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Physical review / A 104(5), 053102 (2021). doi:10.1103/PhysRevA.104.053102, The effects of postcollision interaction (PCI) in the time evolution of photoinduced Auger decay, assisted by a terahertz (THz) field, is theoretically investigated. We propose a time-dependent model which is based on the quantum-mechanical description of the photoinduced Auger process and quasiclassical description of the PCI. The suggested model is used for calculating the PCI distortion of the Auger spectrum at different temporal overlaps of the exciting extreme ultraviolet pulse and the streaking THz pulse., Published by Inst., Woodbury, NY

Journal of physics / B 53(18), 184004 (2020). doi:10.1088/1361-6455/ab9c38, Ultra-short extreme ultraviolet pulses from the free-electron laser FLASH are characterized using terahertz-field driven streaking. Measurements at different ultra-short extreme ultraviolet wavelengths and pulse durations as well as numerical simulations were performed to explore the application range and accuracy of the method. For the simulation of streaking, a standard classical approach is used which is compared to quantum mechanical theory, based on strong field approximation. Various factors limiting the temporal resolution of the presented terahertz streaking setup are investigated and discussed. Special attention is paid to the cases of very short (∼10 fs) and long (up to∼350 fs) pulses., Published by IOP Publ., Bristol

Intense X-ray free-electron lasers (XFELs) can rapidly excite matter, leaving it in inherently unstable states that decay on femtosecond timescales. As the relaxation occurs primarily via Auger emission, excited state observations are constrained by Auger decay. In situ measurement of this process is therefore crucial, yet it has thus far remained elusive at XFELs due to inherent timing and phase jitter, which can be orders of magnitude larger than the timescale of Auger decay. Here, we develop a new approach termed self-referenced attosecond streaking, based upon simultaneous measurements of streaked photo- and Auger electrons. Our technique enables sub-femtosecond resolution in spite of jitter. We exploit this method to make the first XFEL time-domain measurement of the Auger decay lifetime in atomic neon, and, by using a fully quantum-mechanical description, retrieve a lifetime of $2.2^{ + 0.2}_{ - 0.3}$ fs for the KLL decay channel. Importantly, our technique can be generalised to permit the extension of attosecond time-resolved experiments to all current and future FEL facilities., Comment: Main text: 20 pages, 3 figures. Supplementary information: 17 pages, 6 figures

Angle resolved spectra of photoelectrons generated by multiphoton ionization in the presence of THz-light fields were measured. We observed a strong electron modulation for different THz phases, which we attribute to momentum transfers caused by rescattering at the ionic core.

Optics express 29(21), 32739 - 32754 (2021). doi:10.1364/OE.432761, A fast and robust, yet simple, method has been developed for the immediate characterization of x-ray pulse durations via IR/THz streaking that uses the center of energy (COE) of the photoelectron spectrum for the evaluation. The manuscript presents theory and numerical models demonstrating that the maximum COEs shift as a function of the pulse duration and compares them to existing data for validation. It further establishes that the maximum COE can be derived from two COE measurements set at a phase of $��$/2 apart. The theory, model, and data agree with each other very well, and they present a way to measure pulse durations ranging from sub-fs to tens of fs on-the-fly with a fairly simple experimental setup., Published by Soc., Washington, DC

A simple and very fast method for reconstruction of temporal structure of linearly polarized FEL pulses from the THz streaking spectra of photoelectrons is suggested. The method is based on a quantum mechanical approach within the strong-field approximation. The method is suitable for online retrieval of the temporal characteristics of the FEL pulses. It can be applied for any photon frequency in a broad range of FEL pulse duration with a proper selection of the streaking field. To enhance its accuracy, it is suggested to simultaneously analyze the streaking spectra for several emission angles.

International Conference on Spectral Lines Shapes, Dublin, Ireland, 17 Jun 2018 - 22 Jun 2018; Journal of physics / Conference Series Conference Series 1289, 012030 - (2019). doi:10.1088/1742-6596/1289/1/012030, In this work, we present measurements of the intensity-dependent photoelectron spectrum of Kr irradiated by the FLASH FEL tuned to a photon energy of 25.8 eV. Intensity dependent photoelectron spectra were obtained with the aid of a Velocity Map Imaging (VMI) spectrometer. As the FEL photon energy is close to threshold, two photon sequential double ionization is favoured. The number of open channels is kept to a minimum and leading to a simple description of the process., Published by IOP Publ., Bristol

Attosecond time-resolved photoemission from the differently terminated BiTeCl surfaces yield a photoelectron streaking that cannot be explained by bulk propagation effects alone. Instead, the atomic scale electronic structure and dynamical screening for both surface terminations have to be taken into account.

