High Resolution Magnetic Circular Dichroism Spectrum of Cs2ZrCl6:Ir4

High resolution, liquid helium temperature MCD, and absorption spectra over the range 17 000–24 500 cm−1 are presented for the Ir4+ ion doped into the cubic host Cs2ZrCl6 in which it exhibits extensive fine structure. The spectra have been interpreted as arising almost exclusively from ligand-to-metal charge-transfer transitions on the basis of intensity arguments and an analysis of the MCD spectrum. There is considerable evidence that the band at ∼23 000–24 500 cm−1 assigned as Eg″(2T2g)→ Eu″(2T2u)+Uu′(2T2u) shows a large excited state Ham effect (quenching of spin-orbit coupling by a dynamic Jahn-Teller effect) so that the no-phonon lines of the two spin—orbit components (Eu″ and Uu′), rather than being split by ∼(3/4) ζC1=440 cm−1, are actually separated by ≲ 6 cm−1. The other intense absorption region, ∼19 000–20 500 cm−1, is assigned to Eg″(2T2g)→ Uu′(2T1u)+Eu′(2T1u). The Eu′ component is orbitally forbidden but very probably borrows intensity from Uu′. There is also some possibility of a Ham effect in this region, but uncertainties in the location of band origins prevents any definite conclusions. To the red of the strong absorption, at ∼17 500–19 000 cm−1, are some broad, much weaker bands which are assigned to the parity-forbidden charge-transfer transitions, Eg″(2T2g)→ Ug′(2T1g)+Eg′(2T1g). The region 20 800–22 000 cm−1 shows a complex spectrum which we attribute primarily to the ligand-to-metal excitation t2g(π)6t2g5→ t2g(π)5t2g6. Again, a Ham effect is possible, but we are not able to offer a detailed analysis. A series of weak lines in the range 22 000–22 600 cm−1 are tentatively attributed to ligand-field transitions. Our interpretations differ in almost every particular from those offered previously by Douglas.