(a) Tong et al. produce ions in selected vibrational and rotational states inside an ion trap by two-photon excitation to the first electronic state, and then a third photon ionizes the . (b) To detect the rotational and vibrational populations, Tong et al. excite the from specific rotational states to the first electronically excited state (A). A known portion of these decay by fluorescence to vibrational levels in the ground state (X), and these state-selected ions undergo charge transfer with argon in the trap. This neutralizes the nitrogen ions, and their removal can be captured in images, yielding quantitative measurements of the population in each rotational-vibrational state.