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Electron transport on the attosecond timescale, through several atomic layers, has now been observed in single-crystal tungsten in real time. This development was made possible by extension of established attosecond spectroscopic techniques developed in gas-phase targets. In tungsten, an isolated attosecond XUV pulse was used to excite photoelectrons inside the crystal, while a waveform controlled, few-cycle NIR laser pulse was used to probe their emission. As the relative delay between XUV and NIR pulses is varied, the underlying field of the NIR pulse, which modulates the measured photoelectron kinetic energy, is revealed in both the 4f core-state and conduction-band photoemission peaks. Comparison of these “streaking” curves revealed that photoelectrons originating from the 4f core-states were emitted from the tungsten surface approximately 110 attoseconds after those from the delocalized conduction-band states. Delayed emission is found to be a consequence of the material’s electron transport properties, which can only be accessed in the time-domain.
In general, this result indicates that time-domain observation of attosecond charge dynamics in solids and surfaces – including charge transfer, charge screening, image charge formation and decay, electron-electron scattering, and collective electronic motion – is now feasible. However, observing these phenomena through attosecond spectroscopy is nontrivial and will at least require management of the broad XUV bandwidth needed to support the attosecond pulses, as well as, detailed consideration of the effect of the NIR probe field on the material itself. |
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