Abstract:
With the recent progress in high-order harmonic generation (HHG) using femtosecond lasers, laboratory photoelectron spectroscopy with an ultrafast, widely tunable vacuum-ultraviolet light source has become available. Whereas HHG-based photoemission experiments at kilohertz repetition rates have been severely limited by the space-charge effects in the past, the new development of compact HHG light sources with megahertz repetition rates allows for efficient photoemission and double photoemission experiments as is demonstrated here [1-7].
I will present momentum-resolved photoemission experiments with photon energies between 14 and 40 eV that demonstrate the high performance of the setup [3,4,6]. In addition, the combination of two time-of-flight spectrometers with coincidence detection electronics opens the way for efficient and long-term stable double photoemission experiments at variable photon energies [1,6-8]. For the noble metal (001) surface of Ag, we present a detailed analysis of double photoemission data and will compare them with similar data for the NiO(001) surface. The electron-electron pair distribution shows a sharp sum-energy onset, which corresponds to one hole in the Ag 3d band (4.5 eV below the Fermi level) and a second excitation from the Ag sp band. Simultaneously, an intense energy sharing between the electrons in the pair is visible indicating strong electron-electron correlations [5]. For thin films of C60, a molecular-orbital resolved correlation energy is determined based on double photoemission data at various photon energies.

References:
[1]    M. Huth, C.-T. Chiang, A. Trützschler, F. O. Schumann, J. Kirschner, and W. Widdra; Applied Physics Letters 104, 061602 (2014).
[2]    A. Blättermann, C.-T. Chiang, and W. Widdra; Physical Review A 89, 043404 (2014).
[3]    C.-T. Chiang, M. Huth, A. Trützschler, M. Kiel, F. O. Schumann, J. Kirschner, and W. Widdra, New Journal of Physics 17, 013035 (2015).
[4]    C.-T. Chiang, M. Huth, A. Trützschler, F. O. Schumann, J. Kirschner, and W. Widdra, Electron Spectroscopy and Related Phenomena 200, 15-21(2015).
[5]     A. Trützschler, M. Huth, C.-T. Chiang, R. Kamrla, F. O. Schumann, J. Kirschner, and W. Widdra, Phys. Rev. Lett. 118, 136401(2017).
[6]    M. Huth, A. Trützschler, C.-T. Chiang, R. Kamrla, F. O. Schumann, and W. Widdra, J. Appl. Phys. 124, 164504 (2018).
[7]     C.-T. Chiang, A. Trützschler, M. Huth, R. Kamrla, F. O. Schumann, and W. Widdra, Prog. Surf. Sci. 95, 100572 (2020).

Speaker Bio: Prof. Wolf Widdra is a Professor at the the Institute of Physics, Martin-Luther-Universität Halle-Wittenberg, Halle, Germany.