Table of contents

MQDT studies of channel interactions reported for systems involving 3 non-degenerate and many degenerate closed channels but limited to one open channel were extended to many open channel systems.

We apply a pump-probe approach to study the process of single-photon two-electron ionization of the helium atom. We demonstrate that this approach provides a complete characterization of the double photoionization process, allowing one to reconstruct both the magnitude and phase of the ionization amplitudes.

Experimental evidence is presented for confinement resonances predicted in photoionization of the endohedral Xe@C₆₀⁺ molecular ion at photon energies corresponding to the giant 4d resonance of Xe.

The left-right asymmetry parameters of Kr 4p photoelectrons were investigated experimentally in the photon energy range of the 3d⁻¹ → np resonant excitations. An interference effect was observed between an unknown photoionization channel and the resonant excitation participator Auger decay processes.

The angular distribution of Kr 4p photoelectrons was investigated experimentally in the photon energy range of the 3d⁻¹ → np resonant excitations. The experimental dipole and non-dipole anisotropy parameters were determined for the spin-orbit components of the Kr 4p shell. An interference effect was observed between the direct photoionization and the resonant excitation participator Auger decay processes in the photon energy dependence of the experimental anisotropy parameters.

Using the configuration interaction Pauli-Fock including core polarization (CIPFCP) approach we have calculated absolute total and partial cross sections for photoionization of the excited mp⁵(m+1)p J = 0 − 3 levels in Ar, Kr and Xe (m = 3 − 5) near threshold, including the region of the odd nl' (l' = 0, 2, 4) autoionizing resonances. For several intermediate levels of Ar and Kr, the computed cross sections are compared with laser spec-troscopic measurements; the theoretical and experimental lineshapes and branching ratios for different resonances show good mutual agreement.

Photoionization cross-section measurements for atomic nitrogen in the region of the K-edge have been made at the Advanced Light Source for the first time and are suitably reproduced by R-matrix calculations.

First experimental measurements of both the Ba 5s photoionization cross section and photoelectron angular asymmetry parameter in the 5s correlation minimum following the 4d giant resonance are presented along with the corresponding theoretical interpretation.

The effect of interchannel coupling and confinement on the photoionization of the Kr atom is examined. It is found that the position of the Cooper minimum is sensitive to interchannel coupling in both free and confined Kr. The strengths of the confinement oscillations are also sensitive to interchannel coupling.

Spectacular trends in the modification of the Xe 5s photoionization via interchannel coupling with confinement resonances emerging in the Xe 4d giant resonance upon photoionization of the Xe@C₆₀, Xe@C₂₄₀ and Xe@C₆₀@C₂₄₀ endo-fullerenes are theoretically unraveled and interpreted.

Based on a well optimized quasi-complete basis sets, full relativistic calculations have been performed on the energy levels of 2p⁴ns, np, nd (n=3, 4) states of atomic fluorine up to higher than the first ionization potential corresponding to F⁺ (³P₂). In addition, the oscillator strengths for transitions from 2p⁴(¹D)4s ²D and 2p⁴(¹S)3s ²S to 2p⁵²P_3/2 are given.

Fully differential cross sections of double core hole photoionization of Be with fixed valence electrons

The ionic single-molecule-magnet [Mn^III₆Cr^III]³ with corresponding three counterions has been deposited on different surfaces and studied with respect to its structure and its electronic and magnetic properties. This is the first time that spin polarization of photoelectrons ejected by means of circularly polarized synchrotron radiation has been measured in a single-molecule-magnet.

A simple model for the photodetachment of dipole-bound anions is proposed where non-adiabatic coupling of vibrational states leads to a Fano resonance in the spectrum. It is found that the shape of the photodetachment spectrum depends significantly on the parameter representing molecular polarizability. The model is also applied to a Fano profile observed in the photodetachment of small water cluster anions.

Single-photon dissociative photoionization of H₂/D₂ in the Q₁, Q₂ doubly excited states resonance regions, where ultrafast electronic and nuclear dynamics are coupled, is studied using the vector-correlation method with single selected femtosecond high-order harmonic and synchrotron radiation in the VUV. Results are compared at the level of electron-ion kinetic energy correlation diagrams, asymmetry parameters, and the molecular frame photoelectron angular distributions.

