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

摘要

We present the results of experiments demonstrating the spectroscopicdetection of F\"{o}rster resonance energy transfer from NH$_3$ in the$X\,^1A_1$ ground electronic state to helium atoms in 1s$n$s\,$^3$S$_1$ Rydberglevels, where $n=37$ and $n=40$. For these values of $n$ the1s$n$s\,$^3$S$_1\rightarrow$1s$n$p\,$^3$P$_J$ transitions in helium lie closeto resonance with the ground-state inversion transitions in NH$_3$, and can betuned through resonance using electric fields of less than 10~V/cm. In theexperiments, energy transfer was detected by direct state-selective electricfield ionization of the $^3$S$_1$ and $^3$P$_J$ Rydberg levels, and bymonitoring the population of the $^3$D$_J$ levels following pulsed microwavetransfer from the $^3$P$_J$ levels. Detection by microwave spectroscopicmethods represents a highly state selective, low-background approach to probingthe collisional energy transfer process and the environment in which theatom-molecule interactions occur. The experimentally observed electric-fielddependence of the resonant energy transfer process, probed both by directelectric field ionization and by microwave transfer, agrees well with theresults of calculations preformed using a simple theoretical model of theenergy transfer process. For measurements performed in zero electric field withatoms prepared in the 1s40s\,$^3$S$_1$ level the transition from a regime inwhich a single energy transfer channel can be isolated for detection to one inwhich multiple collision channels begin to play a role has been identified asthe NH$_3$ density was increased.