Probing resonant energy transfer in collisions of ammonia with Rydberg helium atoms by microwave spectroscopy.

We present the results of experiments demonstrating the spectroscopic detection of Förster resonance energy transfer from NH³ in the X¹A₁ ground electronic state to helium atoms in 1sns ³S₁ Rydberg levels, where n = ³7 and n = 40. For these values of n, the 1sns ³S₁ →1snp ³P_J transitions in helium lie close to resonance with the ground-state inversion transitions in NH³ and can be tuned through resonance using electric fields of less than 10 V/cm. In the experiments, energy transfer was detected by direct state-selective electric field ionization of the ³S₁ and ³P_J Rydberg levels and by monitoring the population of the ³DJ 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 performed using a simple theoretical model of the energy transfer process. For measurements performed in zero electric field with atoms prepared in the 1s40s ³S₁ 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 NH³ density was increased. [ABSTRACT FROM AUTHOR]