Fig. 2: Photophysical properties of [Eu₂] in the solid-state.

a Mechanism of PL sensitization in Eu(III) complexes: the Eu(III)-based ⁵D_J (J = 0–2) receiving levels are populated after a series of excitation, intersystem crossing (ISC), and T₁ → ⁵D_J energy transfer (ET) processes. The co-populated excited ⁵D₂ and ⁵D₁ levels non-radiatively (NR) relax to ⁵D₀ level (inset); radiative relaxation of ⁵D₀ level to ground ⁷F_J (J = 0–6) crystal field levels manifests as line-like luminescence. The ⁵D₀ → ⁷F₀ transition is suited for QIP applications due to its narrow linewidth and long coherence lifetimes of the nuclear spin states—±5/2, ±3/2, ±1/2 (I = 5/2 for ¹⁵¹Eu/¹⁵³Eu)—associated with the ⁷F₀ ground-state level. b Photoluminescence spectrum showing the ⁵D₀ → ⁷F_J (J = 0–6) transitions in the visible and near-IR range (λ_exc. = 330 nm). c Photoluminescence excitation (PLE) spectrum of the ⁵D₀ → ⁷F₀ transition of [Eu₂] measured at 1.4 K. An inhomogeneous linewidth (Γ_inh) of 50 GHz is calculated for the main peak centered at 580.185 nm (vac.) d Luminescence decay of the ⁵D₀ excited-state of [Eu₂] measured at 20 K under resonant excitation at 580.185 nm. An excited-state lifetime (τ_obs) of 880 μs was obtained from a single exponential fit of the decay.