Ham effect in the 2T2u charge-transfer excited state of octahedral IrCl62−

The $E_g{\mathrm{″\hspace{0.17em}(}}^2{T}_{\text{2g}}{\mathrm{)\to \; E}}_u{\mathrm{″(}}^2{T}_{\text{2u}}{\mathrm{)+U}}_u{\mathrm{\prime (}}^2{T}_{\text{2u}})$ allowed ligand‐to‐metal charge‐transfer bands in octahedral ral ${\mathrm{IrCl}}_6^{\text{2−}}({\mathrm{Cs}}_2{\mathrm{ZrCl}}_6:{\mathrm{Ir}}^{\text{4+}})$ which correspond to the excitation t_2u(π)→t_2g(d) show a highly resolved complex vibronic pattern over the region 22 900 – 24 500 cm⁻¹ at liquid helium temperature. In this paper, it is shown that this spectrum can be interpreted in detail on the basis of a strong Jahn‐Teller effect in the t_2g molecular mode. The treatment is carried out in a basis spanning all vibronic functions through the first 11 quanta of the undisplaced t_2g harmonic oscillator functions. All matrix elements involving both the first order Jahn‐Teller Hamiltonian and spin‐orbit coupling are included in a complete diagonalization. A large Ham effect quenches the spin‐orbit coupling so that the no‐phonon lines of the two spin‐orbit components ($E_u″$ and U′_u) are separated by ∼5–6 cm⁻¹ rather than the expected value of ∼(3/4)ζ_Cl≈440 cm⁻¹. The calculated results explain semiquantitatively the over‐all band spectrum in both absorption and MCD as well as the behavior of the no‐phonon lines as a function of magnetic field (up to ∼70 kG) and temperature. There is additional low energy vibrational structure, undoubtedly due to lattice vibrations, the most important of which is probably Jahn‐Teller active. The e_g molecular vibration appears weakly, if at all. An alternative treatment assuming a strong Jahn‐Teller effect in the e_g mode does not satisfactorily account for the observed spectrum. Arguments are suggested to account for t_2g rather than e_g Jahn‐Teller activity.