

A new setup has been designed and built to study organometallic complexes in gas phase at the third-generation Synchrotron radiation sources. This setup consists of a new homemade computer-controlled gas cell that allows us to sublimate solid samples by accurately controlling the temperature. This cell has been developed to be a part of the high-resolution X-ray emission spectrometer permanently installed at the GALAXIES beamline of the French National Synchrotron Facility SOLEIL. To illustrate the capabilities of the setup, the cell has been successfully used to record high-resolution Kα emission spectra of gas-phase ferrocene Fe(C5H5)2 and to characterize their dependence with the excitation energy. This will allow to extend resonant X-ray emission to different organometallic molecules.

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.

Subfemtosecond Control of Molecular Fragmentation by Hard X-Ray Photons.

Tuning hard x-ray excitation energy along Cl 1s→σ^{*} resonance in gaseous HCl allows manipulating molecular fragmentation in the course of the induced multistep ultrafast dissociation. The observations are supported by theoretical modeling, which shows a strong interplay between the topology of the potential energy curves, involved in the Auger cascades, and the so-called core-hole clock, which determines the time spent by the system in the very first step. The asymmetric profile of the fragmentation ratios reflects different dynamics of nuclear wave packets dependent on the photon energy.

Potential Energy Surface Reconstruction and Lifetime Determination of Molecular Double-Core-Hole States in the Hard X-Ray Regime.

A combination of resonant inelastic x-ray scattering and resonant Auger spectroscopy provides complementary information on the dynamic response of resonantly excited molecules. This is exemplified for CH_{3}I, for which we reconstruct the potential energy surface of the dissociative I 3d^{-2} double-core-hole state and determine its lifetime. The proposed method holds a strong potential for monitoring the hard x-ray induced electron and nuclear dynamic response of core-excited molecules containing heavy elements, where ab initio calculations of potential energy surfaces and lifetimes remain challenging.

Double-Core-Hole States in Neon: Lifetime, Post-Collision Interaction, and Spectral Assignment.

Using synchrotron radiation and high-resolution electron spectroscopy, we have directly observed and identified specific photoelectrons from K^{-2}V states in neon corresponding to simultaneous 1s ionization and 1s→valence excitation. The natural lifetime broadening of the K^{-2}V states and the relative intensities of different types of shakeup channels have been determined experimentally and compared to ab initio calculations. Moreover, the high-energy Auger spectrum resulting from the decay of Ne^{2+}K^{-2} and Ne^{+}K^{-2}V states as well as from participator Auger decay from Ne^{+}K^{-1}L^{-1}V states, has been measured and assigned in detail utilizing the characteristic differences in lifetime broadenings of these core hole states. Furthermore, post collision interaction broadening of Auger peaks is clearly observed only in the hypersatellite spectrum from K^{-2} states, due to the energy sharing between the two 1s photoelectrons which favors the emission of one slow and one fast electron.

Vibrationally Resolved B 1s Photoionization Cross Section of BF3.

Photoelectron diffraction is a well-established technique for structural characterization of solids, based on the interference of the native photoelectron wave with those scattered from the neighboring atoms. For isolated systems in the gas phase similar studies suffer from orders of magnitude lower signals due to the very small sample density. Here we present a detailed study of the vibrationally resolved B 1s photoionization cross section of BF3 molecule. A combination of high-resolution photoelectron spectroscopy measurements and of state-of-the-art static-exchange and time-dependent DFT calculations shows the evolution of the photon energy dependence of the cross section from a complete trapping of the photoelectron wave (low energies) to oscillations due to photoelectron diffraction phenomena. The diffraction pattern allows one to access structural information both for the ground neutral state of the molecule and for the core-ionized cation. Due to a significant change in geometry between the ground and the B 1s(-1) core-ionized state in the BF3 molecule, several vibrational final states of the cation are populated, allowing investigation of eight different relative vibrationally resolved photoionization cross sections. Effects due to recoil induced by the photoelectron emission are also discussed.

High-resolution photoelectron spectroscopy with angular selectivity - a tool to probe valence-Rydberg states and couplings in HCl(+).