Dissociative photoionization of carbon monoxide with angle resolved photoelectron-photoion fragment detection has yielded recoil frame photoelectron angular distributions. Comparisons with accurate photoionization calculations have lead to the identification of the inner-valence ion states and estimates of their predissociative lifetimes.

We present a combined experimental and theoretical study focused on the ionic states of the N₂O₄ molecule. Experimental results regarding photoionization induced by the synchrotron radiation SOLEIL in the 13.5-15.5 eV energy range were obtained using the electron-ion velocity vector correlation method. The potential energy curves for the dissociation of the N-N bond were computed within ab initio multireference wave functions based methods (CASSCF and CASPT2) for the first electronic states of N₂O₄ and N₂O₄⁺.

Energetically overlapping resonances in N₂O of the differently localized 1s core electrons of the center and terminal N atoms may interfere. Indications for such interference have been found in angularly and spectrally resolved fragment fluorescence spectra after inner shell excitation of overlapping resonances of the 1s electrons of the central (N_c) and terminal (N_t) nitrogen atom of N₂O.

In this work, circular dichroism in H₂ (D₂) photoionization is studied in detail. We have selected several photon energies for a case study: 19 eV for which only direct ionization to the 1sσ_g ionization channel is present, 27 eV where autoionization of Q₁¹Σ⁺_u doubly excited states takes place, and 32.5 eV for which autoionization from Q₁ and doubly excited states and direct ionization to 1sσ_g and 2pσ_u channels strongly interfere. The latter case shows clear evidence of different behavior of the photoionization against radiation helicity.

We investigate theoretically the single ionization of neon by an attosecond XUV pulse, aiming at a better understanding of the outgoing electron wave-packet in the early stages of its detachment. To do so, we integrate the one-electron time-dependent Schrödinger equation numerically. The non-local interaction with the spectator electrons in the time-dependent hamiltonian is accounted for with a configuration-averaged effective Hartree-Fock potential.

We have kinematically complete measured photo double ionization of Helium at E_γ=800 eV and observed recoiling ions with nearly 0 momentum that agree well with theoretical predictions of a new photo double ionization mechanism.

With the first X-ray free-electron laser (FEL), the Linac Coherent Light Source (LCLS), multiphoton ionization has been pushed to a new regime, where atoms and molecules are not just ionized by a series of valence ionizations but "from the inside out". At unprecedented high intensities and short pulse durations in the soft X-ray regime, a series of inner-shell photoionizations followed by cascades of Auger decays was observed to lead to highly charged final states in rare gases such as Ne, Ar, Kr, and Xe. Ion time-of-flight and fluorescence spectra were recorded for different FEL pulse energies and pulse lengths and compared to theoretical models to explain the underlying processes that lead to unexpectedly high charge states in Xe.

We study the Cooper minimum in HHG from argon atoms using long wavelength laser pulses. We find that the minimum in high harmonic spectra is systematically shifted with respect to total photoionization cross section measurements. We use a semi-classical theoretical approach based on Classical Trajectory Monte Carlo and Quantum Electron Scattering methods (CTMC-QUEST) to model the experiment. Our study reveals that the shift between photoionization and high harmonic emission is due to several effects: the directivity of the recombining electrons and emitted polarization, and the shape of the recolliding electron wavepacket.

We report the results of extensive R-matrix calculations for the X-ray photoabsorption of all magnesium ions in the vicinity of the K-edge.

Resonant triple-differential cross-section (TDCS) measurements on atomic magnesium on the 2p → 3d resonance are presented. These measurements are similar to recent measurements made in strontium at the 4p → 4d resonance [1].

Photoionization cross section of endohedrally confined calcium is computed using R-matrix method.

Single photon double K-shell ionization of small molecules (N₂, O₂, CO, CO₂) has been observed by coincidence detection of two photoelectrons with two Auger electron and reveals properties of hollow molecules.

We study the dissociative photoionization of D₂ by an attosecond pulse train (APT) in the presence of a near-infrared (IR) field. Strong oscillations in the D+ kinetic energy release spectrum with a half period of the optical cycle of the infrared field are observed and attributed to interferences between ionization pathways involving different harmonic orders of the APT due to the IR-induced coupling between the 1sσ_g and 2pσ_u ionization channels.

We doubly ionize H₂O by single photon absorption at 43 eV leading to H⁺ + OH⁺. A direct double ionization and a sequential process in which single ionization is followed by rapid dissociation into a proton and an autoionizing OH^* are identified. We obtain information about the internuclear distance at which the decay of the OH^* occurs from the angular distribution of the autoionization electron.

