Ionization in a frozen Rydberg gas with attractive or repulsive   potentials
  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. 
Matthieu Viteau (LAC), Amodsen Chotia (LAC), Daniel Comparat (LAC),   Duncan A. Tate (LAC), Thomas F. Gallagher (LAC), Pierr...