Abstract:
Recent development of femtosecond lasers has allowed laboratory-based angle-resolved photoelectron spectroscopy (ARPES) and photoelectron microscopy (PEEM) to study ultrafast electron dynamics at surfaces and nanostructures. In this talk, the advanced laser-based photoelectron methods including the two-photon photoemission (2PPE) and the double photoemission (DPE) will be introduced.
In 2PPE, photoelectrons are emitted from surfaces due to the absorption of twice of the incident photon energy. As a result, 2PPE provides a unique opportunity to probe the unoccupied electronic states above the Fermi-level. As an example, spin-resolved and magnetic dichroic 2PPE on the unoccupied quantum well states within the few layers of cobalt films will be presented [1]. In these 2PPE processes, spin-dependent transitions and spin-orbit coupling play important roles [2-3], which allow magnetic domain imaging in PEEM.
In the DPE process, the incident photon energy is shared by two photoelectrons. Therefore, DPE provides the possibility to directly analyze the electron-electron interaction in solids. In this talk, a newly developed vacuum-ultraviolet high-order harmonic light source dedicated to ARPES and DPE will be introduced [4,5]. By combining this light source with a pair of time-of-flight photoelectron spectrometers, a band-resolved analysis of correlated electron pairs is realized for DPE on Ag(001) and Cu(111) [6]. The signatures of two-electron states in the two-dimensional DPE spectra will be discussed in terms of electron-electron interaction.
[1] Magnetic dichroism from optically excited quantum well states C.-T. Chiang et al., Phys. Rev. Lett. 103, 077601 (2009).
[2] Spin-orbit coupling in unoccupied quantum well states: experiment and theory for Co/Cu(001) C.-T. Chiang, et al., Phys. Rev. B. 81, 115130 (2010).
[3] Spin-selective pathways in linear and nonlinear photoemission from ferromagnets C.-T. Chiang, et al, Phys. Rev. B. 85, 165137 (2012).
[4] Boosting laboratory photoelectron spectroscopy by megahertz high-order harmonics C.-T. Chiang, et al., New J. Phys. 17, 013035 (2015).
[5] Efficient and tunable high-order harmonic light sources for photoelectron spectroscopy at surfaces C.-T. Chiang et al., J. Electron Spectrosc. Relat. Phenomena, 200, 15 (2015).
[6] Band-resolved double photoemission spectroscopy on correlated valence electron pairs in metals
A. Trützschler, M. Huth, C.-T. Chiang (corresponding author), R. Kamrla, F. O. Schumann, J. Kirschner, and
W. Widdra, Phys. Rev. Lett. 118, 136401 (2017).
Short Bio: Dr. Cheng-Tien Chiang, born in 1984 in Taipei, Taiwan, obtained his Bachelor and Master degrees in the Department of Physics, National Taiwan University, Taipei (2005, 2007), and PhD in the Max Planck Institute of Microstructure Physics and the Martin-Luther-University Halle-Wittenberg in Halle (Saale), Germany (2011). His research focuses on the electronic structure at surfaces and interfaces of transition metals and their oxides. In these systems, the interaction between localized d electrons as well as the interplay between the d and sp electrons play dominant roles. To observe these effects directly in experiments, he uses laser-based photoelectron spectroscopy and microscopy with femtosecond time-resolution.