  We present an experimental study of the dynamics following the
photo-excitation and subsequent photo-ionization of single Cs atoms on the
surface of helium nanodroplets. The dynamics of excited-Cs-atom desorption and
re-adsorption as well as CsHe exciplex formation are measured using femtosecond
pump-probe velocity-map-imaging spectroscopy and ion-time-of-flight
spectrometry. The timescales for the desorption of excited Cs atoms off helium
nanodroplets as well as the timescales for CsHe exciplex formation are
experimentally determined for the 6p states of Cs. For the 6p $^2\Pi_ {1/2}$
state, our results confirm that the excited Cs atoms only desorb from the
nanodroplet when the excitation wavenumber is blue-shifted from the $6p\,^2\Pi_
{1/2} \leftarrow 6s\,^2\Sigma_ {1/2}$ resonance. Our results suggest that the
dynamics following excitation to the 6p $^2\Pi_ {3/2}$ state can be described
by an evaporation-like desorption mechanism, whereas the dynamics arising from
excitation to the 6p $^2\Sigma_ {1/2}$ state is indicative for a more impulsive
desorption process. Furthermore, our results suggest a helium-induced
spin-orbit relaxation from the the 6p $^2\Sigma_ {1/2}$ state to the 6p $^2\Pi_
{1/2}$ state. Our findings largely agree with the results of time-dependent
$^4$He-density-functional theory (DFT) simulations published earlier [Coppens
et al., Eur. Phys. J. D 73, 94 (2019)].