Dissociation of perfluorocyclobutane into cation pairs and neutral fragments following the C and F K-shell excitation was investigated using the photoelectron–photoion–photoion coincidence (PEPIPICO) technique. Based on the analysis of islands in the PEPIPICO spectra, fragmentation pathways of the molecular dication were determined. Two characteristic series of the pathways, i.e., fission of F atom(s) followed by charge separation and elimination of CF₂ or CF₃ followed by charge separation, were identified. The symmetric fission of the molecular dication was found only for the C 1s excitation.

The photoelectron asymmetry parameter for single photon ionization of H₂ between h_ν = 31 eV and 35 eV has been measured. At these photon energies the doubly excited Q₁ and Q₂ states are energetically accessible and lie within the Franck-Condon absorption range. For electron energies greater than ~6 eV, the β parameter oscillates strongly, mainly due to interferences in autoionization decay of the Q₁¹Σ_u⁺ and Q₂¹Π_u doubly excited states. The experimental results are compared with close coupling calculations.

In the dissociative photoionization (DPI) process, hν + O₂ → O + O⁺ + e⁻, ionisation and dissociation both occur (either as a direct or indirect process) following photoabsorption. The Threshold Photoelectron Photoion Coincidence (TPEPICO) technique, i.e. measuring the coincidence yield between threshold photoelec-trons and photoions is a powerful way of studying the dynamics involved. The c⁴Σ_u⁻ state in O⁺₂ at ~ 24.56 eV has a shallow minimum in its potential that supports two distinct quasi-bound vibrational levels (v = 0, 1). We have investigated the angular distributions of 2 eV O⁺(⁴S) ions produced from DPI of O ⁺₂ c⁴Σ_u⁻ (v = 0, 1) using the TPEPICO technique.

Vibrationally resolved cross section for the valence shell photoionization of N₂ and CO has been accurately measured experimentally up to 300 eV. Series of ratios between two vibrational cross sections (v-ratios) are compared to theoretical results obtained from a new ah initio DFT method that includes nuclear motion. It reveals strong oscillations around the Franck-Condon factor which are the fingerprints of double-slit like interferences. To support this point, a simple model (based on the Cohen-Fano formula) will be presented at the conference.

We combine new accurate calculations, asymptotic relations, and available experimental data to construct an H⁻ photodetachment cross section reliable for a large range of photon energies.

High-resolution photoionization cross-section measurements for the Cℓ-like ion Ar⁺ made at the Advanced Light Source are compared with 48-level semi-relativistic Breit-Pauli R-matrix calculations.

The negative ion of cerium has been investigated with tunable infrared laser photodetachment spectroscopy over selected photon energy ranges between 0.56 − 0.70 eV. The spectrum reveals several sharp peaks due to negative ion resonances and possible bound-bound transitions in Ce⁻. The newly observed transitions, together with our previous measurements [1], provide insight into the rich near-threshold spectrum of this lanthanide negative ion.

Relative cross sections for photoionization of W^q+ ions q = 1, ..., 5 were measured at energies up to 90 eV employing the photon-ion merged-beam technique at the Advanced Light Source in Berkeley. While the cross sections for the lower charge states are dominated by broad resonance features in the range of 4f and 5p excitations, the cross section for W⁵⁺ shows much narrower peak features.

High-resolution photoionization measurements of Xe⁺ ions have been performed at the Advanced Light Source in Berkeley, California, USA. The experimental cross sections are compared with results from Dirac-Coulomb R-matrix calculations.

Absolute single-photon multi-electron ionization and fragmentation cross sections for C⁻₆₀ ions are measured for the first time. It is determined that the excess electron in the anion causes a significant increase in cross section and an overall energy scaling, but otherwise has little effect on the observed structure. Appearance energies (apparent thresholds) of the fragmentation products are also observed to be significantly lower for the anion, as compared to the neutral molecule.

New techniques at synchrotrons can not only drive higher accuracy, structural information and insight in photoabsorption and inelastic scattering of the photon, but also inelastic scattering of the electron. This is important for fields such as XAFS, XANES and powder diffraction, but has also initiated new fields of nanoroughness measurement, measurement of electron inelastic mean free paths, bonding information at an accuracy of crystallographic determination, and similar advances for fluorescence and scattering investigations. We summarise and present some key opportunities for research and development.

