Mechanism of Electron Spin Decoherence in a Partially Deuterated Glassy Matrix

Long electron spin coherence lifetimes are essential for applications in quantum information science and electron paramagnetic resonance, for instance, for nanoscale distance measurements in biomolecular systems using double electron-electron resonance. We experimentally investigate the decoherence dynamics under the Hahn echo sequence of the organic radical d₁₈-TEMPO in a variably deuterated frozen water:glycerol matrix. The coherence time (phase memory time) T_M scales with proton concentration as [¹H]^-0.65. For selectively deuterated matrices, decoherence is accelerated in the presence of proton clustering, that is, substantial short-range density in the proton-proton radial distribution functions (<3 Å). Simulations using molecular dynamics and many-body spin quantum dynamics show excellent agreement with experiment and show that geminal proton pairs such as CH₂ and OH₂ groups are major decoherence drivers. This provides a predictive tool for designing molecular systems with long electron spin coherence times.