Due to strong electron correlation effects and electron coupling with nuclear motion, the molecular inner-valence photoionization is still a challenge in electron spectroscopy, resulting in several interesting phenomena such as drastic changes of angular dependencies, spin-orbit induced predissociation, and complex interplay between adiabatic and nonadiabatic transitions. We investigated the excited electronic states of HCl(+) in the binding energy range 27.5-30.5 eV using synchrotron radiation based high-resolution inner-valence photoelectron spectroscopy with angular resolution and interpreted the observations with the help of ab initio calculations. Overlapping electronic states in this region were disentangled through the analysis of photoelectron emission anisotropies. For instance, a puzzling transition, which does not seem to obey either an adiabatic or a nonadiabatic picture, has been identified at ∼28.6 eV binding energy. By this study, we show that ultrahigh-resolution photoelectron spectroscopy with angular selectivity represents a powerful tool to probe the highly excited ionic molecular electronic states and their intricate couplings.

Intramolecular photoelectron diffraction in the gas phase.

We report unambiguous experimental and theoretical evidence of intramolecular photoelectron diffraction in the collective vibrational excitation that accompanies high-energy photoionization of gas-phase CF4, BF3, and CH4 from the 1s orbital of the central atom. We show that the ratios between vibrationally resolved photoionization cross sections (v-ratios) exhibit pronounced oscillations as a function of photon energy, which is the fingerprint of electron diffraction by the surrounding atomic centers. This interpretation is supported by the excellent agreement between first-principles static-exchange and time-dependent density functional theory calculations and high resolution measurements, as well as by qualitative agreement at high energies with a model in which atomic displacements are treated to first order of perturbation theory. The latter model allows us to rationalize the results for all the v-ratios in terms of a generalized v-ratio, which contains information on the structure of the above three molecules and the corresponding molecular cations. A fit of the measured v-ratios to a simple formula based on this model suggests that the method could be used to obtain structural information of both neutral and ionic molecular species.

Nonstoichiometric intensities in core photoelectron spectroscopy.

X-ray photoemission spectroscopy is used in a great variety of research fields; one observable is the sample's stoichiometry. The stoichiometry can be deduced based on the expectation that the ionization cross sections for innershell orbitals are independent of the molecular composition. Here we used chlorine-substituted ethanes in the gas phase to investigate the apparent carbon stoichiometry. We observe a nonstoichiometric ratio for a wide range of photon energies, the ratio exhibits x-ray-absorption fine structure spectroscopy (EXAFS)-like oscillations and hundreds of eV above the C1s ionization approaches a value far from 1. These effects can be accounted for by considering the scattering of the outgoing photoelectron, which we model by multiple-scattering EXAFS calculations, and by considering the effects of losses due to monopole shakeup and shakeoff and to intramolecular inelastic scattering processes.

Experimental observation of rotational Doppler broadening in a molecular system.

The first experimental evidence of rotational Doppler broadening in photoelectron spectra, reported here, show good agreement with recently described theoretical predictions. The dependence of the broadening on temperature and photoelectron kinetic energy is quantitatively predicted by the theory. The experiments verify that the rotational contributions to the linewidth are comparable to those from translational Doppler broadening and must be considered in the analysis of high-resolution photoelectron spectra. A classical model accounting for this newly observed effect is presented.

Study of the dissociation of nitrous oxide following resonant excitation of the nitrogen and oxygen K-shells.

A photochemistry study on nitrous oxide making use of site-selective excitation of terminal nitrogen, central nitrogen, and oxygen 1s-->3pi excitations is presented. The resonant Auger decay which takes place following excitation can lead to dissociation of the N2O+ ion. To elucidate the nuclear dynamics, energy-resolved Auger electrons were detected in coincidence with the ionic dissociation products, and a strong dependence of the fragmentation pathways on the core-hole site was observed in the binding energy region of the first satellite states. A description based on the molecular orbitals as well as the correlation between the thermodynamical thresholds of ion formation and the first electronic states of N2O+ has been used to qualitatively explain the observed fragmentation patterns.