Understanding dynamics on ultrafast timescales enables unique and new insights
into important processes in the materials and life sciences. In this respect,
the fundamental pump-probe approach based on ultra-short photon pulses aims at
the creation of stroboscopic movies. Performing such experiments at one of the
many recently established accelerator-based 4th-generation light sources such
as free-electron lasers or superradiant THz sources allows an enormous
widening of the accessible parameter space for the excitation and/or probing
light pulses. Compared to table-top devices, critical issues of this type of
experiment are fluctuations of the timing between the accelerator and external
laser systems and intensity instabilities of the accelerator-based photon
sources. Existing solutions have so far been only demonstrated at low
repetition rates and/or achieved a limited dynamic range in comparison to
table-top experiments, while the 4th generation of accelerator-based light
sources is based on superconducting radio-frequency technology, which enables
operation at MHz or even GHz repetition rates. In this article, we present the
successful demonstration of ultra-fast accelerator-laser pump-probe
experiments performed at an unprecedentedly high repetition rate in the few-
hundred-kHz regime and with a currently achievable optimal time resolution of
13 fs (rms). Our scheme, based on the pulse-resolved detection of multiple
beam parameters relevant for the experiment, allows us to achieve an excellent
sensitivity in real-world ultra-fast experiments, as demonstrated for the
example of THz-field-driven coherent spin precession
