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

Abstract

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 1s$n$s\,$^3$S$_1$ Rydberg levels, where $n=37$ and $n=40$. For these values of $n$ the 1s$n$s\,$^3$S$_1\rightarrow$1s$n$p\,$^3$P$_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 detected by direct state-selective electric field ionization of the $^3$S$_1$ and $^3$P$_J$ Rydberg levels, and by monitoring the population of the $^3$D$_J$ levels following pulsed microwave transfer from the $^3$P$_J$ 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\,$^3$S$_1$ 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.