The geometry of the X̃2A1 state of PF3+: experiment and theory

The fluorescence excitation spectrum of PF3 in the energy range 9–13 eV has been recorded using monochromatised synchrotron radiation. Transitions to the (8a1)−1np (n = 4, 5, 6) Rydberg states are observed which converge on the ground state of PF3+. The transitions show long progressions in the ν2 vibrational ladder, peaking at ν2 ≈ 13. The value of ν2 in these Rydberg states is 458 ± 24 cm−1. The analysis of the Franck-Condon intensity distribution suggests that the FPF bond angle increases by 14 ± 1° upon photoexcitation. Since the bond angle in neutral PF3 is 97.8°, the ground state of PF3+ therefore remains of pyramidal geometry. These experimental results are confirmed by ab initio calculations. The equilibrium bond angle in PF3 and PF3+ is calculated to be 97.5° and 109.0°, with barriers to planarity of 3.84 and 2.08 eV, respectively. This paper resolves a longstanding disagreement in the literature whether the ground state of PF3+ is pyramidal or planar, and whether the barrier to planarity is greater than or less than the value of ν2 in the cation.