Fluorescence Imaging of Individual Ions and Molecules in Pressurized Noble Gases for Barium Tagging in ¹³⁶Xe

The imaging of individual Ba²⁺ ions in high pressure xenon gas is one possible way to attain background-free sensitivity to neutrinoless double beta decay and hence establish the Majorana nature of the neutrino. In this paper we demonstrate selective single Ba²⁺ ion imaging inside a high-pressure xenon gas environment. Ba²⁺ ions chelated with molecular chemosensors are resolved at the gas-solid interface using a diffraction-limited imaging system with scan area of 1×1~cm² located inside 10~bar of xenon gas. This new form of microscopy represents an important enabling step in the development of barium tagging for neutrinoless double beta decay searches in ¹³⁶Xe, as well as a new tool for studying the photophysics of fluorescent molecules and chemosensors at the solid-gas interface.