  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 space-charge-free 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 $ > 10^{11}$ photons/s in isolated harmonics to the sample over
a broad photon energy range from 18 to 37 eV with a spot size of $58 \times 100
\; \mu$m$^2$. 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 space charge distortions of
the photoelectron spectra, and excellent isolation of individual harmonic
orders allow us to observe the laser-assisted photoelectric effect with
sideband amplitudes as low as $6 \times 10^{-4}$, enabling time-resolved XUV
photoemission experiments in a qualitatively new regime.
