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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

In this joint experimental and theoretical study we demonstrate coherent control of the optical field emission and electron transport in plasmonic gaps subjected to intense single-cycle laser pulses. Our results show that an external THz field or a minor dc bias, orders of magnitude smaller than the optical bias owing to the laser field, allows one to modulate and direct the electron photocurrents in the gap of a connected nanoantenna operating as an ultrafast nanoscale vacuum diode for lightwave electronics. Using time-dependent density functional theory calculations we elucidate the main physical mechanisms behind the observed effects and show that an applied dc field significantly modifies the optical field emission and quiver motion of photoemitted electrons within the gap. The quantum many-body theory reproduces the measured net electron transport in the experimental device, which allows us to establish a paradigm for controlling nanocircuits at petahertz frequencies., The support from the Department of Education of the Basque Government through Project No. PI2017-30 (J.A., A.K.K., and G.A.) and Project No. IT1164-19 for con solidated groups (J.A.) is gratefully acknowledged. A.K.K. acknowledges support from Project No. FIS2016-76617- P of the Spanish Ministry of Economy and Competitiveness. D.B. and A.L. acknowledge support of the Deutsche Forschungsgemeinschaft through the Emmy Noether programme (No. BR 5030/1-1) and the collaborative research center SFB 767. D.B. acknowledges the support by the European Research Council through Grant No. 819871 (UpTEMPO).

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.

We report measurements of the temporal dynamics of the valence band photoemission from the magnesium (0001) surface across the resonance of the Γ surface state at 134 eV and link them to observations of high-resolution synchrotron photoemission and numerical calculations of the time-dependent Schrödinger equation using an effective single-electron model potential. We observe a decrease in the time delay between photoemission from delocalized valence states and the localized core orbitals on resonance. Our approach to rigorously link excitation energy-resolved conventional steady-state photoemission with attosecond streaking spectroscopy reveals the connection between energy-space properties of bound electronic states and the temporal dynamics of the fundamental electronic excitations underlying the photoelectric effect., We thank F. Siegrist for experimental assistance, and we acknowledge financial support by the Munich Centre for Advanced Photonics (MAP). R. K. acknowledges an ERC Consolidator Grant “AEDMOS” (ERC-2014-CoG AEDMOS). D. P. acknowledges support from the “MEDEA” (H2020- MSCA-ITN-2014-641789-MEDEA).

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

Time-dependent Schrodinger equation simulations for a one-dimensional model potential reveal that the delay extracted from a streaking spectrogram does not reflect the photoemission time if the streaking field inside the solid cannot be neglected.

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.

We use classical electrodynamics, time-dependent density functional theory, and random-phase approximation to study the gap plasmons propagating in the nm-wide gap between metal surfaces. Particular emphasis is given to the quantum effects emerging when the junction is functionalized with a nanostructure supporting unoccupied gap localized electronic states. With the example of a quantum well (QW) introduced in the junction we show that the optically assisted electron transport across the junction via the gateway QW localized electronic states might strongly affect the lifetime and the propagation length of the gap plasmon. The coupling to the single-particle electron-hole excitations from occupied electronic states at metal surfaces into the QW-localized electronic states provides an efficient decay channel of the gap plasmon mode. Different from the through-gap electron tunneling discussed in the plasmonics literature, the electron transport involving the gateway electronic state is characterized by the threshold behavior with plasmon frequency. As a consequence, the dynamics of the gap plasmon can be controlled by varying the binding energy of the QW-localized electronic state. In more general terms, our results demonstrate strong sensitivity of the gap plasmons to the optically assisted electron transport properties of the junction which opens further perspectives in design of nanosensors and integrated active optical devices.

We report on the theoretical study of the second and third harmonic generation in plasmonic dimer nanoantennas with narrow gaps. Our study is based on the time dependent density functional theory. This allows us to address the nonlinear response of a tunneling junction in a subnanometric plasmonic gaps with a quantum calculation, which goes beyond conventional classical local and nonlocal treatments. We demonstrate that the nonlinear electron transport in a plasmonic junction, associated to the corresponding strong field enhancement in the narrow gap, allows to reach orders of magnitude enhancement in the efficiency of the second and third harmonic generation. Depending on the size of the junction and the frequency of the fundamental incident wave, we show that the frequency conversion in plasmonic dimer gaps can be determined by (i) the intrinsic nonlinearity of each individual nanoparticle, (ii) the nonlinear ac tunneling current across the gap, and (iii) the resonant excitations of the plasmon modes of the dimer. The study of the nonlinear response of plasmonic gaps within a full quantum treatment allows us to understand the fundamental mechanisms of nonlinearity in nanoplasmonics., G.A., R.E., and J.A. acknowledge the projects FIS2016-80174-P from Spanish MINECO, project 70NANB15H32 from U.S. Department of Commerce, National Institute of Standards and Technology. G.A., R.E., and A.K.K. acknowledge project PI2017-30 of the Department of Education of the Basque Government.