Using momentum-resolved electron-ion multicoincidence, we have investigated interatomic Coulombic decay (ICD). We observed ICD processes in NeAr dimers following Ar LMM and Ne KLL Auger decay. From Auger final state of Ar²⁺(3s⁻²), we observed ICD where three electrons participate in.

Intermolecular Coulombic decay (ICD) is an ultrafast relaxation mechanism of a highly excited system embedded in a chemical environment. Our theoretical investigation of ammonia clusters shows that the ICD efficiency can be regulated by protonation or deprotonation. It is suggested that by varying ICD rate through changing the environmental pH value a control over the relaxation dynamics can be achieved.

We report the first observation of electron-transfer-mediated decay (ETMD) and interatomic Coulombic decay (ICD) from the triply charged states with an inner-valence vacancy, using the Ne₂, NeAr, and Ar₂. These ETMD and ICD processes are unambiguously identified by electron-ion-ion coincidence spectroscopy in which the kinetic energy of the ETMD or ICD electron and the kinetic energy release between the two fragment ions are measured in coincidence.

Alkali-halide molecular clusters were formed from the vaporized molecules in collisions with an intense beam of argon clusters. Photoionization of the alkali-halide clusters as the result of interaction with x-ray radiation has been utilized in a photoelectron spectroscopy study of cluster bonding mechanisms. The changes in the energy structure due to the transformation from molecules to the solid salt were observed both in the inner and outer shells of constituent elements (metal and halogen) and were used to deduce the geometric structure of the clusters in question.

Absolute cross sections of three competing decay channels, namely ionization, dissociation and fluorescence, were determined after excitation of superexcited states of the hydrogen molecule.

A short overwiev of recent results on x-ray emission obtained by JSI high resolution spectrometer is given. The report is about photoexcitation of atomic and molecular targets and deals with topics such as isolation of multiple excitation channels, the resonant Raman effect in radiative Auger decay, the interference of absorption-emission paths, and the sensitivity of resonant emission spectra to molecular relaxation.

Photon yields resulting from radiative cascades of photoexcited helium singly and doubly excited states in a dc electric field are presented. The combined effect of the spin-orbit and the field-induced mixing on the yields are examined.

We have performed a spectroscopic study of the methylene blue cation (MB⁺) stored in an electrostatic ion storage ring in Tokyo Metropolitan University. Isolated MB⁺ ions were extracted from an electrospray ionization source and introduced into the storage ring. An excitation spectrum of isolated MB⁺ was obtained from the intensity of delayed fragmentation observed for about 1 ms after laser irradiation in the visible range.

We present here an experiment to precisely determine the 1s²2s2p (³P₁-³P₂) level splitting using laser excitation of Be-like Ar, at the Stockholm electron beam ion trap (R-EBIT). The wavelength of the above transition is in visible region of electromagnetic spectrum and accessible using a tunable laser system. The aim of the experiment is to develop precision laser spectroscopy of trapped highly charged ions.

We have calculated the effective Lande-g factors, g^* for the entangled states in Hydrogen and Hydrogen-like atoms. We show that g^* takes integer values such as 0,1,2,3,... .For (s − s) entanglement, we have g^* = 0. For (s − p) entanglement, we have g^* = 1 and so on. In general, for the entanglement of two states with angular momentum quantum numbers l₁ and l₂ we have g^* =l₁+l₂.

We investigate the spatio-temporal dynamics of short laser pulse train propagating in an absorbing medium under the conditions of electromagnetically induced transparency.

Multiphoton intrashell resonances are observed and measured quantitatively for coherent elliptic states, prepared in orthogonal DC electric and magnetic fields and perturbed by a linearly polarized RF-field. Strong suppressions of the resonances are found at certain values of the eccentricity e of the initial states.

We apply the convergent close-coupling (CCC) and time-dependent close- coupling (TDCC) methods to describe energy and angular resolved double photoionization (DPI) of lithium at arbitrary energy sharing. By doing so, we are able to evaluate the recoil ion momentum distribution of DPI of Li and make a comparison with recent measurements of Zhu et al. [Phys. Rev. Lett. 103, 103008 (2009)].

Our study of the ionization of the 1s state of the hydrogen atom through the absorption of two identical photons covers a region of higher energies than those considered up to now; it aims to establish the region of validity of nonrelativistic dipole approximation for the total generalized cross section.