The unprecedented combination of transient absorption and ion mass spectroscopy with attosecond resolution is used to study and control the complex multidimensional excitation and decay cascade of an ultrafast Auger process in krypton.

Journal of physics / B 54(8), 085601 (2021). doi:10.1088/1361-6455/abf154, Auger electron spectra were simulated in the presence of a terahertz streaking field to study post-collision interaction (PCI) effects in the time-evolution of photoinduced Auger decays. The PCI is characterized by the ratio of the spectral line width for Auger electron emission in opposite directions with respect to the THz-field. These calculations have been performed using the analytical semiclassical model developed by Bauch and Bonitz (2012 Physical Review A 85 053416). The results are shown for Ne(KLL), Ar(LMM), Kr(MNN) and Xe(NOO) Auger transitions whereby different possible experimental conditions were evaluated., Published by IOP Publ., Bristol

The photoelectron angular distribution and the circular dichroism in two-colour XUV+NIR above-threshold ionization of helium atoms have been investigated both experimentally and theoretically. Circularly polarized XUV pulses from the free electron laser FERMI have been spatially and temporally overlapped with circularly polarized optical pulses in the interaction region with an atomic helium jet. The emitted electrons were energy and angle analyzed by means of a velocity map imaging spectrometer. Asymmetry parameters of the angular distribution were determined and compared to theoretical predictions based on the strong field approximation and perturbation theory, respectively. For low NIR intensities, the ratio of the partial waves in the two-photon ionization process and their relative phase could be deduced. For high NIR intensities, the influence of multi-photon processes is discussed. Circular dichroism was revealed in both cases and is in good agreement with the results of the strong field approximation.

Two-color photoionization of atomic He has been investigated by angle-integrated and angle-resolved electron spectroscopy. The combined action of intense radiation pulses from the XUV free-electron laser (FEL), FERMI or FLASH, and a synchronized optical laser on the target atom gives rise to a rich sideband structure in the photoemission spectrum. Measurements of the angular distribution parameters and the determination of the circular and linear dichroism for the two-color photoionization enable a detailed analysis of the symmetry of the outgoing electron waves and of the dynamics underlying the multi-photon processes. The experimental results are in excellent agreement with theoretical results obtained using perturbation theory (low intensity regime) and the strong field approximation. For the particular case of two-photon ionization the measurements represent an ideal tool for characterizing certain FEL parameters, here for example the degree and the sign of circular polarization. Finally, new features of the dichroism are theoretically predicted originating from the non-dipole contribution into the photoionization amplitudes.

We present a review of theoretical studies of the simplest nonlinear photoprocesses detected in the XUV range with the use of free electron lasers: sequential double and triple ionization of atoms by two and three XUV photons. Photoelectron angular distributions and angular correlations between emitted electrons are considered. A comparison of the calculated results with recent angle-resolved photoelectron spectroscopy experiments is discussed.

Ultrafast spectroscopy with attosecond resolution has enabled the real time observation of ultrafast electron dynamics in atoms, molecules and solids. These experiments employ attosecond pulses or pulse trains and explore dynamical processes in a pump–probe scheme that is selectively sensitive to electronic state of matter via photoelectron or XUV absorption spectroscopy or that includes changes of the ionic state detected via photo-ion mass spectrometry. Here, we demonstrate how the implementation of combined photo-ion and absorption spectroscopy with attosecond resolution enables tracking the complex multidimensional excitation and decay cascade of an Auger auto-ionization process of a few femtoseconds in highly excited krypton. In tandem with theory, our study reveals the role of intermediate electronic states in the formation of multiply charged ions. Amplitude tuning of a dressing laser field addresses different groups of decay channels and allows exerting temporal and quantitative control over the ionization dynamics in rare gas atoms., Photoionization of atoms and molecules is a complex process and requires sensitive probes to explore the ultrafast dynamics. Here the authors combine transient absorption and photo-ion spectroscopy methods to explore and control the attosecond pulse initiated excitation, ionization and Auger decay in Kr atoms.

