Attosecond Dynamics of $sp$-Band Photoexcitation

Attosecond Dynamics of $s p$ -Band Photoexcitation

Johann Riemensberger, Stefan Neppl, Dionysios Potamianos, Martin Schäffer, Maximilian Schnitzenbaumer, Marcus Ossiander, Christian Schröder, Alexander Guggenmos, Ulf Kleineberg, Dietrich Menzel, Francesco Allegretti, Johannes V. Barth, Reinhard Kienberger, Peter Feulner, Andrei G. Borisov, Pedro M. Echenique, and Andrey K. Kazansky

Phys. Rev. Lett. 123, 176801 – Published 21 October 2019

Abstract

We report measurements of the temporal dynamics of the valence band photoemission from the magnesium (0001) surface across the resonance of the $\bar{Γ}$ 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.

Received 26 February 2019
Revised 3 June 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.176801

Physics Subject Headings (PhySH)

Electronic structure Surface states Ultrafast optics

Attosecond laser spectroscopy Photoemission spectroscopy

Atomic, Molecular & OpticalCondensed Matter, Materials & Applied Physics

Authors & Affiliations

Johann Riemensberger ^1,2,*,†, Stefan Neppl^1,†, Dionysios Potamianos^1,2, Martin Schäffer¹, Maximilian Schnitzenbaumer¹, Marcus Ossiander^1,2, Christian Schröder¹, Alexander Guggenmos^2,3,‡, Ulf Kleineberg^2,3, Dietrich Menzel¹, Francesco Allegretti¹, Johannes V. Barth¹, Reinhard Kienberger¹, and Peter Feulner¹

¹Physik Department, Technische Universität München, James-Franck-Str 1, 85748 Garching, Germany
²Max-Planck Institut für Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany
³Fakultät für Physik, Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany

Andrei G. Borisov^4,5, Pedro M. Echenique⁵, and Andrey K. Kazansky^5,6

⁴Institut des Sciences Moléculaires d’Orsay (ISMO), UMR 8214, CNRS, Université Paris Sud, Université Paris-Saclay, bât 520, F-91405 Orsay, France
⁵Material Physics Center CSIC-UPV/EHU; Donostia International Physics Center DIPC, Paseo Manuel de Lardizabal 5 20018, Donostia-San Sebastián, Spain
⁶IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain

^*To whom correspondence should be addressed. johann.riemensberger@tum.de Present address: École Polytechnique Fédérale de Lausanne (EPFL), Laboratory of Photonics and Quantum Measurements (LPQM), Lausanne CH-1015, Switzerland.
^†Present address: Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 12489 Berlin, Germany.
^‡Present address: Ultrafast Innovations GmbH, Am Coulombwall 1, 85748 Garching, Germany.

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Vol. 123, Iss. 17 — 25 October 2019

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Images

Figure 1
(a) Experimental geometry of attosecond and synchrotron photoemission experiments. XUV radiation and NIR radiation (time-resolved experiment only) are focused on a Mg(0001) surface. The emission angle $θ$ is varied by tilting the sample (laser experiment) or rotation of the detector (synchrotron); the angular resolution $Δ θ$ is determined by the electron energy analyzer. (b) (Left) Occupied part of Mg(0001) band structure perpendicular to the surface. The surface state lies at the lower edge of the band gap and is indicated in dark red. (Right) Experimental photoemission spectrogram recorded with narrow band synchrotron radiation in normal emission geometry. The resonant photoemission bands arise from direct bulk transitions conserving the band momentum in the reduced zone scheme. Red arrows mark the resonant photoemission of the surface state in the extended zone scheme. (c) ARPES at 130 eV photon energy displaying the parabolic dispersion of the Shockley-type $\bar{Γ}$ surface state. (d) ARPES recorded with isolated attosecond pulses. See main text for details.
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Figure 2
(a) Schematic of the attosecond streaking spectroscopy. XUV (purple) and IR (red) pulses with controlled time delay $Δ t$ are focused on the Mg(0001) surface and expel electrons (blue) from the Mg $2 p$ states and valence band (VB) levels. The steady-state photoelectron spectrum, recorded at 124.4 eV photon energy, is indicated in blue on the right. (b) Attosecond streaking spectrogram recorded at 124.4 eV. (c) Energy-dependent relative attosecond time delays $Δ τ$ of the Mg $2 p$ core level for small ( $\pm 2 °$ , blue) and large ( $\pm 22 °$ , red) electron detection regions and relative time delays obtained from numerical calculations with the effective single-electron model potentials (hard potential [10] (green), soft potential [37] (gray)). Classical transport theory predicts nearly vanishing time delays for purely ballistic electron propagation [14, 38] (gray, dashed). The light red point represents the result from Ref. [28].
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Figure 3
(a) Effective single-electron potential $U (z)$ (green) of the electron-metal interaction as a function of distance from the metal surface in units of the Mg(0001) interlayer spacing (in monolayers, ML). Insets: Band structure of occupied states for a 68 layer Mg(0001) slab. States marked in blue and red are plotted in Fig. 3. Gray shaded area marks the projected band gap. (b) Wave functions of occupied states around the $\bar{Γ}$ -band gap. The symmetric surface state is plotted in dark red. The wave function of the state with highest energy below the gap is plotted in dark blue. The first and second states above the projected band gap are plotted in light colors. (c) Comparison of experimental (left) and simulated (right) photoemission spectrograms in normal emission geometry around the 2nd resonance of the $\bar{Γ}$ surface state, taking into account the finite collection angle of the analyzer. (d) (Left) Normalized partial intensity of calculated photocurrent emitted from each individual bound state $k_{⊥}$ [color scale same as Fig. 3]. The strong emission at $k_{⊥} = 1.21 Å^{- 1}$ marks the $\bar{Γ}$ surface state in the projected band gap. (Right) Relative time delay of Mg $2 p$ photoemission with respect to individual bound states. Positive values indicate the VB electrons leaving the crystal first.
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