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.

The optical response of small charged metallic nanodisks of one atomic monolayer thickness is analyzed under the excitation by an incident plane wave and by a localized pointlike dipole. Using the time-dependent density functional theory (TDDFT) and classical electrodynamical calculations we identify the bright and dark plasmon modes and study their evolution under external charging of the nanostructure. For neutral nanodisks, despite their monolayer thickness, the in-plane optical response, as obtained from TDDFT, is in agreement with classical electromagnetic results. The optical response for an incident wave polarized perpendicular to the nanostructure cannot be retrieved classically as it reflects a discrete energy structure of electronic levels. This latter situation appears most sensitive to external charging while the energy of the in-plane plasmon with dipolar character is nearly charge independent., J.A. and M.Z. acknowledge support from the Spanish Ministry of Economy and Competitiveness MINECO through project FIS2016-80174-P, as well as support from NIST Grant No. 70NANB15H32 of the US Department of Commerce. A.K.K acknowledges financial support from the project FIS2016-76617-P of MINECO. M.Z. also acknowledges the hospitality of the Institute des Sciences Moléculaires d’Orsay.

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.

We use a time-dependent density functional theory approach to study the optical response of a hybrid nanostructure where the junction between thin metallic films is functionalized with a quantum well (QW) structure. We show that an unoccupied QW-localized electronic state opens the possibility of the active electrical control of the photoassisted electron transport through the junction and of the absorption at optical frequencies. Control strategies based on an applied bias or an external THz field are demonstrated., A.K.K. acknowledges partial support by Spanish Ministerio de Economia y Competitividad (MINECO) through Project No. FIS2016-76617-P.

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.

The nonlinear response of metallic nanoparticles is obtained from quantum time dependent density functional theory calculations. Without any aprioristic assumption our calculations allow us to identify high-order harmonic generation in canonical plasmonic structures such as spherical single particles and dimers. Furthermore, we demonstrate that under currently available experimental conditions, the application of an external polarizing field to the nanoparticles allows to actively control even-order harmonic generation in otherwise symmetry forbidden situations. Our quantum calculations provide quantitative access to the high-order response of metallic nanoantennas, which is of utmost importance in the design, control, and exploitation of optoelectronic devices as well as in the generation of extreme ultraviolet radiation., J.A. acknowledges financial support from the Department of Education of the Basque Government through Project IT756/13 for consolidated groups, Project FIS2016-80174-P from the Spanish Ministry of Economy and Competitiveness, and Federal Grant 70NANB15H321 from the National Institute of Standards and Technology (NIST). R.E. acknowledges support from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through FEDER funds “Una manera de hacer Europa”. A.K.K. acknowledges financial support from Project FIS2016-76617-P of the Spanish Ministry of Economy and Competitiveness.

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

With an example of the periodic plasmonic dolmen structure we performed a theoretical study of the effect of the conducting water layer on the plasmon resonances of the system with narrow gaps. Using the scanning tunneling microscopy studies of the conductance of the water junctions as inputs, we show that water layer(s) should affect plasmon modes of the systems with nm and sub nm gaps in two ways. The frequency of the plasmon modes shifts because of the dielectric screening, as commonly used in plasmonic sensors, and the corresponding resonance in the optical spectra looses intensity and broadens because of the resistive tunneling current. The water layer in the junction lowers potential barrier for electron tunneling, so that quantum effects in plasmon response appear for the junction width at least twice larger as compared to the vacuum gaps.

