The 5f16d1→5f2 luminescence spectrum of U4+ in Cs2GeF6 crystals-A quantum chemical study

The 5f16d(t2g)1 emission spectrum of Cs2GeF6:U4+ single crystals has been calculated using the relativistic ab initio model potential (AIMP) embedded cluster method. It consists of a series of broad, intense bands in the UV followed by broad and increasingly weaker bands in the visible and near infrared spectral range, which appear to be associated with emissions from the lowest 5f16d(t2g)1 excited state, 1Eu, to a number of levels of the 5f2 manifold. The electric dipole allowed emission of highest energy, 1Eu→1T1g, is found to have very low oscillator strength, -0.115×10-2; this value is only 4% of the most intense electronic transition: 1Eu→1T2g whose electronic origin is found to be at 34200cm-1. A very large Stokes shift of 6200cm-1 is obtained, which should result in low luminescence quenching due to reabsorption and should favourably contribute to the emission quantum efficiency. The lifetime of the spontaneous emission from 1Eu has been calculated to be in the nanoseconds range: 33 ns.