The second emission of the uranyl ion in aqueous solution

Abstract

The second lower-energy emission (LEE), weaker than the normal higher-energy luminescence (HEE) of UO²⁺₂(aq) and previously assigned to the ^*(OUOHOUO)⁴⁺(aq) exciplex in the exciplex-formation model (EXFM) and also recently attributed to the π³_uδ_u(X^*) emission in the π³_uΦ_u(U*)⇌π³_uδ_u(X^*) reversible-crossing model (RCM), has been investigated. Results from detailed emission studies as well as spectral data from other authors, show that, contrary to what is advanced in the RCM, the LEE at pH ≈ 3 (attributed to the X^* state in the RCM) is not identical to the LEE at pH ⩽ 2. It is unambiguously shown that the LEE at pH = 3, assigned to the π³_uδ_u(X^*) state in the RCM, is actually the emission of the well defined dinuclear species (UO₂)₂(OH²⁺₂(aq). The exciplex-formation model is further corroborated by the influence of perchlorate and methanol on the emission of UO²⁺₂(aq).