摘要： In the past two decades, attosecond science has led to important insights into ultrafast electron dynamics. Whilst extreme-ultraviolet (XUV) photon flux attained from high harmonic generation (HHG) has increased significantly over this time, space-charge effects limit the maximum allowable number of photons per pulse for many attosecond experiments, especially in multidimensional – i.e., either angularly (ARPES) or spatially (PEEM) resolved – photoelectron spectroscopy (PES) on solids [1,2]. To allow for higher average flux without increasing space-charge impairments, HHG sources at higher repetition rates are needed. We combine a fibre-based ytterbium amplifier system with 4.5-μJ, 40-fs and 1030-nm pulses at 18.4 MHz repetition rate with intra-cavity HHG with an enhancement of 35 (Fig. 1a). Geometric output coupling through a pierced mirror affords high photon energies of 25-60 eV at a flux of 9×1012 XUV photons per second [3], which is unprecedented at this photon energy and MHz repetition rates. The HHG beam is focused on a tungsten target and the generated photoelectrons are detected by an angle-resolving time-of-flight (ToF) spectrometer. Simultaneous stabilization of cavity length and oscillator repetition rate result in excellent intensity stability of the system and enable long-term measurements with three orders of magnitude less integration time at equal space charge conditions in comparison to state-of-the-art kHz HHG sources. An evaluation of the photoelectron statistics of the setup by observing the temporal evolution of the relative standard deviation σ of the photoelectron spectrum proves the aptitude of our setup for high-precision, long-term PES experiments, with a statistical behaviour for up to 3×109 shots (see Fig. 1b). By measuring photoelectron spectra at different XUV flux, we show that no space charge within the instrumental resolution of 0.3 eV can be observed in a 10-μm-diameter focus spot. Our experiments reveal the XUV-IR-delay dependence of sidebands in the photoelectron spectrum generated by laser-assisted photoemission. This represents the first measurement with attosecond precision at MHz repetition rate.