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 $\text{∼23\hspace{0.17em}000–24\hspace{0.17em}500\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ assigned as $E_g{\mathrm{″(}}^2{T}_{\text{2g}}{\mathrm{)\to \; E}}_u{\mathrm{″(}}^2{T}_{\text{2u}}{\mathrm{)+U}}_u{\mathrm{\prime (}}^2{T}_{\text{2u}})$ 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 ${\mathrm{(E}}_u\mathrm{″\hspace{0.17em}}\mathrm{and}{\mathrm{\hspace{0.17em}U}}_u\mathrm{\prime )}$ , rather than being split by ${\text{∼(3/4)\hspace{0.17em}ζ}}_{\mathrm{C}1}\text{=440\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ , are actually separated by $\text{≲ 6\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ . The other intense absorption region, $\text{∼19\hspace{0.17em}000–20\hspace{0.17em}500\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ , is assigned to $E_g{\mathrm{″(}}^2{T}_{\text{2g}}{\mathrm{)\to \; U}}_u{\mathrm{\prime (}}^2{T}_{\text{1u}}{\mathrm{)+E}}_u{\mathrm{\prime (}}^2{T}_{\text{1u}})$ . The $E_u\prime$ component is orbitally forbidden but very probably borrows intensity from $U_u\prime$ . 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 $\text{∼17\hspace{0.17em}500–19\hspace{0.17em}000\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ , are some broad, much weaker bands which are assigned to the parity‐forbidden charge‐transfer transitions, $E_g{\mathrm{″(}}^2{T}_{\text{2g}}{\mathrm{)\to \; U}}_g{\mathrm{\prime (}}^2{T}_{\text{1g}}{\mathrm{)+E}}_g{\mathrm{\prime (}}^2{T}_{\text{1g}}\mathrm{).}$ The region 20 800–22 000 cm⁻¹ shows a complex spectrum which we attribute primarily to the ligand‐to‐metal excitation $t_{\text{2g}}{\mathrm{(\pi )}}^6{t}_{\text{2g}}^5{\mathrm{\to \; t}}_{\text{2g}}{\mathrm{(\pi )}}^5{t}_{\text{2g}}^6.$ 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.