Trihalides, hypersensitive transitions

A number of studies of rare-earth spectra in condensed phases (2, 4, 17, 18) have confirmed that the theory of Judd and of Ofelt can account for the observed intensities of electric-dipole transitions within the 4f configuration on the basis of three phenomenological parameters t2, t4, and tq. These studies have also shown that the parameter t2 is very sensitive to the rare-earth ion environment while t4 and tq are relatively insensitive to the environment. In the earlier gas-phase neodymium trihalide study, it was found that X2 is enhanced to such an extent relative to the t4 and tq parameters that the oscillator strength of the hypersensitive %/2 — C 5/2 transition is a factor of 10 higher than in condensed phases (JJ). 

Plausible though this mechanism is, it came under criticism (7) because, inter alia, it could not account for the intense hypersensitive transitions of the gaseous rare-earth trihalides (8). However, there is recent evidence that the halides are not planar (9,JL0), as had been previously supposed. If this is in fact the case, the importance of the mechanism based on Y terms in V remains undecided. 

Mechanisms of f-f hypersensitive transition intensities of lanthanide trihalide molecules a spin-orbit configuration interaction study... 

Table 14 summarizes the trivalent lanthanide halide vapor complexes which have been studied by means of spectrophotometry. Among the binary systems involving Group-IIIA trihalides, chlorides are by far the most extensively studied systems. The overall spectra of vapor complexes are similar to those observed in aqueous solutions (Carnall 1979) or in other (e.g. glass) media (Reisfeld 1975) in terms of transition energies but the hypersensitive transition intensities were foimd to vary. However, the hypersensitive intensity ( G5/2 <— 19/2) of Nd-Al-Cl vapor complexes compares well with the result... 

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