The effects of weak electric fields on resonant energy transfer between
NH3 in the
X 1A1 ground electronic state and Rydberg He atoms in triplet states with principal quantum numbers
n=36--41 have been studied in a crossed-beam apparatus. For these values of
n, electric dipole transitions between the Rydberg states that evolve adiabatically to the $|ns\ensuremath{\rangle}$ and $|np\ensuremath{\rangle}$ states in zero electric field can be tuned into resonance with the ground-state inversion transitions in
NH3 using electric fields, with energy transfer occurring via F\"orster resonance. In the experiments the Rydberg He atoms, traveling in pulsed supersonic beams, were prepared by resonant two-photon excitation from the metastable
1s2s3S1 level and crossed an effusive beam of
NH3 before being detected by state-selective pulsed-electric-field ionization. The resonant-energy-transfer process was identified by monitoring changes in the ionization signal from the $|ns\ensuremath{\rangle}$ and $|np\ensuremath{\rangle}$ Rydberg states for each value of
n. The electric-field dependence of the experimental data is in good agreement with the results of calculations in which the resonant dipole-dipole coupling between the collision partners was accounted for.
Long-range interactions between cold Rydberg atoms, which are used in many important applications, can be enhanced using F\"orster resonances between collective many-body states controlled by an external electric field. Here we report on the first experimental observation of highly resolved radio-frequency-assisted F\"orster resonances in a few cold Rb Rydberg atoms. We also observed radio-frequency-induced F\"orster resonances which cannot be tuned by a dc electric field. They imply an efficient transition from van der Waals to resonant dipole-dipole interaction due to Floquet sidebands of Rydberg levels appearing in the rf field. This method can be applied to enhance the interactions of almost arbitrary Rydberg atoms with large principal quantum numbers.
We provide evidence for the dipole blockade of Rydberg excitation by examining the distributions of the number of Rydberg excitations created in ensembles of interacting Rydberg atoms. We show that, over a range of densities, the Rydberg excitation number distributions become significantly narrower when an applied electric field is used to tune the
85Rb interaction channel $45{D}_{5∕2}+45{D}_{5∕2}\ensuremath{\rightarrow}43F+47{P}_{3∕2}$ into F\"orster resonance. Substantially sub-Poissonian distributions that agree with published theory were achieved using very dense atom samples tuned into F\"orster resonance.