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1.INTRODUCTIONTabletop high-harmonic generation (HHG) produces attosecond pulse trains with the unique characteristics of good energy resolution (≈100-300meV) and sub-fs time resolution, making HHG an ideal source for time-resolved photoemission studies [1,2]. In combination with angle-resolved photoemission spectroscopy (ARPES), it is now possible to extract detailed information about electron dynamics over the entire Brillouin zone [3]. In recent work [4] taking advantage of laser-assisted photoemission [5], we harnessed attosecond pulse trains to directly and unambiguously measure the difference in lifetimes between photoelectrons born into free-electron-like states and those excited into unoccupied excited states in the band structure of Ni(111). A significant increase in lifetime of 212±30 as occurs when the final state coincides with an unoccupied excited state in the Ni band structure. Moreover, a strong dependence of lifetime on emission angle is directly related to the final-state band dispersion as a function of electron transverse momentum. In further work, we have directly extracted the time-domain influence of scattering and screening on the ejected photoelectrons by the electrons near the Fermi level. By taking advantage of the polarization-and angle-resolved sensitivity of photoemission, we can clearly distinguish different photoelectron lifetimes from individual occupied valence bands of Ni and Cu with unprecedented energy and time resolution. This allows us to distinguish different attosecond electron screening and scattering dynamics in Ni(111) and Cu(111)— in the time domain for the first time. We note that our results are distinctly different from previous time-delay measurements in solids, in which multiple valence bands were probed using broad bandwidth isolated attosecond pulses, thus necessarily integrating over multiple bands and photoemission features [2]. 2.MethodologyThis work uses a system consisting of a high-harmonic (HHG) source driven by a ti:sapphire laser, and a UHV chamber with a hemispherical analyzer. The physics behind the experiment is explained in Fig. 1A. A comb of linearly polarized high harmonics is focused collinearly with infrared laser pulses (~26 fs, 780 nm) onto an atomically clean Ni(111) or Cu(111) surface. We use the entire spectrum of harmonic orders (11th – 41st) as illumination, and any band within the material thus manifests itself as a ladder of direct photoemission bands (Fig. 1B). Addition of the 780 nm laser results in phase-locked laser-assisted photoemission that modulates the photoelectron spectra as a function of relative delay between the EUV pump and IR probe fields (τd). This allows us to extract photoelectron dynamics on attosecond time scales and Å length scales, by analyzing the attosecond-time-scale beating due to the coherent interference of two-photon quantum pathways that lead to the same final photoelectron energy (in a technique combining laser-assisted photoemission and RABBITT) [5-7]. Fig. 1.(A) Using HHG, different photoelectron final states can be accessed, corresponding to free-electron-like states or excited states in the band structure. (B) Static ARPES excited by s-polarized HHG generated in different gas targets. (C) Band structure along Γ-L extracted from our data (open symbols) compared to previous experiments (solid lines) and DFT calculations (dashed lines). The final state resonance in (B) is highlighted. ![]() 3.ResultsWe first probed the band structure of Ni(111) by studying how the static photoelectron spectra depend on the EUV photon energy and polarization, as shown in Fig. 1B. Due to photoemission selection rules, we can unambiguously assign the two photoemission peaks that are excited by s-polarized light to the two valence bands with Λ3 symmetry ( Fig. 2.(A) Measured timing of photoemission ![]() By using the phase-locked laser-assisted photoemission, we can directly measure photoemission from the surface as a function of time. To take into account the influence of any chirp in the relative time of emission of the harmonics themselves, we use the non-resonant photoemission from Compared to Ni, Cu also has an fcc lattice structure, with similar lattice constants. The electronic structure of Cu(111) is also close to Ni(111) (Fig. 1C), but with the Fermi level approximately 2.2 eV higher, due to one more d electron in Cu. Because of the difference in Fermi level, Ni has a half-filled d band, while Cu has a filled one [8]. We repeated the laser-assisted photoemission measurements on Cu(111); the lifetime of photoelectrons from Cu Fig. 3.(A) Lifetime of photoelectrons from ![]() Figure 3 also shows that the time of photoemission for the 4.ConclusionsOur results highlight the importance of the material band structure on the timing of photoelectron emission from a surface, and represents the first experimental study of the effects of electron-electron scattering and dynamic screening in metals on attosecond time scales. Thus, it opens new possibilities for ultrafast studies of materials. REFERENCESS. Eich et al.,
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