Capturing Exciton-Proton Collisions in Confined Water

<title>Abstract</title> Excitons—electron-hole pairs that dominate the excited states of semiconductors—are exquisitely sensitive to molecules and ions. However, these quasiparticles are prone to environmental noise, limiting their selectivity as chemical probes. Here, we report the first direct observation of exciton-proton collisions in confined water, enabled by a quantum defect “trap-in-a-pore” architecture that isolates excitons from unintended perturbations. When protons encounter excitons, we observe discrete, reversible photoluminescence spectral switching manifested as intensity steps, revealing stochastic proton trapping and de-trapping events at the same defect site. These single-ion traps enable optical interrogation of spectroscopically elusive ions, such as hydrated H+ and D+, and their dynamics at the single-particle level.