Generation of cold polyatomic cations by reactive two-body ion-atom collisions

The frontiers of chemical research have shifted from all-neutral cold chemistry to charged-neutral cold chemistry with the development of laser cooling and charged particle trapping technologies. One of the goals of cold chemical reactions is the formation of cold molecules containing three or more atoms. However, no polyatomic cations have been generated in a laboratory through reactive ion-atom collisions. Here, we report the first creation of a series of cold ($\leq$ mK) polyatomic cations $^{87}$Rb$_\text{N}^+$ (N = 2, 3,..., 18) in the two-step CW-laser photoionization of laser-cooled $^{87}$Rb atoms in the ion-atom hybrid trap. These polyatomic cations occur in a large number ($\sim 10^6$ for $^{87}$Rb$_\text{N}^+$, N = 2,3,...,7) thanks to the experimental realization of spatial overlap of a dense ionic cloud and an atomic cloud as well as the storage of charged reaction products. We have for the first time directly observed these reaction products by combining time-of-flight mass spectrometry (TOF-MS) and resonant-excitation mass spectrometry (REMS). By controlling the interaction time of ion-atom collisions and the storage time of ions, we have experimentally validated the cascaded generation and dissociation characteristics of this series of polyatomic cations. The techniques for assembling and observing homonuclear polyatomic cations developed by us are applicable to any experiment involving an ion-atom hybrid trap. Our work paves the way for the investigation of cold-controlled ion-atom chemistry and the evolution of the interstellar medium.Comment: 4 figures and 1 supplemental figure