  Ultrafast electron dynamics drive phenomena such as photochemical reactions,
catalysis, and light harvesting. To capture such dynamics in real-time,
femtosecond to attosecond light sources are extensively used. However, an exact
match between the excitation photon energy and a characteristic resonance is
crucial. High-harmonic generation sources are exceptional in terms of pulse
duration but limited in spectral tunability in the VUV range. Here, we present
a monochromatic femtosecond source continuously tunable around 21 eV photon
energy utilizing the second harmonic of an OPCPA laser system to drive
high-harmonic generation. The unique tunability of the source is verified in an
experiment probing the interatomic Coulombic decay in doped He nanodroplets
across the He absorption bands. Moreover, we achieved intensities sufficient
for driving non-linear processes using a tight focusing of the VUV beam. We
demonstrated it on the observation of collective autoionization of multiply
excited pure He nanodroplets.
