Ionization of Rb Rydberg atoms in the attractive nsnp dipole-dipole potential

We have observed the ionization of a cold gas of Rb Rydberg atoms which occurs when nsns van der Waals pairs of ns atoms of n approximate to 40 on a weakly repulsive potential are transferred to an attractive dipole-dipole nsnp potential by a microwave transition. Comparing the measurements to a simple model shows that the initial 300-mu K thermal velocity of the atoms plays an important role. Excitation to a repulsive dipole-dipole potential does not lead to more ionization on a 15-mu s time scale than leaving the atoms in the weakly repulsive nsns state. This observation is slightly surprising since a radiative transition must occur to allow ionization in the latter case. Finally, by power broadening of the microwave transition, to allow transitions from the initial nsns state to the nsnp state over a broad range of internuclear spacings, it is possible to accelerate markedly the evolution to a plasma.