  Radio-frequency (rf) fields in the MHz range are used to induce resonant
energy transfer between cold Rydberg atoms in spatially separated volumes.
After laser preparation of the Rydberg atoms, dipole-dipole coupling excites
the 49s atoms in one cylinder to the 49p state while the 41d atoms in the
second cylinder are transferred down to the 42p state. The energy exchanged
between the atoms in this process is 33 GHz. An external rf-field brings this
energy transfer into resonance. The strength of the interaction has been
investigated as a function of amplitude (0-1 V/cm) and frequency (1-30 MHz) of
the rf-field and as a function of a static field offset. Multi-photon
transitions up to fifth order as well as selection rules prohibiting the
process at certain fields have been observed. The width of the resonances has
been reduced compared to earlier results by switching off external magnetic
fields of the magneto-optical trap, making sub-MHz spectroscopy possible. All
features are well reproduced by theoretical calculations taking the strong
ac-Stark shift due to the rf-field into account.