Attosecond time-resolved photoemission spectroscopy reveals that photoemission from solids is not yet fully understood. The relative emission delays between four photoemission channels measured for the van der Waals crystal tungsten diselenide (WSe) can only be explained by accounting for both propagation and intra-atomic delays. The intra-atomic delay depends on the angular momentum of the initial localized state and is determined by intra-atomic interactions. For the studied case of WSe, the photoemission events are time ordered with rising initial-state angular momentum. Including intra-atomic electron-electron interaction and angular momentum of the initial localized state yields excellent agreement between theory and experiment. This has required a revision of existing models for solid-state photoemission, and thus, attosecond time-resolved photoemission from solids provides important benchmarks for improved future photoemission models., This work was supported by the German Research Foundation (DFG) within the Collaborative Research Center (SFB) 613 (F.S., P.B., W.P., and U.H.), the Priority Programs SPP 1931 (C.S., M.H., and W.P.), and SPP 1840 (St.F., S.N., and W.P.); the Basque Government (grant IT-756-13 UPV/EHU) (V.M.S., E.E.K., R.D.M., P.M.E., and A.K.K.); and the Spanish Ministerio de Economía y Competitividad (grants FIS2016-76617-P and FIS2016-76471-P) (V.M.S., E.E.K., R.D.M., P.M.E., and A.K.K.) and Fondo Europeo de Desarrollo Regional (FEDER) (CTQ2016- 80375-P) (M.T.-S.). N.M.K. acknowledges hospitality and financial support from the theory group in cooperation with the small quantum systems (SQS) research group of European XFEL.

We report on precise measurements of absolute nonlinear ionization probabilities obtained by exposing optically trapped ultracold rubidium atoms to the field of an ultrashort laser pulse in the intensity range of $1 \times 10^{11}$ to $4 \times 10^{13}$ W/cm$^2$. The experimental data are in perfect agreement with ab-initio theory, based on solving the time-dependent Schr\"odinger equation without any free parameters. Ultracold targets allow to retrieve absolute probabilities since ionized atoms become apparent as a local vacancy imprinted into the target density, which is recorded simultaneously. We study the strong-field response of $^{87}$Rb atoms at two different wavelengths representing non-resonant and resonant processes in the demanding regime where the Keldysh parameter is close to unity., Comment: 13 pages, 7 figures. This is a pre-print of an article published in Communications Physics. The final authenticated version is available online at: https://doi.org/10.1038/s42005-018-0032-5

Physical review letters 118(1), 013002 (2017). doi:10.1103/PhysRevLett.118.013002, Intense, circularly polarized extreme-ultraviolet and near-infrared (NIR) laser pulses are combined to double ionize atomic helium via the oriented intermediate He+(3p) resonance state. Applying angle-resolved electron spectroscopy, we find a large photon helicity dependence of the spectrum and the angular distribution of the electrons ejected from the resonance by NIR multiphoton absorption. The measured circular dichroism is unexpectedly found to vary strongly as a function of the NIR intensity. The experimental data are well described by theoretical modeling and possible mechanisms are discussed., Published by APS, College Park, Md.

A method of angular streaking with a rotating terahertz electric field for photoelectrons produced by femtosecond extreme ultraviolet pulses is suggested and theoretically analyzed. The method can be used for free electron laser (FEL) pulse characterization on a shot-to-shot basis. It is shown that in related measurements an interesting phenomenon appears: formation of very bright and sharp features in the angular resolved electron spectra measured in the plane perpendicular to the collinear beam direction. These features are similar to the conventional caustics in the wave propagation. The caustics are accompanied by a well-developed interference structure. The intensity distribution along the caustic is determined by the envelope of the FEL pulse.

