  We report the creation of an interacting cold Rydberg gas of strontium atoms.
We show that the excitation spectrum of the inner valence electron is sensitive
to the interactions in the Rydberg gas, even though they are mediated by the
outer Rydberg electron. By studying the evolution of this spectrum we observe
density-dependent population transfer to a state of higher angular momentum l.
We determine the fraction of Rydberg atoms transferred, and identify the
dominant transfer mechanism to be l-changing electron-Rydberg collisions
associated with the formation of a cold plasma.