We study interference effects in Compton scattering of a photon from positronium. Interference effects, which are due to the binding of an electron and a positron, are important at lower energies up to several keV. In our study we use full nonrelativistic electron/positron-photon interaction.

Decay of Kr 3p⁻¹ vacancy through cascade of Auger transitions is studied. Extended basis of interacting configurations is employed in every step of the cascade. Configuration interaction strength is used to set basis of interacting configurations for every ionization stage. Auger spectra from all steps of cascade are obtained.

The angular distributions of electrons emitted in the photoionisation of rare gas atoms using one and two photons are presented. The one-photon results show that these differential measurements can provide complementary information on the photoionisation event with respect to the measurement of the total absorption cross section while the two photon ionization allows additional parameters to be extracted from the experiments thus permitting a more complete description of the photoionisation dynamics.

Distinct oscillations in vibrationally resolved cross section ratios for the photoionization of CH₄ from the C 1s orbital at photon energies as high as 1keV are predicted. The oscillations are attributed to the different relative vibrational excitation due to the scattering of the photoelectron by the peripheral hydrogen atoms. The latter effect is also responsible for the well known EXAFS oscillations in the integrated photoelectron spectrum. The calculations are performed with an ab-initio DFT method [1], as well as with a single-particle semi-analytical model, which incorporate both the effect of the nuclear recoil and of the Coulomb corrections.

Molecular fragmentation following absorption of X-ray radiation is one of the effects of radiation damage. To investigate this process in a class of organic molecules of biological interest the inner shell excitation, ionisation, electron decay and fragmentation of the pyrimidine molecule have been fully characterised by electron and ion spectroscopies and theoretical calculations. These techniques enabled us to observe site-selective molecular fragmentation of the N and C (1s) core excited states and to explain that it is governed by the final singly charged ion state populated by resonant Auger electron decay.

Vibrational wave packets in the excited electronic states of H₂ created by a single UV pulse are probed by a time-delayed second pulse of the same characteristics. The motion of the vibrational wave packet is traced by analyzing the signature of the 2pσ_u dissociative ionization channel in the proton kinetic energy distributions at different time delays between the pulses. Moreover, since the pump and probe pulses have the same characteristics, replicas of the vibrational wave packet are created at different times leading to interference patterns in the vibrational distributions of the excited states that are probed in the ionization channels.

We consider vibrational branching ratios in the valence photoionization of N₂, CO, and XCN, with X=F, Cl, Br, and I. Non-resonant geometry dependence of the matrix elements is seen to lead to the breakdown of the Franck-Condon approximation. The geometry dependence can be due to changing molecular orbitals or changing nodal positions in the continuum wave functions.

In contrast with atomic photoionization or molecular direct photoionization, in the autoionization region, electron and proton kinetic-energy differential probabilities resulting from a short pulse cannot be reconstructed by the incoherent superposition of those resulting from long pulses.

The properties of the metallofullerene A@C₆₀ (A = Mg, Ca, Sc, Sr and Y) have been investigated. The fullerene environment is modeled by a confined attractive potential V_c(r). The electronic orbits of the atoms are obtained by solving numerically the Dirac-Fock equations including V_c(r). Modification of electronic structure by attractive well result in the remarkably redistribution of the oscillator strength. The photoionization of the ground level (3s²¹S₀) of Mg in the Mg@C₆₀ also has been calculated by using relativistic R-matrix method.

A theoretical study is presented on the dissociation of H^+..F⁻ and K^+.. Cl⁻ heavy-Rydberg ion pairs induced by a ramped electric-field, a technique used experimentally to detect and probe ion-pair states.

An experiment is presented for the precise determination of magnetic moments by double resonance spectroscopy of highly charged ions confined in a Penning trap

Double Auger decay after core-level photo excitation and after ionization through synchrotron radiation in gas phase carbon monoxide has been studied. We report the first experiment where both Auger electrons in double Auger decay have been measured in coincidence with the ionic fragments.

We calculate the selection rules for single photonic transitions in hydrogen- like atoms. We show that for the first part the reduced selection rules read: . For the second part the reduced selection rules read:

We calculate the spin dependent selection rules for photonic transitions in hydrogen-like atoms by taking the eigenfunctions of Dirac hydrogen-like atoms. It is shown that photonic transitions occur when [Δj = 0,±l,±2 while Δm_j = 0,±1,±2]. We apply the present selection rules to the so-called parity violating transition (such as 6s → 7s in atomic Cesium Cs) and show that it is possible according to the present selection rules.