Using the time-dependent density functional theory, we perform quantum calculations of the electron dynamics in small charged metallic nanoparticles (clusters) of spherical geometry. We show that the excess charge is accumulated at the surface of the nanoparticle within a narrow layer given by the typical screening distance of the electronic system. As a consequence, for nanoparticles in vacuum, the dipolar plasmon mode displays only a small frequency shift upon charging. We obtain a blue shift for positively charged clusters and a red shift for negatively charged clusters, consistent with the change of the electron spill-out from the nanoparticle boundaries. For negatively charged clusters, the Fermi level is eventually promoted above the vacuum level leading to the decay of the excess charge via resonant electron transfer into the continuum. We show that, depending on the charge, the process of electron loss can be very fast, on the femtosecond time scale. Our results are of great relevance to correctly interpret the optical response of the nanoparticles obtained in electrochemistry, and demonstrate that the measured shift of the plasmon resonances upon charging of nanoparticles cannot be explained without account for the surface chemistry and the dielectric environment., M.Z. acknowledges financial support from the Departamento Administrativo de Ciencia, Tecnologia e Innovacion-COLCIENCIAS and Facultad de Ciencias from Universidad de los Andes. M.Z. also gratefully acknowledges the hospitality of the Donostia International Physics Center and the Materials Physics Center in San Sebastian. J.A. acknowledges support from the Spanish Ministry of Economy and Competitiveness through project FIS2013-41184-P and project 2015CD0010 of the CSIC scientific cooperation program for development “ICOOP LIGHT” 2015.

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.

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.

The Cs/Cu(1 1 1) and Na/Cu(1 1 1) systems exhibit a transient excited electronic state localized on the adsorbate. Photo-excitation of this state triggers a motion of the alkali adsorbate away from the surface, leading to vibrational excitation of the adsorbate and possibly to desorption. A theoretical study of these photo-induced processes in the case of an exciting fs laser pulse is reported, based on a time-dependent approach of the adsorbate motion. The mean energy transfer from the laser photon energy to the adsorbate motion is shown to be weak, about 1% of the photon energy. Correspondingly, the vibrational excitation to high lying levels is very weak as well as the desorption process. The initial electronic state of the photo-induced process belongs to a continuum and vibrational excitation and desorption are found to vary rapidly with the energy of the initial electronic state. Initial vibrational excitation of the alkali adsorbate is also found to efficiently favour the desorption process, leading to a drastic variation of the desorption probability with the vibrational temperature of the adsorbate. The present results for the two systems are discussed and compared, in connection with available experimental data on these systems and on similar ones.

A new interferometric multi-photon photoemission scheme has been recently introduced by Gudde et al. [1] and applied to a study of the n=1 image state on Cu(100). We report on the results of a theoretical modelling of this photoemission scheme based on a wave function representation. It incorporates both resonant transitions and two-photon non-resonant transitions. In the case of the n=1 image state on Cu(100), the initial state of the photoemission belongs to a continuum. Summation of the contributions to the photoemission yield from the various initial states is shown to drastically blur the interference structure, acting as an efficient dephasing process. The sensitivity of the new scheme and of the one-colour TR-2PPE scheme to the system dynamical properties is discussed.

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license., The ability of localized surface plasmons to squeeze light and engineer nanoscale electromagnetic fields through electron-photon coupling at dimensions below the wavelength has turned plasmonics into a driving tool in a variety of technological applications, targeting novel and more efficient optoelectronic processes. In this context, the development of active control of plasmon excitations is a major fundamental and practical challenge. We propose a mechanism for fast and active control of the optical response of metallic nanostructures based on exploiting quantum effects in subnanometric plasmonic gaps. By applying an external dc bias across a narrow gap, a substantial change in the tunneling conductance across the junction can be induced at optical frequencies, which modifies the plasmonic resonances of the system in a reversible manner. We demonstrate the feasibility of the concept using time-dependent density functional theory calculations. Thus, along with two-dimensional structures, metal nanoparticle plasmonics can benefit from the reversibility, fast response time, and versatility of an active control strategy based on applied bias. The proposed electrical manipulation of light using quantum plasmonics establishes a new platform for many practical applications in optoelectronics., J.A. acknowledges support from the Spanish Ministry of Economy and Competitiveness through projects FIS2013-41184-P and 2015CD0010 of the Consejo Superior de Investigaciones Científicas scientific cooperation program for development “I-COOP LIGHT” 2015. P.N. acknowledges support from the Robert A. Welch Foundation (grant C-1222) and the Air Force Office of Science and Research (grant FA9550-15-1-0022). M.Z. acknowledges financial support from the Departamento Administrativo de Ciencia, Tecnología e Innovación–COLCIENCIAS and Facultad de Ciencias from Universidad de los Andes.

