Physikalische Chemie - Direktor: Prof. Dr. Martin Wolf
Department Seminar
Host: R. Ernstorfer

Thursday, March 12, 2020, 2:00 pm
Willstätter House, Faradayweg 10
Prof. Thomas Allison
Stony Brook University, NY
Time-resolved ARPES at 88 MHz repetition rate with full 2π collection - Video Conference (for conference link write to Ralph Ernstorfer)
Angle-resolved photoemission spectroscopy (ARPES) is often considered the best way to experimentally determine the ground-state electronic structure of materials. However, although applying ARPES to short-lived excited states via the pump/probe method (tr-ARPES) demands orders of magnitude more data than ground-state ARPES studies, measurements have been forced to work with orders of magnitude lower data rates due to the limits imposed by the repetition rate of available short-pulse extreme-ultraviolet (XUV) light sources and the collection efficiency of photoelectron analyzers. These limitations have severely restricted the range of samples and phenomena that can be studied with ultrafast tr-ARPES.
At Stony Brook, we have constructed a next-generation tr-ARPES instrument that addresses this data-rate challenge. We realize a high repetition rate ultrashort-pulse XUV light source using frequency comb methods and the technique of cavity-enhanced high harmonic generation CE-HHG. With a relatively simple and compact setup, the light source delivers to the sample >1011 photons/s in well-isolated harmonics over a broad photon energy tuning range of 18-37 eV with a spot size of 58 × 100 μm2 [1-3]. The high repetition rate enables space-charge-free tr-ARPES experiments with nano-Ampere sample currents. From photoelectron spectroscopy data, we place conservative upper limits on the XUV pulse duration and photon-energy bandwidth of 93 fs and 65 meV, respectively. The light source has now been integrated with a time-of-flight momentum microscope [4], enabling efficient, parallel, momentum-resolved detection of the full 2π emission hemisphere from the surface under study.
I will discuss preliminary results with this new instrument, ongoing tr-ARPES experiments, and also continuing development of the instrument. In particular, I will focus on tr-ARPES measurements from micron-scale sample areas and tr-ARPES measurements with perturbative pump excitation to study the intrinsic quasi-particle dynamics of materials.
[1] X. L. Li, M. A. R. Reber, C. Corder, Y. Chen, P. Zhao, and T. K. Allison. Rev. Sci. Inst. 87, 093114 (2016).
[2] C. Corder, P. Zhao, J. Bakalis, X. L. Li, M. D. Kershis, A. R. Muraca, M. G. White, and T. K. Allison, Structural Dynamics 5, 054301 (2018).
[3] C. Corder, P. Zhao, J. Bakalis, X. L. Li, M. D. Kershis, A. R. Muraca, M. G. White, and T. K. Allison. Development of a tunable high repetition rate XUV source for time-resolved photoemission studies of ultrafast dynamics at surfaces. Proceedings SPIE 10519, LAMOM XXIII, 105190B (2018). DOI:10.1117/12.2295232.
[4] K. Medjanik, O. Fedchenko, S. Chernov, D. Kutnyakhov, M. Ellguth, A. Oelsner, B. Schonhense, T. R. F. Peixoto, P. Lutz, C. H. Min, F. Reinert, S. Daster, Y. Acremann, J. Viefhaus, W. Wurth, H. J. Elmers, and G. Schonhense. Nat. Mater. 16, 615–621 (2017).