We calculate the Zeeman-fine energies of atomic potassium by using the energy eigenvalues for hydrogen-like atoms for n < 11. We find 18 different energy values and 153 wavelengths some of which are the same. The calculated wavelengths are in good agreement with the observed values for potassium atom.

We study the state characteristics and conditional dynamics of interacting Rydberg atoms mutually excited to a given principal quantum number. The intershell transitions inside Rydberg atoms exposed to femtosecond laser pulses are also subject of investigation.

Optical absorption spectrum for the Ce@C₈₂ endohedral fullerene has been studied using the density-functional theory and the many-body Green's-function approaches. The geometry optimization was performed using the DMol₃ software. A plane wave approach as implemented in the ABINIT package has been used to solve the Kohn-Sham equation. Self-energy was evaluated by the GW approximation. Finally the optical absorption spectra have been calculated using the random phase approximation (RPA), RPA plus self-energy correction (RPA-GW) and the Bethe-Salpeter equation (BSE-GW).

After an initial few-cycle UV pump pulse, the time-dependent spatial locations of holes and particles produced in the valence level can be probed by exciting core electrons.

We have investigated both experimentally and theoretically the post collision interaction effect on Argon 2s photoelectrons in the case of cascade double Auger decay. New features of this effect are revealed.

The 2p photoelectron spectra of ground-state sodium atoms have been investigated. By means of large scale MCDF calculations, it is shown that the agreement between our theoretical results and the measured spectra is good if the rearrangement of the residual bound-state electron density is taken into account.

Photoionization of Xe and Xe@C₆₀ have been studied using the random phase approximation with exchange method and our C₆₀ model potential. The Xe atom was described by relaxed orbitals including overlap integrals. The Schrödinger equation for Xe@C₆₀ was solved using both regular and irregular solutions and the continuous boundary conditions at inner and outer radii of the C₆₀ shell. Better photoionization results for both Xe atom and Xe@C₆₀ molecule have been obtained and compared with the experimental data.

Dipole and quadrupole photoionization parameters for intermediate subshells of mercury have been studied using the relativistic-random-phase-approximation (RRPA). Multiple Cooper minima are observed in 5d quadrupole cross sections, even at the level of intrashell coupling. It has been found that the effect of interchannel coupling is much more significant for dipole photoionization parameters than for quadrupole parameters.

The R-matrix method was implemented to treat molecular photoionization problem based on the UK R-matrix codes. This method was formulated to treat photoionization process long before, however, its application has been mostly limited to photoionization of atoms. Application of the method to valence photoionization as well as inner-shell photoionization process will be presented.

The photofragmentation of free thymidine molecule has been studied using combined electron- and ion spectroscopy. The results show that valence- and core ionization processes produce very different fragmentation patterns and that regardless of the photon energy, the photoionization leads almost always to dissociation of the thymidine molecule.

Double Auger decay following HBr(3d) inner-shell ionization by synchrotron radiation has been used to investigate HBr³⁺ potential energy curves. The direct double Auger decay reveals the residual binding energy sustained by theoretical calculation. Sequential processes are also observed and are characterized through observation of Br^+* autoionizing states following HBr²⁺ fragmentation.

We study the pump-laser-induced nuclear dynamics in diatomic molecules in different electronic states by calculating the evolution of nuclear wave packets, starting with an initial Franck-Condon distribution.

We performed multielectron coincidence measurements for inner-shell photoionizations of ethyl trifluoroacelate molecules (C₄H₅F₃O₂) using a magnetic bottle electron spectrometer. From a two dimensional coincidence map between a photoelectron and Auger electron for C 1s ionizations, we extracted site-specific Auger electron spectra for each carbon site and corresponding binding energy of doubly charged states.

We present a detailed study of the partial and total cross section for photon induced electron emission from H⁺₂. Employing approximate continuum and bounded wavefunctions for one and two center function basis we can deduce the origin and position of the Cooper like minima in the partial cross section and the relationship with the Young-type interference.

