Colloquium
Department of Physics, NCU
Photoelectron spectroscopy with femtosecond lasers
Prof. 江正天 (Cheng-Tien Chiang)
Institute of Atomic and Molecular Sciences, Academia Sinica
Date 2021.10.05 (Tue)
Place S4-625
Time 14:00-16:00
Photoelectron spectroscopy has become a general tool to study the electronic structure of materials. In combination with femtosecond lasers and nonlinear optics, laser-based photoemission experiments at surfaces have allowed to study the ultrafast intertwined dynamics of electrons, phonons, and spin excitations [1,2]. As a significant extension of conventional photoemission experiments, there has been significant progress of double photoemission (DPE) spectroscopy at surfaces recently. In DPE experiments, a pair of photoelectrons are emitted upon the absorption of one single photon [3]. By using a pair of electron spectrometers, these two photoelectrons can be analyzed with energy and momentum resolution. Therefore DPE can provide unique access to directly characterize interacting electrons in solids.
In this talk, an overview of the recently developed laser-based DPE experiments will be introduced [4]. To provide a sufficiently high photon energy to excite a pair of electrons, high-order harmonic light sources operating at megahertz repetition rates with a widely tunable photon energy range were built [5]. In combination with a pair of time-of-flight photoelectron spectrometers, photoelectron pairs from Ag(001), Cu(111), as well as the strongly correlated NiO(001) and CoO(001) surfaces have been observed. Specifically on the Ag and Cu surfaces, photoelectron pairs can be categorized according to their valence band character for the first time [6]. In strong contrast, on NiO and CoO indications of multiple excitations of d electrons were observed. These results will be discussed in terms of the characteristic electron-electron interaction in these materials.
[1] S. Geber et al., Science 357, 71 (2017).
[2] K. Gillmeister, D. Golež, C.-T. Chiang et al., Nat. Commun. 11, 4095 (2020).
[3] J. Berakdar, Phys. Rev. B 58, 9808 (1998).
[4] C.-T. Chiang et al., Prog. Surf. Sci. 95, 100572 (2020).
[5] C.-T. Chiang et al., J. Electron Spectrosc. Relat. Phenomena 200, 15 (2015).
[6] A. Trützschler, M. Huth, C.-T. Chiang et al., Phys. Rev. Lett. 118, 136401 (2017).