Electrically tuned Förster resonances in collisions of NH3 with Rydberg He atoms

Abstract

The effects of weak electric fields on resonant energy transfer between ${\mathrm{NH}}_{3}$ in the $X$ ${^{1}A}_{1}$ 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 ${\mathrm{NH}}_{3}$ 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 $1s2s\phantom{\rule{0.16em}{0ex}}{^{3}S}_{1}$ level and crossed an effusive beam of ${\mathrm{NH}}_{3}$ 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.