Abstract
Time- and Angle-resolved photoelectron spectroscopy from surfaces can be used to record the dynamics of electrons and holes in condensed matter on ultrafast time scales. However, ultrafast photoemission experiments using extreme-ultraviolet (XUV) light have previously been limited by either space-charge effects, low photon flux, or limited tuning range. In this article, we describe XUV photoelectron spectroscopy experiments with up to 5 nA of average sample current using a tunable cavity-enhanced high-harmonic source operating at 88 MHz repetition rate. The source delivers >1011 photons/s in isolated harmonics to the sample over a broad photon energy range from 18 to 37 eV with a spot size of 58 × 100 μm2. 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 high photocurrent, lack of strong space charge distortions of the photoelectron spectra, and excellent isolation of individual harmonic orders allow us to observe laser-induced modifications of the photoelectron spectra at the 10−4 level, enabling time-resolved XUV photoemission experiments in a qualitatively new regime.
Highlights
Time- and Angle-resolved photoelectron spectroscopy from surfaces can be used to record the dynamics of electrons and holes in condensed matter on ultrafast time scales
Angle-resolved photoelectron spectroscopy (ARPES) using synchrotron radiation has become an essential tool for condensed matter physics and surface science
Pump-probe experiments in condensed matter physics can broadly be divided into nonperturbative “high-fluence” and perturbative “low-fluence” experiments
Summary
Angle-resolved photoelectron spectroscopy (ARPES) using synchrotron radiation has become an essential tool for condensed matter physics and surface science. Even at the high repetition rate of 100 kHz and the coarse resolution of 100 meV, space charge constraints still limit the sample current to 760 pA. Second is that the signal of interest is inherently small, since only a fraction of the sample’s electrons are excited by the pump Combined, these two factors result in the need for orders of magnitude more data than ground state studies done at synchrotrons, but space charge limits the data rate to be orders of magnitude lower. By performing experiments with high flux at 88 MHz repetition rate, nanoamperes of sample current can be generated from a sub-100 lm laser spot with space charge effects estimated to be less than 10 meV, comparable to synchrotron-based ARPES experiments.. IV, we compare this work to previous efforts and discuss how the system can be further improved
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