  We present a combined ion imaging and density functional theory study of the
dynamics of the desorption process of rubidium and cesium atoms off the surface
of helium nanodroplets upon excitation of the perturbed $6s$ and $7s$ states,
respectively. Both experimental and theoretical results are well represented by
the pseudodiatomic model for effective masses of the helium droplet in the
desorption reaction of m_eff/m_He~10 (Rb) and 13 (Cs). Deviations from this
model are found for Rb excited to the 6p state. Photoelectron spectra indicate
that the dopant-droplet interaction induces relaxation into low-lying
electronic states of the desorbed atoms in the course of the ejection process.
