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Tabletop-scale coherent EUV generated through high-harmonic generation (HHG) produces light in the form of an attosecond pulse train that uniquely combines characteristics of good energy resolution (≈100-300meV) with sub-fs time resolution. This makes HHG an ideal source for studying the fastest dynamics in materials. Furthermore, using angle-resolved photoemission spectroscopy (ARPES), it is possible to extract detailed information about electron dynamics over the entire Brillouin zone. In recently published work, we combined HHG with ARPES to identify a sub-femtosecond excited-state lifetime for the first time. Photoemission occurs as a three-step process: 1) An electron is photoexcited from the valence band to far above the Fermi energy; 2) it transports to the surface, and 3) it overcomes the work function and exits. If the electron is promoted into a highlyexcited unoccupied band in the material (as opposed to a free-electron-like state), we observe the electron emission lifetime to increase in a measurable way—the Ni band 22 eV above the Fermi level has a lifetime of 212±30 attoseconds. Furthermore, by comparing photoemission from Cu and Ni, we reveal the influence of attosecond-timescale electron screening vs scattering by the electrons near the fermi surface. This work for the first time demonstrates the relevance of attosecond spectroscopy to the study of intrinsic properties and band structure in materials, as opposed to the strong-field induced dynamics studied extensively to-date.
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