Review of scientific instruments 87(8), 083113(2016). doi:10.1063/1.4961470, A non-destructive diagnostic method for the characterization of circularly polarized, ultraintense, short wavelength free-electron laser (FEL) light is presented. The recently installed Delta undulator at the LCLS (Linac Coherent Light Source) at SLAC National Accelerator Laboratory (USA) was used as showcase for this diagnostic scheme. By applying a combined two-color, multi-photon experiment with polarization control, the degree of circular polarization of the Delta undulator has been determined. Towards this goal, an oriented electronic state in the continuum was created by non-resonant ionization of the O$_2$ 1s core shell with circularly polarized FEL pulses at hν ≃ 700 eV. An also circularly polarized, highly intense UV laser pulse with hν ≃ 3.1 eV was temporally and spatially overlapped, causing the photoelectrons to redistribute into so-called sidebands that are energetically separated by the photon energy of the UV laser. By determining the circular dichroism of these redistributed electrons using angle resolving electron spectroscopy and modeling the results with the strong-field approximation, this scheme allows to unambiguously determine the absolute degree of circular polarization of any pulsed, ultraintense XUV or X-ray laser source., Published by American Institute of Physics, [S.l.]

The ability to fully characterize ultrashort, ultra-intense X-ray pulses at free-electron lasers (FELs) will be crucial in experiments ranging from single-molecule imaging to extreme-timescale X-ray science. This issue is especially important at current-generation FELs, which are primarily based on self-amplified spontaneous emission and radiate with parameters that fluctuate strongly from pulse to pulse. Using single-cycle terahertz pulses from an optical laser, we have extended the streaking techniques of attosecond metrology to measure the temporal profile of individual FEL pulses with 5 fs full-width at half-maximum accuracy, as well as their arrival on a time base synchronized to the external laser to within 6 fs r.m.s. Optical laser-driven terahertz streaking can be utilized at any X-ray photon energy and is non-invasive, allowing it to be incorporated into any pump–probe experiment, eventually characterizing pulses before and after interaction with most sample environments.

The cascaded Auger decay following ionization or excitation with 94 eV soft-X-ray pulses from the 3d subshell in krypton has been energy-and-time-resolved for the first time. The decay time of the 4s −14p −2 nl → 4p −3 + e and/or 4s −14p −1 nl → 4p −2 + e transition is measured to be 74 ± 20 fs. Furthermore, our data shows that the electrons with a kinetic energy around 25 eV (generally assigned as 3d −1 np → 4s −2 n′p + e) emitted after core excitation are emitted in a second decay step suggesting an alternative assignment of the form 4s −2 np 4p −2 + e.

A simple semi-empirical description is proposed for atomic photoionization parameters with attosecond sweep on the basis of the sudden perturbation approximation. The results of model calculations are in good agreement with the complete quantum-mechanical description of the process based on the numerical solution of the time-dependent Schrodinger equation, including the field of the atom as well as the fields of VUV and IR pulses.

X-ray free-electron lasers (XFELs) may allow us to employ the single-particle imaging (SPI) method to determine the structure of macromolecules that do not form stable crystals. Ultrashort pulses of 10 fs and less allow us to outrun complete disintegration by Coulomb explosion and minimize radiation damage due to nuclear motion, but electronic damage is still present. The major contribution to the electronic damage comes from the plasma generated in the sample that is strongly dependent on the amount of Auger ionization. Since the Auger process has a characteristic time scale on the order of femtoseconds, one may expect that its contribution will be significantly reduced for attosecond pulses. Here we study the effect of electronic damage on the SPI at pulse durations from 0.1 to 10 fs and in a large range of XFEL fluences to determine optimal conditions for imaging of biological samples. We analyzed the contribution of different electronic excitation processes and found that at fluences higher than ${10}^{13}--10{}^{15} \mathrm{photons}/\ensuremath{\mu}{\text{m}}^{2}$ (depending on the photon energy and pulse duration) the diffracted signal saturates and does not increase further. A significant gain in the signal is obtained by reducing the pulse duration from 10 to 1 fs. Pulses below a duration of 1 fs do not give a significant gain in the scattering signal in comparison with 1-fs pulses. We also study the limits imposed on SPI by Compton scattering.

The energy and angular distributions of photoelectrons (the double differential cross sections, DDCS) for attosecond-pulse induced photoemission from atoms in the presence of a strong laser field are theoretically considered. We suggest a comparatively simple computational method based on the strong field approximation for calculation of the DDCS of short pulse photoionization which explicitly includes competing processes of interference and dephasing of the electronic wave packets. The method can be useful for analysis of attosecond streaking experiments. It is tested by comparison with the exact quantum-mechanical computation based on a numerical solving of the time-dependent Schrodinger equation. An excellent agreement between the two computational methods is demonstrated in a wide range of the pulse durations.

