  Since their first experimental observation, ultralong-range Rydberg molecules
consisting of a highly excited Rydberg atom and a ground state atom have
attracted the interest in the field of ultracold chemistry. Especially the
intriguing properties like size, polarizability and type of binding they
inherit from the Rydberg atom are of interest. An open question in the field is
the reduced lifetime of the molecules compared to the corresponding atomic
Rydberg states. In this letter we present an experimental study on the
lifetimes of the ^3\Sigma (5s-35s) molecule in its vibrational ground state and
in an excited state. We show that the lifetimes depends on the density of
ground state atoms and that this can be described in the frame of a classical
scattering between the molecules and ground state atoms. We also find that the
excited molecular state has an even more reduced lifetime compared to the
ground state which can be attributed to an inward penetration of the bound
atomic pair due to imperfect quantum reflection that takes place in the special
shape of the molecular potential.
