Energy shift of the 4f¹³6s¹ excited states of Yb²⁺ from gas phase to the CsCaBr₃ solid

Ab initio calculations on Yb²⁺-doped CsCaBr₃ show a large increase (20000 cm⁻¹) of the 4f ¹⁴ → 4f ¹³6s ¹ transitions from gas phase to CsCaBr₃, due to large embedding effects originated beyond the first bromide coordination shell. The 4f ¹³6^s 1 states of (YbBr₆)^4- in vacuo undergo a sudden energy and electron density change at short Br-Yb distances resulting in the formation of an Yb-trapped exciton, which evolves to full ionization, making Yb(II) unstable in hexabromide coordination in gas phase. Embedding in CsCaBr₃ localizes the 6s electron inside the Br₆ cage, which increases the 6s-Br₆ repulsion and increases the energy of the 4f ¹³6s ¹ states.