A non-stationary method of calculation of energy distributions of emitted electrons in near-threshold resonant (sequential) double photoionization of atoms by attosecond pulses is developed. The method is based on the numerical solution of a system of time-dependent equations describing photoionization and the following Auger decay. As an example, a double-differential cross section of the sequential double photoionization of Ne is considered. It is shown that the shape of the electron energy distribution strongly depends on the pulse duration.

Electron spectra from radiationless decay of a Ne 1s vacancy are measured with excitation energies immediately at the 1s ionization threshold. Special attention is given to the excess energy region where Γ is the lifetime width of the 1s vacancy. Spectra are dominated by recapture of photoelectrons into Rydberg states of Ne+ above the threshold, and by a smooth transition into the spectator resonant Auger spectrum below the threshold. High resolution of the present experiment allowed us to clearly observe Rydberg states up to n = 8 and to determine the individual probabilities for populating of these states by photoelectron recapture. Results are compared with calculations based on the semiclassical theory of post-collision interaction extended to high Rydberg excitations of the primary photoexcited electron, as well as with a non-stationary quantum mechanical theory of near-threshold photoionization with subsequent Auger decay.

We employ a novel technique in which highly excited Rydberg states of Ne+ 2p4np are populated via PCI-induced recapture of the near-threshold 1s photoelectron (De Fanis et al 2004 Phys. Rev. A 70 040702) to investigate valence inter-multiplet Auger transitions. The following series of the transitions have been observed: Ne+ 2p4(1D)np 2L → Ne2+ 2p4 3PJ, Ne+ 2p4(1S)np 2P →Ne2+ 2p4 3PJ and Ne+ 2p4(1S)np 2P →Ne2+ 2p4 1D. Their energy positions, quantum defects and the anisotropy parameters of the Auger electron emission have been determined. Experimental results are in good agreement with multi-configuration Dirac–Fock calculations carried out as a part of this study. The importance of interference effects for decays via naturally overlapping fine-structure components of the intermediate state is discussed.

Under certain conditions specified in this work, a two-particle orbital orientation of bound states can be detected by means of double photoemission with polarized light. These conclusions are based on a tensorial analysis of the dipole transition amplitudes and are illustrated by a numerical study of double photoemission from helium oriented by laser-pumping. The analytical and numerical results demonstrate the interrelation between two-particle correlation and orbital polarization.

We present the results of high-resolution measurements of the low-energy part of the Auger electron spectrum generated by photoexcitation of the 1s −1 3p resonance in Ne. The spectrum was measured in the energy range of 10– 33 eV where mainly the electrons from the second-step Auger transitions of the cascades 1s −1 3p → 2s2p 5 np → 2s 2 2p 4 and 1s −1 3p → 2s 0 2p 6 np → 2s2p 5 (n = 3, 4) are detected. High resolution allows us to study the multiplet structure of the transitions and to determine the widths of the intermediate states of the Ne + ion. Anisotropy of the angular distribution of Auger electrons is determined for all of the lines observed. The results are in reasonably good agreement with MCDF calculations.

Double ionization of a polarized atom by a single photon with an arbitrary polarization is theoretically considered. A general expression for the triple differential cross section is obtained and analysed. We point out the existence of a circular and a linear dichroism in the angular distributions of photoelectrons. Generally two types of sources for the dichroism are distinguished: one connected with the initial polarization of the target, and the other related to the two-electron final-state properties. We show how these two types of dichroic effects can be experimentally investigated separately by choosing the detection geometry appropriately. As an example, the photo-double ionization of 2P excited and polarized states of atomic helium is considered. The calculations reveal a measurable dichroism in the angular correlations of two emitted electrons. Possible applications of circular dichroism measurements to the study of superconductors and orbitally ordered materials are discussed.

General relations between dynamical parameters describing the spin polarization and the angular distribution of Auger or autoionization electrons emitted from atoms have been established. They are derived on the basis of known properties of the angular momentum coupling coefficients, therefore they are exact and independent of the decay dynamics. All similar relations previously found for certain Auger transitions as well as for photoionization follow, as particular cases, from the general ones.