We use extremely intense, ultrashort soft X-Ray pulses generated by the LCLS X-Ray Free Electron Laser to investigate the production of molecular double core-hole states by sequential two-photon X-Ray absorption. The effect of critical LCLS parameters such as the number of photons per pulse, the pulse duration, and the focal spot size on the photoelectron and Auger spectra is modeled in detail and the results of these simulations are used as an aid in the interpretation of the experimental spectra obtained. The emphasis is on double core-hole formation in small polyatomic molecules such as CO₂.

Auger decay following C 1s core ionization leads to dissociating doubly ionized molecular states of CO₂ and CS₂. Detailed dynamics of these three- and two-body processes is investigated using electron-ion multicoincidence spectroscopy and Monte-Carlo simulations of various dissociation scenarios.

We have used soft X-ray radiation from the Advanced Light Source (ALS) in combination with Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) to examine the correlated momenta of the photoelectron and molecular fragments arising from the dissociative photoionization of methane. In particular, we are studying (1) molecule frame photoelectron angular distributions as a stringent test of theory and (2) the interplay between electronic and nuclear dynamics through the correlated measurement of vibrationational state population and fragmentation branching ratios and kinetic energy release.

Coherent two-center photoelectron emission has been predicted for all electron subshells. So far, however, is has been only proven for the innermost 1s-subshell only. We present first results for the valence photoionization of N₂ and O₂ showing oscillatory behavior in both, the partial cross section σ and the angular distribution asymmetry parameter β.

The valence electrons of fullerenes may be regarded as spherical distributions with a finite width of a jellium-like potential giving rise to collective motions of this orange peel electron cloud. They cause strong enhancement of the photoionization cross section, a resonant behavior phenomenon know as plasmon excitations. The number and characteristic features of these excitations will be discussed.

If coherent superpositions such as the symmetry eigenstates gerade and ungerade are in turn coherently superimposed, localization occurs. This effect is studied by the intensities of the Doppler components of electrons emitted from dissociating fragments of superexcited O₂. The measurements show clear evidence for such coherent localization.

Photoionization of an atom A, in the presence of a neighboring atom B, can proceed via resonant excitation of B with subsequent energy transfer to A through two-center electron-electron correlation. We demonstrate that this two-center mechanism can strongly outperform direct photoionization at nanometer internuclear distances and possesses characteristic features in its time development and the spectrum of emitted electrons.

We consider two different Sturmian Function (SF) expansions to describe three-body break-up wave functions having outgoing flux behavior: one in hyper-spherical coordinates (ρ and α) and one in spherical coordinates (r₁ and r₂). We compare the convergence rates of the two treatment, and relate the analysis to geometry aspects of the problem.

Secondary electron spectra emitted from polycrystalline Al have been measured in coincidence with 500 eV electrons reflected after undergoing a given energy loss. Contributions from bulk- and surface-plasmon decay have been observed, as well as contributions from LVV Auger electrons and electron-electron scattering.

Time-dependent double photoionization of 2s² valence electrons of Be

In this work we present the application of the generalized Sturmian basis to the single photon - double ionization of atoms.

We observed an interference originating from coincidence detection of two indistinguishable electron pairs emitted upon photoionization of 4d electron in Xe. At 89.9 eV photon impact the energy of photoelectron ejected from 4d_5/2 orbital equals energy of Auger electron emitted in decay of 4d_3/2 hole into the [5s5p¹P] state, and for the same final state the energy of the 4d_3/2 photoelectron equals the Auger electron energy in decay of 4d_5/2 hole. An angle-integrated coincidence yield as a function of photon energy is measured with the magnetic bottle time-of-flight spectrometer and shows a peak at the expected energy position.

L_2,3 − M²_2,3 Auger decay in argon is an extremely well studied subject from the experimental, as well as the theoretical side. We add an additional insight by observing the process at photon excitation energy that introduces quantum coherence among different pairs of reaction paths or induces an essential exchange uncertainty along the same path. Coincidence measurements of ejected electron pairs are performed with ten independent electron analysers in plane perpendicular to the incoming photon beam.

Attosecond streaking is one of the most fundamental processes in attosecond science allowing for a mapping of temporal (i.e. phase) information to the energy domain. We show that on the single particle level attosecond streaking time shifts contain spectral phase information associated with the Eisenbud-Wigner-Smith time delay, as long as the influence of the streaking infrared field is properly accounted for.

Re-evaluated parameters for the square-well potential model for photoionization of endo-fullerenes are proposed and employed to reveal the spectacular modifications in the Xe 4d photoionization giant resonance along the path from Xe@C₆₀ to Xe@C₂₄₀ to multi-walled Xe@C₆₀@C₂₄₀.

