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