Vapor complexation in the CsI–HoI3 system up to 1300 K and the f ← f hypersensitive transition intensities of Ho(III) in different coordination geometries

Electronic absorption spectroscopy is used in the temperature range 850–1300 K, to study the vapor species over molten HoI3–CsI (1:1), molten CsI and solid HoI3. Quantitative absorbance measurements are used to calculate the following enthalpies of transition: ΔHsubl(HoI3) = 271 ± 3 kJ mol−1, ΔHvap. (CsHoI4) = 155 ± 2 kJ mol−1 and ΔHvap. (CsI) = 151 ± 2 kJ mol−1. The ligand field components of the 5G6 ← 5I8 hypersensitive transition of Ho(III) for the three different, all iodide, coordination geometries of HoI3(g), CsHoI4(g) and HoI63− (in molten CsI) have been examined in detail. The molar absorptivities (ε) and oscillator strengths (f) increase as the coordination decreases from the “octahedral” HoI63− (ε = 65 L mol−1 cm−1; f = 99 × 10−6) to the distorted tetrahedral HoI4− (ε = 235 L mol−1 cm−1; f = 290 × 10−6) to the trigonal HoI3 (ε = 390 L mol−1 cm−1; f = 500 × 10−6). The main factors affecting the hypersensitive transition intensities are the coordination number and symmetry and the ligand polarizability as well as the Boltzmann population effects on the ground state levels which are responsible for the appearance of “hot” bands in the spectra. A C2v symmetry is anticipated for the CsHoI4(g) with the HoI4− “tetrahedra” distorted towards a square planar symmetry leading to a structure with a pseudo-like inversion center.