Generation of cold polyatomic molecular ions by ion-atom collisions

A goal of studies on ultracold chemical reactions is the formation of ultracold molecules containing three or more atoms. Although the first ultracold polyatomic molecules were formed recently; knowledge of the kinetics of polyatomic molecular ions as reaction products remains limited. Thus, we studied ion-atom reaction collisions in a continuous-wave (CW) laser photoionization of cold atoms in an Rb-Rb$^+$ hybrid trap. A series of polyatomic molecular ions was produced, with precise changes in the atomic number of one rubidium atom. Using resonant-excitation mass spectrometry, we directly observed Rb$_3^+$ and Rb$_4^+$ molecular ions in time-of-flight mass spectrum for the first time. The information of the quantum state of these polyatomic molecular ions and the influence factors was obtained by measuring their lifetimes. The approach is simple, robust, and suitable for all types of laser-coolable elements. Our work paves the way for ultracold ion-atom chemical reactions, introduces the concept of polyatomic molecular ion platforms, and deepens the understanding of ion-atom reaction collisions. It has important implications for astronomical sciences, ultracold neutral plasma, cluster physics, and other disciplines.