  The ac Stark effect was used to induce resonant energy transfer between
translationally cold Rydberg atoms. The Rb Rydberg atoms were obtained by laser
excitation of cold atoms from a magneto-optical trap. Using two-photon
microwave spectroscopy of the 49s_{1/2}-50s_{1/2} transition, the field
strength of a 28.5 GHz dressing field was calibrated. When this dressing field
was set at specific field strengths, the two-atom dipole-dipole process
43d_{5/2} + 43d_{5/2} -> 45p_{3/2} + 41f was dramatically enhanced, due to
induced degeneracy of the initial and final states. A series of observed
resonant field strengths correspond to different magnetic sublevel
possibilities for the initial and final states. These agree well with
calculated resonance fields based on a perturbative ac Stark shift formula.
This method for enhancing interactions is complementary to dc
electric-field-induced resonant energy transfer, but has more flexibility due
to the possibility of varying the applied frequency.
