RESONANT COLLISIONS OF RYDBERG ATOMS EXCITED FROM A MAGNETO OPTICAL TRAP William R. Anderson and T. F. Gallagher, University of Virginia, Charlottesville, VA, USA (NIST address: Building 220, Room B206, NIST, Gaithersburg, MD 20899, 301-975-3744, email:

Atoms with a single electron excited to a state of high principle quantum number (Rydberg states) provide an ideal system in which to study resonant dipole-dipole collisions. Such a collision is necessary to achieve population inversion in some lasers. However, these lasers rely on coincidental resonances of certain atomic or molecular systems. Rydberg atoms, with their immense dipole moments and dense, easily tunable energy levels provide a controllable system in which to study this process. In addition, well proven laser cooling techniques may be used with rubidium to control the relative velocities of the colliding atoms.

We collect rubidium atoms from a thermal vapor with a diode laser magneto optical trap (MOT). We then use pulsed dye lasers to excite trapped atoms to Rydberg states. Electric field plates surrounding the atom cloud allow the atomic energies to be shifted through a resonant dipole-dipole coupling with other Rydberg states. A collision is detected by selectively field ionizing atoms in states which are accessible only through a collision.

Continuously tuning the energy levels reveals the collisional interaction as a function of detuning from resonance. We show a dramatic decrease in the resonance widths when observing slow collisions from the trap as opposed to fast collisions in a room temperature atomic vapor. We have observed collisions which have an intrinsic collision time of at least half a microsecond. This is an eternity for an atomic collision. Ramsey interference is observed by allowing the atoms to collide multiple times with a variable time delay between collisions. The data are consistent with some simple models of the dipole collision interaction. However there is suggestion that some other interaction, possibly multi-body, is present.