Tailored extreme ultraviolet (EUV) multilayer systems have been designed, fabricated and characterized for applications as optical key components for guiding or monochromatizing ultrashort sub-femtosecond EUV pulses emitted from Ti:Sa laser-driven high harmonic sources. While spectral filtering of individual harmonic requires a periodic multilayer mirror exhibiting a small reflection bandwidth

Spin polarization of the resonant Auger electrons is measured following the decay of Kr 3d(-1)5p states resonantly excited by circularly polarized light. A large polarization transfer is found in particular for all strong transitions to the 4s(-1)4p(-1)5p and 4s(-2)5p states. The experimental results are in excellent agreement with calculations carried out by means of the multiconfiguration Dirac-Fock method.

We have studied angular correlations between resonant Auger electrons and successive second-step Auger electrons emitted in a cascade decay after photoexcitation of resonances 3d −1 5/2 5p in Kr and 4d −1 5/2 6p in Xe. Two spectrometers were used for the angle-resolved detection of the two electrons in coincidence. Angular correlations for several pairs of strong lines have been measured in both atoms. The experimental results are compared with multiconfiguration Dirac–Fock calculations which include electron correlations in the initial, intermediate and final states of the cascade. In general, good agreement between experiment and theory is obtained.

Intrinsic parameters describing the spin polarization and the angular distribution of Auger electrons emitted from atoms are considered. Recently it was found (Schmidtke B, Khalil T, Drescher M, M?ller N, Kabachnik N M and Heinzmann U 2000 J. Phys. B: At. Mol. Opt. Phys. 33 5225) that not all of them are independent. We suggest an algorithm for finding explicit relations between the intrinsic parameters for any particular Auger transition. As an example, the Auger decay of a vacancy with the initial angular momentum J = 3/2 is considered. All independent equations relating the intrinsic parameters are found for this case.

Synopsis Intense, circularly polarized extreme-ultraviolet (XUV) and near-infrared (NIR) laser pulses are combined to double-ionize atomic helium via the oriented intermediate He+ (3p) resonance state. A strong and NIR-intensity dependent circular dichroism is accounted to a helicity dependent AC-Stark shift.

A drastic modification of the conventional streaking patterns of double differential cross section for attosecond near-threshold ionization of atoms in a strong infrared (IR) field is theoretically investigated. Rescattering of slow electrons from the ionic core leads to appearance of the interference fringes and strong diffraction maxima in the spectra of photoelectrons at small energies. The computations based on solving the time-dependent Schr\"odinger equation show that both these spectral features strongly depend on the IR intensity and frequency. Investigations of low-energy streaking can provide unique information on slow photoelectron dynamics in combined fields of ionic core and strong IR laser.

We study theoretically the photoelectron angular distributions (PADs) from two-color two-photon near-threshold ionization of hydrogen and noble gas (He, Ne, and Ar) atoms by a combined action of femtosecond extreme ultraviolet (EUV) and near-infrared (IR) laser pulses. By using second-order time-dependent perturbation theory, we clarify how the two-photon ionization process depends on the EUV-IR pulse delay and how it is connected to the interplay between resonant and nonresonant ionization paths. Furthermore, by solving the time-dependent Schr\"odinger equation, we calculate the anisotropy parameters ${\ensuremath{\beta}}_{2}$ and ${\ensuremath{\beta}}_{4}$ as well as the amplitude ratio and relative phase between partial waves characterizing the PADs. We show that, in general, these parameters notably depend on the time delay between the EUV and IR pulses, except for He. This dependence is related to the varying relative role of resonant and nonresonant paths of photoionization. Our numerical results for H, He, Ne, and Ar show that the pulse-delay effect is more pronounced for $p$-shell ionization than for $s$-shell ionization.

Photoelectron angular distributions (PADs) dependence on the time-delay between EUV-FEL and IR laser pulses is observed for ionization of Ne. PADs for Rydberg excited oneand two-IR photon above-threshold ionization of He are measured. Ultrafast Phenomena 2014 © 2014 OSA.

Ultrafast extreme ultraviolet and X-ray free-electron lasers are set to revolutionize many domains such as bio-photonics and materials science, in a manner similar to optical lasers over the past two decades. Although their number will grow steadily over the coming decade, their complete characterization remains an elusive goal. This represents a significant barrier to their wider adoption and hence to the full realization of their potential in modern photon sciences. Although a great deal of progress has been made on temporal characterization and wavefront measurements at ultrahigh extreme ultraviolet and X-ray intensities, only few, if any progress on accurately measuring other key parameters such as the state of polarization has emerged. Here we show that by combining ultra-short extreme ultraviolet free electron laser pulses from FERMI with near-infrared laser pulses, we can accurately measure the polarization state of a free electron laser beam in an elegant, non-invasive and straightforward manner using circular dichroism. © 2014 Macmillan Publishers Limited.