Many electron correlation effects on dipole (E1) and quadrupole (E2) photoabsorption processes are investigated for the Ar isoelectronic sequence from Cl⁻ to Cd³⁰⁺.

The LS R-matrix method was used to compute new photoionization cross sections for Fe II. Results are compared with available experimental data and with previous calculations of the cross section. We also present the first fine-structure photoionization data for this ion obtained with the fully-relativistic DARC codes.

K-shell photoionization cross-section measurements for singly ionized atomic nitrogen made at the SOLEIL Light Source for the first time are compared with MCDF and R-matrix calculations.

Absolute cross sections for K-shell photoionization of Be-like B⁺ ions were measured employing the photon-ion merged-beam technique at the Advanced Light Source in Berkeley. The results are found to be in fair agreement with R-matrix calculations using L-S coupling.

The K-shell photodetachment spectrum of O^- has been investigated using the merged ion-photon beam photo-detachment technique. O^- ions were produced in a Cs sputtered ion source on a new Movable Ion Photon Beamline (MIPB) while the photons were produced at the Advanced Light Source Beamline 8.0.1. Positive oxygen ions formed by multiple detachment were detected as a function of photon energy. Photoexcitation of a 1s electron leads to a short-lived Feshbach resonance ~3 eV below the 1s detachment threshold due to the extra stability of the now full 2p⁶ shell.

A photodetachment experiment has been performed with the formation of atoms in highly excited states of high positive and negative polarizabilities. A semiclassical model has been developed to describe the effect of the repulsive polarization potential in the vicinity of the photoreaction threshold.

An apparatus for measurements of partial photodetachment cross sections has been constructed. In the first measurements a number of previously unobserved resonances in K⁻ were observed.

Recent experimental works report observations of a significant vibrational dependence of the photoelectron angular distributions (PADs) recorded for the O₂(X³Σ_g) ← O⁻₂ (X²Π_g) band. It is the aim of the theoretical model presented here to reproduce the experimental results, allow for a deeper insight into the mechanism of this process and explain the sensitivity of the PAD to vibronic coupling in the anion ground electronic state.

Photodissociation of fluorescein monoanions was studied by using an electrostatic storage ring. The ions are stored in an ion trap and the storage ring before laser irradiation. Photodissociation neutral spectra of the relaxed ions as a function of time consist of components with short and long lifetimes. By comparing their wavelength spectra with those in the liquid phase, it is concluded that the spectra originate from different tautomers of fluorescein monoanions. Moreover, via storage in the storage ring, one of them disappears due to interconversion.

We investigate the photodissociation process for both hot and cold HeH⁺ ions. Detailed comparison with experiments performed at FLASH is made possible by experimentally determining the ro-vibrational distribution of the ions under identical source conditions, and averaging the theoretical cross sections accordingly.

Fragmentation of small organic molecules by intense few-femtosecond X-ray free-electron laser pulses has been studied using Coulomb explosion imaging. By measuring kinetic energies and emission angles of the ionic fragments in coincidence, we disentangle different fragmentation pathways, for certain cases can reconstruct molecular geometry at the moment of explosion, and show how it depends on LCLS pulse duration.

Although it is thought to play an important role in the chemistry of some extra-terrestrial environments, the HeH⁺ cation has not been detected in space so far. We suggest it could be observed in its triplets rather than singlet states and we study the formation by radiative stabiliation and the destruction by photodissociation of the two lowest states of this symmetry.

A molecular thermometer based on the momentum distributions of the ionic fragments has been founded. The excitation temperature of fragmented C₆₀ was determined using the novel thermometer. The results provide a clear evidence that a first-order phase transition in the hot C₆₀ is occurring at the temperature around 6050K ± 250K.

Ab initio and mass-resolved fragmentation study of thermally evaporated selenium clusters Se_n induced by VUV synchrotron radiation.

The photo-fragmentation of ammonia molecular clusters ionized with soft x-rays is studied for photon energies near the N-1s threshold. The fragmentation is studied with a 3D momentum mass spectrometer to access the energy and angular correlations between fragments. By choosing the cluster sizes below or above the critical size of stable dication, we investigate the coulomb explosion dynamics and the interplay between charge delocalization and mobility of molecules in the clusters.