Resonant energy transfer between cold Rydberg atoms was used to determine
Rydberg atom energy levels, at precisions approaching those obtainable in
microwave spectroscopy. Laser cooled Rb atoms from a magneto-optical trap were
optically excited to 32d Rydberg states. The two-atom process 32d(j=5/2) +
32d(j=5/2) -> 34p(j=3/2) + 30g is resonant at an electric field of
approximately 0.3 V/cm. This process is driven by the electric dipole-dipole
interaction, which is allowed due to the partial f character that the g state
acquires in an electric field. The experimentally observed resonant field,
together with the Stark map calculation is used to make a determination of the
Rb ng-series quantum defect: delta_g (n=30) = 0.00405(6)