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.

The energy of core-excited Ne ∗ (1s −1 n l) atoms inside bulk Ne is computed using a joint pseudopotential-wave packet propagation approach. Special care is taken to ensure the proper dielectric properties of bulk Ne together with its 3D-geometric structure. Results are compared with available X-ray absorption experimental results on the (1s −1 n p) excitons. Analysis of the state energy and of the wave-function of the excited n l electron allows to define two different regimes for the perturbation of the outer n l electron by the neighbouring Ne atoms in the crystal: confinement in the cage formed by the first Ne neighbours for low-lying states and Rydberg states in a screened Coulomb potential for the higher-lying states. An effective screened spherically symmetric potential representing well the electron–hole interaction in the Ne ∗ (1s −1 n l) atoms inside bulk Ne is proposed, based on the analysis of the present 3D-modelling.

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.

Adsorption of alkali atoms on the (1 1 1) and (1 0 0) noble metal surfaces has been shown recently to induce long-lived resonances located inside the surface projected band gap. However, the width of these resonances, as it appears in two-photon photo-emission experiments, is much larger than the inverse of their lifetime. We report on a theoretical study of some broadening mechanisms of these resonance lines in the Na/Cu(1 1 1) and Cs/Cu(1 1 1) systems at low coverage, including the homogeneous natural line broadening and the inhomogeneous statistical broadenings due to the distribution of adsorption heights associated to the quantal vibration of the alkali adsorbate and to the lateral disorder of alkali adsorption on the surface. The inhomogeneous mechanisms are shown to induce a very large broadening of the resonance line, in quantitative agreement with experimental results. The most important broadening effect appears to be the effect of the distribution of alkali adsorption heights.

We present a 'virtual detector' method which allows the extraction of the momentum distributions in continuum states from time-dependent wave packet propagation studies with explicitly time-dependent Hamiltonians. It is based on the approach developed by Balint-Kurti et al (1990 J. Chem. Soc. Faraday Trans. 86 1741) for quantum chemistry applications and consists in measuring a signal in the asymptotic region. In that sense, it is close to the semiclassical method proposed recently in this journal (Feuerstein and Thumm 2003 J. Phys. B: At. Mol. Opt. Phys. 36 707). The advantage of the present approach is that it is based on a rigorous quantum mechanical treatment, robust, easy to handle and requires a much smaller grid size. The method is illustrated with the 3D example of electron detachment from the H− ion colliding with a free-electron metal surface.

The local density of states of two model systems formed by an individual adsorbate (Cs or Ar) on a Cu(111) surface is investigated using a wave-packet propagation approach. In both cases a sharp peak appears at the threshold of the 2D Shockley surface-state continuum. This peak is attributed to quasi-stationary states located just below the 2D continuum. Following the existence criterion for a bound state in two dimensions by Simon, such states splitting off the surface-state bottom should appear whenever the surface-state continuum is perturbed by an attractive potential, independently of the existence of atomic-like states bound by the perturbing potential. These findings are discussed in connection with recent observations of peaks located at the surface-state threshold in scanning tunnelling microscopy experiments.

A model is developed to describe the adsorbate coverage dependence of excited state (image states and adsorbate-localised states) on a metal surface. It is applied to the case of Na adsorbates on a Cu(1 1 1) surface. It allows to define in a consistent way (i) an effective potential, which is only function of the electron-surface distance representing the average perturbation introduced by a large set of alkali adsorbates on the surface and (ii) the local perturbation induced around each adsorbate. It can be applied up to moderate coverages, typically for Na-induced work function changes below 1.5 eV. In this range, its predictions reproduce the experimental findings of Fischer et al. [Surf. Sci. 314 (1994) 89] for the energy of the image states and Na-localised state.

A theoretical study of the electron dynamics in image potential states on Cu(1 0 0) surfaces with different types of adsorbates is presented. Scattering of the image state electron by an adsorbate induces inter-band and intra-band transitions leading respectively to the population decay and to the dephasing of the image state. We compare results obtained with low coverage (typically 1 adsorbate atom per 1000 surface atoms) Cs, Ar, and a model electronegative adsorbates. As follows from our results, Cs adsorbates lead to both appreciable dephasing and decay, while electronegative adsorbates mostly affect the dephasing rate. The effect of low coverage Ar adsorbates is small, consistent with their neutrality.

