  Besides the use as cold matrix for spectroscopic studies, superfluid helium
droplets have served as a cold environment for the synthesis of molecules and
clusters. Since vibrational frequencies of molecules in helium droplets exhibit
almost no shift compared to the free molecule values, one could assume the
solvated particles move frictionless and undergo a reaction as soon as their
paths cross. There have been a few unexplained observations that seemed to
indicate cases of two species on one droplet not forming bonds but remaining
isolated. In this work, we performed a systematic study of helium droplets
doped with one rubidium and one strontium atom showing that besides a reaction
to RbSr, there is a probability of finding separated Rb and Sr atoms on one
droplet that only react after electronic excitation. Our results further
indicate that ground-state Sr atoms can reside at the surface as well as inside
the droplet.