Resonant Auger transitions from the 3d-15p photoexcited states in Kr to the 4s-14p-15p and 4s-25p levels and subsequent Auger decay to the 4p4 and 4s4p5 states of Kr2+ are studied by means of angle-resolved electron spectroscopy. The angular anisotropy parameters β are determined for most of the prominent lines in the spectrum of emitted electrons. It is found that the first-step spectator transitions are strongly anisotropic, while the second-step participator transitions are almost isotropic. Extensive multiconfiguration Dirac-Fock calculations are performed which include correlations in all of the states involved. Noticeable improvements are achieved in comparison with early calculations. The energy position of the main spectral lines and their intensities are described satisfactorily. It turns out that the interference effects due to coherent excitation of resonances influence the anisotropy of the Auger emission significantly. The calculated β parameters agree very well with the experiment.

New relations between the intrinsic parameters δk which describe the longitudinal spin polarization of Auger electrons and αk which describe the anisotropy of their angular distribution are found. The relations are valid for arbitrary Auger transitions with initial (Ji) and final (Jf) angular momenta satisfying the condition Ji > Jf.

A new relation between the intrinsic parameters δ1 and ξ1 which describe the spin polarization of Auger electrons emitted in the decay of the oriented j = 1/2 vacancy is found. A similar relation for intrinsic parameters describing the resonant Auger photoemission from j = 1 states is discussed. The implication for a complete experiment in Auger decay is considered.

We report the first measurement of the kinetic energy release distributions of multiply charged oxygen ions produced in collisions of O2 molecules with 5.9 MeV/amu Xe18+ and Xe43+ ions. The slow Oq+ ions generated in the collision are detected in coincidence by a time- and position sensitive multi- particle detector. An analysis of correlated momenta of molecular fragments provides the kinetic energy release distribution for various reaction pairs O2 → Oq1+Oq2+ with a total charge up to q = q1 + q2 = 11. The position of the maximum and the width of the distributions are well described by the recently suggested Average Bunch Statistical (ABS) model.

The intrinsic parameters β1 and γ1 were determined separately for the first time by explicit measurement of the angular distribution of the transferred spin polarization of Xe N4O2,3O2,3 3P1 Auger electrons after primary excitation with circularly polarized light from the BESSY-II helical undulator UE56/1. The feasibility of a complete experiment for the Auger decay, i.e. extraction of Coulomb matrix elements from the experimental data, is discussed and reveals a new fundamental equation connecting the intrinsic parameters of Auger decay. An analysis procedure is performed, yielding a well confined solution space for amplitude ratios and phase shift differences.

The spectra of Auger electrons are measured for the cascade decay of the photoexcited 1s-13p resonance in Ne. Angular distributions of the Auger electrons for the prominent peaks of the spectra are determined. It is shown that in the first step of the cascade which corresponds to the spectator (shake-modified spectator) transitions 1s-13p→2s-12p-13p(4p) and 1s-13p→2s-23p(4p) the electrons are emitted almost isotropically. In contrast, the majority of the second-step transitions 2s-12p-13p(4p)→2p-2 and 2s-23p(4p)→2s-12p-1 shows strong anisotropy. A qualitative explanation of the observed results is given.

We present measurements of the resonant Auger electron emission at an electron kinetic energy of ~810 eV in the Ne atom following 1s → 3p excitation at an excitation photon energy of 867.12 eV, with a photon band pass of 60-68 meV, an electron energy resolution of 13 meV, and the Doppler width due to thermal motion of the sample Ne atoms of 79 meV. Under these conditions, the overall resolution of 100-105 meV (FWHM) is much smaller than the natural width, 250 meV, of the Ne 1s hole state, and the resonant Auger final states 2p-2(1D2)3p (or 4p) 2P, 2D, 2F are completely (or partially) resolved. The obtained β values and branching ratios are in good agreement with first principles calculations by means of the multi-configuration Dirac-Fock methods within the framework of the two-step model.