Image states at surfaces are very sensitive to the presence of impurities on the surface. A theoretical study of the effect of alkali adsorbates on the image state dynamics is performed for the Cs/Cu(1 0 0) and Na/Cu(1 1 1) systems using a wave-packet propagation approach. Scattering of the image state electron by an alkali adsorbate is shown to induce large inter-band and intra-band transitions leading respectively to the population decay and to the dephasing of the image state. Even very low alkali coverage in the 10−3 monolayer range can effectively affect the image states, reducing their lifetime and broadening their photoemission peaks. The present theoretical results are discussed and compared with available experimental results on the perturbation of image states by various adsorbates.

The mutual angular distributions of the two ejected electrons following direct photodouble ionization have been measured for D2 at an excess energy (E) of 25 eV using linearly polarized light. These (γ ,2 e) ‘t riple’ differential cross sections (TDCSs) were obtained for asymmetric electron energy conditions with energy sharing ratios (R = E2/E1) of R = 24, 11.5, 4a nd2.57. In all cases the ‘reference’ electron (energy = E1 )w as orie nted along the direction of the electric field vector (e )a nddetected in coincidence with a second electron (energy = E2) coplanar with e and the photon beam direction (kγ ). For comparison, helium TDCSs were obtained for the same E and R values under nearly identical spectrometer conditions. These show very good agreement with the results of hyperspherical-� -matrix with semi-classical outgoing waves calculations, thus providing even more confidence in the D2 TDCSs where there is as yet no accurate ab initio theory. The similarities and differences between the experimental results associated with the two targets are qualitatively discussed in terms of Feagin’s model (Feagin J M 1998 J. Phys. B: At. Mol. Opt. Phys. 31 L729).

A theoretical study of the electron dynamics in image potential states on Cu(1 1 1) and Cu(1 0 0) surfaces with alkali adsorbates is presented. It is shown that scattering of the image potential state electron by the adsorbates yields a sizable contribution to the decay of the image state population. Even for extremely low coverages (1 adsorbate atom per 1000 surface atoms) the decay rates due to the adsorbate induced scattering might be comparable to the decay rates due to the electron–electron interactions. We demonstrate that the inter-band transitions to other 2D continua of image or surface states is an efficient decay channel close to the Γ point.

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) from two-photon near-threshold ionization of Ne atoms by the combined action of femtosecond pulses from an extreme ultraviolet (EUV) free-electron laser and infrared (IR) laser have been studied experimentally and theoretically. Solutions of the time-dependent Schr\"odinger equation indicate that the PADs strongly depend on the time delay between EUV and IR pulses. The experimental results obtained for two extreme cases of completely overlapping and nonoverlapping pulses fully confirm the prediction, illustrating that the measurements of the time-delay dependence of the PAD provide a tool for investigating the fundamental problem of the relative importance of the resonant and nonresonant pathways in the two-color two-photon processes.

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.

One-electron and multielectron contributions to the decay of transient states in the Cs/Cu(111) and (100) systems are studied by a joined wave-packet propagation and many-body metal response approach. The long lifetime of these states is due to the Cu $L$ and $X$ band gaps which reduce the electron tunneling between Cs and Cu. In the (111) case, the decay is mainly by inelastic $e\ensuremath{-}e$ interaction, whereas in the (100) case, electron tunneling is dominating. This accounts very well for the experimental findings [Bauer et al., Phys. Rev. B 55, 10 040 (1997) and Ogawa et al., Phys. Rev. Lett. 82, 1931 (1999)].

A representation of the diatomic vibrational distribution function in energy and phase variables yields a transparent physical picture of collision-induced vibrational relaxation. In this way, the energy— and phase—relaxation processes can be separated. A simple parametrization of the phase distribution allows for the extraction of the dephasing time and the reconstruction of autocorrelation functions and femtosecond pump–probe signals. It is shown that the dephasing time can be derived from the Fourier spectrum of the pump–probe signal in favorable cases.

The hyperspherical R-matrix method with semiclassical outgoing waves introduced by us previously [Phys. Rev. A 60, 3667 (1999)] is applied to the one-photon double ionization of the He atom. The absolute differential cross sections obtained are in excellent agreement with experiment for the very challenging dynamical situations considered. This method is the first one to compute the final double continuum state accurately over the entire configuration space, from the vicinity of the nucleus to the asymptotic region.