  Clouds of ultracold strontium 5s48s 1S0 or 5s47d 1D2 Rydberg atoms are
created by two photon excitation of laser cooled 5s2 1S0 atoms. The spontaneous
evolution of the cloud of low orbital angular momentum (low-l) Rydberg states
towards an ultracold neutral plasma is observed by imaging resonant light
scattered from core ions, a technique that provides both spatial and temporal
resolution. Evolution is observed to be faster for the S-states, which display
isotropic attractive interactions, than for the D-states, which exhibit
anisotropic, principally repulsive interactions. Immersion of the atoms in a
dilute ultracold neutral plasma speeds up the evolution and allows the number
of Rydberg atoms initially created to be determined.
