The near-infrared band system of rhenium hexafluoride

The infrared A E _{{5}/{2} g} ← X G _{{3}/{2} g} electronic band system of ReF ₆ near 2μ has been recorded in the vapor and polycrystalline solid phase. Features of the vibrational analysis are: (i) the system has a magnetic-lipole-allowed origin hand, accompanied by electric-dipole-vibronic origins associated with single quanta of the enabling vibrations ν₄( f_{1 u}) and ν₆( f_{2 u}); (ii) short progressions occur in the totally-symmetric mode ν₁( a_{1 g}), indicating a small Franck-Condon shift; (iii) upper state ν₅'( f_{2 g}) progressions of 2-3 members are present in both the magnetic-dipole and electric-dipole vibronic components; and (iv) weak 1-0 transitions in ν₂ ( e_g) are observed only in the electric lipole component. The ν₅' progressions are harmonic and provide def nite physical evidence of a trigonal Jahn-Teller effect, involving the mode ν₅ in the ground ( G _{3}/{2g}) state. Analysis of relative intensities of the members of three such progressions, using a linear coupling model for the Jahn-Teller effect, is found to be consistent with a "mild" coupling strength, 0.2 < D₅ < 0.4. ReF ₆ possesses a single ν₅ Raman fundamental at 247 cm ^-1, although two are allowed by the Jahn-Teller effect. This interval reappears in the electronic spectrum in a way favoring one of these two assignments, and this permits an independent estimate of D₅ = 0.4 ₂. The inferred potential energy stabilization due to trigonal distortion is 74 cm ^-1, while the lowest vibronic level occurs well above this at 114 cm ^-1. Vibration frequencies deduced for the A E _{5}/{3g} state from the 15°K spectrum are: ν₁( a_g) = 711, ν₂( e_g) = 661, ν₄( f_{1 u}) = 266, ν₅( f_zg) ≡ 297, and ν₆( f_{2 u}) = 178 cm ^-1.