  We report clear evidence of the role of dipole-dipole interaction in Penning
ionization of Rydberg atoms, leading to the formation of an ultracold plasma.
Penning ionization of np Rydberg Cesium atoms is prevented for states with in n
< 42, which correspond to a repulsive potential, but it does not occur for n
larger than 42, corresponding to an attractive potential. Blackbody radiation
is mostly responsible for the background and initial ionization, although
ion-Rydberg collisions and population transfer due to limited superradiance may
have to be considered.
