  The desorption dynamics of rubidium dimers (Rb_2) off the surface of helium
nanodroplets induced by laser excitation is studied employing both nanosecond
and femtosecond ion imaging spectroscopy. Similarly to alkali metal atoms, we
find that the Rb_2 desorption process resembles the dissociation of a diatomic
molecule. However, both angular and energy distributions of detected Rb_2^+
ions appear to be most crucially determined by the Rb_2 intramolecular degrees
of freedom rather than by those of the Rb_2He_N complex. The pump-probe
dynamics of Rb_2^+ is found to be slower than that of Rb^+ pointing at a weaker
effective guest-host repulsion for excited molecules than for single atoms.
