Attosecond Time-Resolved Photoemission from Core and Valence States of Magnesium

We report on laser-assisted attosecond photoemission from single-crystalline magnesium. In strong contrast to the previously investigated transition metal tungsten, photoelectron wave packets originating from the localized core level and delocalized valence-band states are launched simultaneously from the solid within the experimental uncertainty of 20 as. This phenomenon is shown to be compatible with a heuristic model based on free-particle-like propagation of the electron wave packets generated inside the crystal by the attosecond excitation pulse and their subsequent interaction with the assisting laser field at the metal-vacuum interface.

Figure 1

Stationary photoemission from the Mg(0001) surface. (a) Comparison of PE spectra recorded with the $\sim 118\mathrm{eV}$ sub-fs XUV pulses (dotted line) and 120 eV narrow-bandwidth synchrotron radiation (gray shaded area). A convolution of the synchrotron spectrum with the reflectivity response of the employed multilayer XUV mirror is depicted as a red solid line. (b) The PE spectrum after Shirley-background subtraction (dotted line) is compared to a PE spectrum reconstructed from a superposition of individual Gaussian time-domain wave packets (green solid line). The green vertical bars indicate the central energies and amplitudes of the considered transitions. Insets provide an enlarged view of the valence-band region.

Figure 2

Attosecond time-resolved PE spectra obtained from the clean Mg(0001) surface. Positive delays correspond to XUV arriving first. The false-color representation of the measured streaking spectrogram (a) is compared to a spectrogram (b) reconstructed by the quantum-mechanical fit procedure (see text for details).

Figure 3

Timing analysis of the VB and $2p$ core-level photoemission from Mg(0001). Calculated COEs for the VB and $2p$ region of the spectrogram (depicted as crosses) are fitted to a pair of parametrized analytic functions describing the vector potential of the NIR streaking pulse [comp. Eq. (1)]. The absence of a relative shift between the fitted functions, shown as red solid line for the VB and blue dotted line for the $2p$ emission, provides evidence for a synchronous release of these electrons from the metal surface.

Figure 4

Analysis of the spatial origin of the emitted photoelectrons. (a) Average escape depth $\lambda$ of electrons in Mg as a function of their kinetic energy according to Ref. 32 (red line). Values calculated from the surface-to-bulk ratios in the $2p$ emission using a layer-attenuation (LA) model are depicted as dots along with a linear fit (dashed line). (b) High resolution PE spectra of the $2p$ emission for different photon energies. The deconvolution in surface (light green shaded) and bulk contributions (dark blue shaded) is performed by fitting with two spin-orbit doublets of Doniach-Šunjić functions.