We present the results of experiments demonstrating the spectroscopic detection of Förster resonance energy transfer from NH_3 in the X ^1A_1 ground electronic state to helium atoms in 1sns ^3S_1 Rydberg levels, where n=37 and n=40. For these values of n the 1sns ^3S_1→1snp ^3P_J transitions in helium lie close to resonance with the ground-state inversion transitions in NH_3, and can be tuned through resonance using electric fields of less than 10 V/cm. In the experiments, energy transfer was

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... tected by direct state-selective electric field ionization of the ^3S_1 and ^3P_J Rydberg levels, and by monitoring the population of the ^3D_J levels following pulsed microwave transfer from the ^3P_J levels. Detection by microwave spectroscopic methods represents a highly state selective, low-background approach to probing the collisional energy transfer process and the environment in which the atom-molecule interactions occur. The experimentally observed electric-field dependence of the resonant energy transfer process, probed both by direct electric field ionization and by microwave transfer, agrees well with the results of calculations preformed using a simple theoretical model of the energy transfer process. For measurements performed in zero electric field with atoms prepared in the 1s40s ^3S_1 level the transition from a regime in which a single energy transfer channel can be isolated for detection to one in which multiple collision channels begin to play a role has been identified as the NH_3 density was increased.