High Resolution Magnetic Circular Dichroism and Absorption Spectra of Cs2ZrBr6:Ir4

The high resolution absorption and MCD spectra of Ir4+ doped into the cubic host Cs2ZrBr6 are reported over the range ∼ 11 000–20 000 cm−1 at liquid helium temperature and as a function of temperature. This host has not been used previously for optical studies and yields an extraordinarily well resolved Ir4+ spectrum with individual vibronic linewidths in the range ∼ 3–15 cm−1. A study of the detailed vibronic structure of three of the bands including the associated ``hot'' absorption proves very helpful in making definitive assignments. It is shown clearly that the strong bands at ∼ 17 000 and 19 000 cm−1 correspond to the allowed ligand-to-metal charge-transfer transitions, Eg ′′ (2T2g) → Eu ′′ (2 T2u ) and Eg ′′ (2T2g) → Uu ′ (2 T2u ), and that in each case the extensive fine structure is associated with a single allowed electronic transition. The third band which shows sharp vibronic structure, at ∼ 14 500 cm−1, is very likely the parity-forbidden charge-transfer transition Eg ′′ → Eg ′ (2 T1g ) whose lower spin—orbit component, Eg ′′ → Ug ′ (2 T1g ), corresponds to a pair of weak absorptions ∼ 12 000 cm−1. The present results are consistent with our previous assignment of the intense band system ∼ 13 000 cm−1 to the ligand-to-metal charge-transfer transition Eg ′′ → Uu ′ (2 T1u ) split by Jahn—Teller interaction. The other component of this transition, Eg ′′ → Eu ′ (2 T1u ), is orbitally forbidden but is nominally assigned to an intense band overlapping the high energy side of the Eg ′′ → Eu ′′ (2 T2u ) transition with which it is strongly mixed. Aside from some very weak absorption in the 15 000 cm−1 region, there seems to be no basis for assigning any of the spectral features observed to ligand field transitions.