New insights into the laser-assisted photoelectric effect from solid-state surfaces

Photoemission from a solid surface provides a wealth of information about the electronic structure of the surface and its dynamic evolution. Ultrafast pump-probe experiments are particularly useful to study the dynamic interactions of photons with surfaces as well as the ensuing electron dynamics induced by these interactions. Time-resolved laser-assisted photoemission (tr-LAPE) from surfaces is a novel technique to gain deeper understanding of the fundamentals underlying the photoemission process. Here, we present the results of a femtosecond time-resolved soft X-ray photoelectron spectroscopy experiment on two different metal surfaces conducted at the X-ray Free-Electron Laser FLASH in Hamburg. We study photoemission from the W 4f and Pt 4f core levels using ultrashort soft X-ray pulses in combination with synchronized infrared (IR) laser pulses. When both pulses overlap in time and space, laser-assisted photoemission results in the formation of a series of sidebands that reflect the dynamics of the laser-surface interaction. We demonstrate a qualitatively new level of sideband generation up to the sixth order and a surprising material dependence of the number of sidebands that has so far not been predicted by theory. We provide a semi-quantitative explanation of this phenomenon based on the different dynamic dielectric responses of the two materials. Our results advance the understanding of the LAPE process and reveal new details of the IR field present in the surface region, which is determined by the dynamic interplay between the IR laser field and the dielectric response of the metal surfaces.Comment: 18 pages, 3 figure

Single-shot temporal characterization of XUV pulses with duration from ~10 fs to ~350 fs at FLASH

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 (similar to 10 fs) and long (up to similar to 350 fs) pulses.We want to acknowledge the work of the scientific and technical team at FLASH. NMK acknowledges the hospitality and financial support from DESY and from the theory group in cooperation with the SQS research group of the European XFEL (Hamburg). KW and MD acknowledge support by the SFB925-A1. UF and AD acknowledge support by the excellence cluster `The Hamburg Center for Ultrafast Imaging-Structure, Dynamics and Control of Matter at the Atomic Scale' (DFG)-EXC 1074 project ID 194651731. SW acknowledges support by the DFG Forschergruppe FOR 1789. Editoria

Angular momentum–induced delays in solid-state photoemission enhanced by intra-atomic interactions

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.Peer Reviewe

Theoretical description of atomic photoionization by an attosecond XUV pulse in a strong laser field: effects of rescattering and orbital polarization

Kazansky AK, Kabachnik NM. Theoretical description of atomic photoionization by an attosecond XUV pulse in a strong laser field: effects of rescattering and orbital polarization. Journal of Physics. B, Atomic, Molecular and Optical Physics. 2007;40(11):2163-2177.A theoretical description of atomic photoionization by attosecond pulses in the presence of an intense laser pulse is presented. It is based on the numerical solving of the non-stationary Schrodinger equation which includes on an equal footing the realistic atomic potential and the electric fields of both pulses. The calculated energy spectra and angular distributions of photoelectrons are compared with those obtained using a simple approximate model based on the strong- field approximation. The agreement is excellent for large energy of photoelectrons. When the energy is small, the rescattering of electrons by the ionic core affects the cross section considerably making the strong- field approximation inadequate. Influence of the electron orbital polarization on the ionization cross section is investigated

An attosecond time-resolved study of strong-field atomic photoionization

Kazansky AK, Kabachnik NM. An attosecond time-resolved study of strong-field atomic photoionization. Journal of Physics. B, Atomic, Molecular and Optical Physics. 2007;40(21):F299-F305.The time evolution of atomic photoionization by an intense few-cycle laser pulse is theoretically investigated. A possible modification of the recent attosecond tunnelling experiment in Uiberacker et al ( 2007 Nature 446 627) is proposed, which consists of measuring the yield of the doubly charged Li ions produced by the combined action of a strong few-cycle infrared pulse and an ultrashort ( attosecond) extreme ultraviolet (XUV) pulse. We predict the results of such an experiment, based on the numerical solution of the time-dependent Schrodinger equation, which describes the atomic electron in a strong laser field. The influence of the XUV pulse is treated in the sudden approximation. It is shown that the dependence of the double ionization cross section on the time delay between the two pulses reflects the time evolution of the strong-field ionization. We demonstrate that even more detailed information can be gained from the forward-backward asymmetry of the photoelectron emission

Theoretical study of attosecond chronoscopy of strong-field atomic photoionization

Kazansky AK, Kabachnik NM. Theoretical study of attosecond chronoscopy of strong-field atomic photoionization. Journal of Physics. B, Atomic, Molecular and Optical Physics. 2008;41(13): 135601.A model which describes the time evolution of strong-field photoionization of atoms is presented. Based on the numerical solution of the non-stationary Schrodinger equation, the model allows one to predict and to interpret the results of experiments on double photoionization of atoms by a combined action of a very short (attosecond) XUV pulse and a few-cycle IR pulse of a powerful laser at various delay times between the two pulses. Depending on the binding energy of the ionized electron, two types of processes are considered. If the electron is tightly bound (Ne case), the XUV pulse ionizes the atom and shakes up another (outer) electron to an excited state, which is subsequently ionized by the strong IR field. For an atom with a weakly bound outer electron (Li case), the IR field ionizes the latter, while the XUV pulse, ionizing the inner shell, terminates (or suppresses) the strong-field ionization. In both cases the yield of doubly ionized ions strongly depends on the delay time between the two pulses, revealing 'steps', oscillations and other features which characterize the time evolution of the ionization process. The presented model describes qualitatively the results of recent experiments on Ne

Theoretical description of atomic photoionization by attosecond XUV pulses in a strong laser field: the case of p-shell ionization

Kazansky AK, Kabachnik NM. Theoretical description of atomic photoionization by attosecond XUV pulses in a strong laser field: the case of p-shell ionization. Journal of Physics. B, Atomic, Molecular and Optical Physics. 2007;40(17):3413-3424.A theoretical description of attosecond photoionization in the presence of a strong laser field, based on the numerical solution of the Schrodinger equation, is extended to the case of p-shell ionization. In particular, Ar(3p) photoionization is considered. The main difference between this case and the previously considered case of s-shell ionization stems from the interference of the two dipole allowed channels of p-shell photoionization, which determines the angular distribution of photoelectrons in the absence of the laser field. The latter additionally distorts the angular distributions. We also extend to the initial p-shell case the model based on the strong-field approximation (SFA), which has been suggested earlier. At high photoelectron energy and low laser intensity both calculations give similar results. However, at low electron energy the SFA is inadequate. The dependence of the angular distribution on the carrier-envelope phase and the effects of orbital polarization are considered

Spin Polarization of Electrons in Two-Color XUV + Optical Photoionization of Atoms

The spin polarization of photoelectrons in two-color XUV + optical multiphoton ionization is theoretically considered using strong field approximation. We assume that both the XUV and the optical radiation are circularly polarized. It is shown that the spin polarization is basically determined by the XUV photoabsorption and that the sidebands are spin polarized as well. Their polarization may be larger or smaller than that of the central photoelectron line depending on the helicity of the dressing field