The photoeffect revisited: Attosecond time-resolved photoelectron spectroscopy
of atoms, nanostructures, and surfaces
Uwe Thumm
Department of Physics, Kansas State University, Manhattan, KS 66506, USA
Executive summary: I plan to discuss different interpretations of photoemission spectra and delay times for
gaseous and solid targets, based on the comparison of calculated photoelectron (PE) spectra with recent
experiments. We examined photoemission time delays and modeled the time-dependent autoionization for the
emission of PEs and Auger electrons from atoms and solid surfaces. We scrutinized the effect of the Coulomb
interaction between PE and residual ion and compared streaking with Wigner time delays. For
photoemission from surfaces and thin films, we found that streaking delays sensitively depend on the electron
mean-free path, IR-skin depth, and surface electronic structure. We further investigated the excitation of bulk
and surface plasmons during the release of PEs from metal surfaces and scrutinized the influence of the
dynamical plasmon response on PE spectra and photoemission time delays.
More details: State-of-the-art streaking spectroscopy experiments enable the resolution in time of photoionization processes at the natural time scale of the motion of valence electrons in atoms [1-4] and solids [510]. This ultrahigh time resolution suggests the unprecedented observation of a "time delay" between the
absorption of XUV photons and subsequent PE emission. Relative delays for photoemission from different
initial states are of the order of tens of attoseconds and thus accurately probe of the entire photoemission
dynamics, including the initial XUV-pulse-triggered release, propagation, and detection of the PE.
XUV photoionization into the laser-dressed continuum of the ionized atom is commonly approximated in
strong-field approximation, neglecting the Coulomb interaction between the emitted PE and the residual ion
[1]. By solving the time-dependent Schrödinger equation (TDSE), we identified a temporal shift in streaked
photoemission spectra that is due to the Coulomb-laser coupling in the final state and exceeds 50 as at small
PE kinetic energies. We expect the examination of this shift to enable (i) the experimental scrutiny of effects
that are due to the combined action of Coulomb and laser forces on the PE and (ii) tests of theoretical
approximations to the exact Coulomb-Volkov state of the PE. Within a semiclassical (eikonal)
approximation, we derived an analytical expression for this effect and assessed its accuracy in comparison
with full TDSE numerical results.
Streaked PE spectroscopy promises to offer a new way to probe collective excitations in nano-particles and
solids, such as, e.g., dynamical image-charge effects. The release of conduction-band electrons from a metal
surface by a sub-femtosecond XUV pulse, and their propagation through and near the solid [5,7,8], provokes
a dielectric response in the solid that acts back on the PE wave packet. We modeled the response of the metal
due to excitation of bulk and surface plasmons, induced by the creation and propagation of PEs in the solid,
in terms of an effective potential that depends on the velocity of the PE. We numerically calculated the
(wake) potential associated with this PE self-interaction and showed that it induces a considerable XUVfrequency-dependent temporal shift in streaked-photoemission spectra, suggesting the observation of the
ultrafast solid-state dielectric response in contemporary streaked photoemission experiments [8].
[1] Electron-ion interaction effects in attosecond time-resolved photoelectron spectra, Zhang & T., Phys. Rev. A 82,
043405 (2010). [2] Streaking and Wigner delays in photoemission from atoms and surfaces, Zhang, U. T., Phys. Rev. A
83, 033401 (2011). [3] Attosecond probing of instantaneous AC Stark shifts in helium atoms, He, Ruiz, Becker, T., J.
Phys. B 44, 211001 (2011). [4] Attosecond time-resolved autoionization of argon, Wang et al., Phys. Rev. Lett. 105,
143002 (2010). [5] Attosecond photoelectron spectroscopy of metal surfaces, Zhang, T., Phys. Rev. Lett. 102, 123601
(2009). [6] Time-resolved core-hole decay in laser-assisted photoemission from adsorbate - covered metal surfaces,
Zhang and T., Phys. Rev. A 80, 032902 (2009). [7] Time-resolved IR laser-assisted XUV photoelectron spectroscopy of
metal surfaces, Zhang & U. T., XXVI ICPEAC, Journal of Physics: Conf. Series 194, 012055 (2009). [8] Probing
dielectric response effects with attosecond time-resolved streaked photoelectron spectroscopy of metal surfaces, Zhang
& U. T., Phys. Rev. A 84, 063403 (2011). [9] Effects of wave-function localization on the time delay in photoemission
from surfaces, Zhang and T., Phys. Rev. A 84, 065403 (2011). [10] Attosecond time-resolved photoelectron dispersion
and photoemission time delays, Liao & T., Phys. Rev. Lett. 112, 023602 (2014) ; Phys. Rev. A 89, 033849 (2014).
Supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office
of Science, U.S. DOE and the U.S. NSF.