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

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Science  22 Sep 2017:
Vol. 357, Issue 6357, pp. 1274-1277
DOI: 10.1126/science.aam9598
  • Fig. 1 Elementary steps in photoemission from the van der Waals crystal WSe2.

    The WSe2 substrate and the principle of attosecond time-resolved streaking spectroscopy are depicted. The surface held at room temperature is illuminated collinearly with a 300-as long EUV pulse with 91-eV center energy and an intense few-cycle IR streaking field (85° angle of incidence, p-polarized). The EUV pulses excite photoelectrons (here shown for the W 4f photoemission) that are then streaked in the IR field, i.e., are gaining or losing kinetic energy depending on the delay tIRtEUV between the EUV and IR pulses. As indicated in the upper left panel, the initial stage of photoemission is dominated by intra-atomic processes: Within the HS approach, the photoelectron wave (green) created by EUV excitation from the W 4f state (blue) is governed by the effective radial potential (red) composed of the HS potential UHS and the centrifugal term. The wave packet propagation in the later stage is schematically depicted in the right panel. It is dominated by a 1D potential that accounts for the inner potential UIP of WSe2 and the interaction with the remaining photohole. The inelastic MFP for the photoelectron is indicated as a horizontal bar and Evac indicates the position of the vacuum energy.

  • Fig. 2 Attosecond time-resolved photoemission spectroscopy from WSe2.

    (A) Long-term stability of the surface over 40 hours. Background-corrected photoemission spectra (fig. S1A) recorded 30 min (black circles) and 40 hours (red circles) after cleaving. The photoelectron peaks for VB, Se 4s, W 4f, and Se 3d are indicated. The spectra are normalized to the total yield after background subtraction. (B) Streaking spectrogram. As a function of the delay between the IR and EUV pulses, the photoemission spectra (after background subtraction) are shown as a density plot. For each delay, the energy positions (overlaid symbols) of the VB, Se 4s, W 4f, and Se 3d emissions and the corresponding simultaneously fitted IR field-time dependence (eq. S1) yielding the delay parameters Δt for each emission channel (continuous overlaid lines) are shown.

  • Fig. 3 Relative photoemission delays.

    The left side shows the relative photoemission delays ΔtVB–Se4s, ΔtSe3d–Se4s, and ΔtW4f–Se4s as a function of time after cleaving for two different crystals indicated by different symbols (circles and squares) using the background subtraction method based on a model spectrum (fig. S1A). The horizontal error bars indicate the time period during which the spectrogram was recorded, and the vertical bars indicate the uncertainty of the delay determination. On the right, the average delays obtained by various background-subtraction procedures are shown: background based on model spectrum (red circle), parabolic background (black circle), combination of parabolic and Shirley background with and without delay-dependent background (blue and green circles, respectively), and delay-dependent background based on model spectrum (magenta circle).

  • Table 1 Comparison between experimental and theoretical photoemission delays.

    The second column specifies the difference between the kinetic energies for X and Se 4s photoelectrons. The third column summarizes the experimental delays as indicated in Fig. 3. The fourth column lists the summed intra-atomic and propagation-induced delays derived by using either the HS or the MCDF approach (fifth column) and 1D TDSE simulation of propagation in the solid (sixth column), respectively. The values derived using the MCDF approach are indicated in squared brackets. For details, see supplementary materials section 2.

    XEmbedded Image
    (eV)
    Experimental
    ΔtX–Se4s (as)
    Theory Σ
    total including
    HS or MCDF
    (as)
    Atomic delay
    HS or MCDF
    (as)
    Propagation delay
    1D TDSE
    (as)
    VB 4p/5d13.512 ± 1012 [12]6 [6]6
    Se 3d–41.328 ± 1029 [25]14 [10]15
    W 4f–19.347 ± 1444 [36]20 [12]24

Supplementary Materials

  • Angular momentumâ€"induced delays in solid-state photoemission enhanced by intra-atomic interactions

    Fabian Siek, Sergej Neb, Peter Bartz, Matthias Hensen, Christian Struüber, Sebastian Fiechter, Miquel Torrent-Sucarrat, Vyacheslav M. Silkin, Eugene E. Krasovskii, Nikolay M. Kabachnik, Stephan Fritzsche, Ricardo Díez Muiño, Pedro M. Echenique, Andrey K. Kazansky, Norbert Müller, Walter Pfeiffer, Ulrich Heinzmann

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

    Download Supplement
    • Materials and Methods
    • Figs. S1 to S9
    • Tables S1 to S3
    • References

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