The energy spread of a focused ion beam causes chromatic aberration that limits the focal spot size and resolution for imaging and fabrication. Ion beams based on photoionization of neutral atoms can have a much smaller energy spread and higher brightness than conventional liquid metal ion sources and are thus capable of higher resolution. We present a method for using selective ionization of Rydberg atoms to reduce the beam energy spread in a cold-atom focused ion beam. We produce experimental maps of the ionization rate of ${}^{85}\mathrm{Rb}$ near the classical ionization threshold, predict the energy spread of an ion beam produced from these states, and demonstrate energy spread reduction in situ in a focused ion beam. We use a novel method to measure the energy spread from images, using only components present in many commercial focused ion beam systems. We choose two states to illustrate the technique and show a 50% difference in energy spread, with opportunities to further reduce the energy spread by constructing a more favorable electric field gradient and finding atomic states with better ionization characteristics.

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• Rydberg atoms & molecules
• Focused ion beam
• Ion beam analysis