Vacuum Ultraviolet Photochemistry of Eco-Friendly Insulating Gas: CF ₃ SO ₂ F

The vacuum ultraviolet (VUV) photochemistry of trifluoromethanesulfonyl fluoride (CF₃SO₂F), a promising eco-friendly insulating gas to replace sulfur hexafluoride (SF₆), was investigated in the 11-14 eV energy range by using synchrotron radiation photoionization mass spectrometry and high-level theoretical calculations. It was found that the CF₃SO₂F⁺ parent ion formed from VUV photoionization is not stable and dissociates into CF₃⁺, SO₂⁺, and SOF⁺ fragment ions. The vertical ionization energy (VIE) of the parent molecule to the repulsive X²A' cationic state was calculated to be 13.46 eV, while the adiabatic ionization energy (AIE) to the bound A²A″ state was determined to be 12.65 eV. Notably, the experimental appearance energies of the CF₃⁺ and SO₂⁺ fragments coincide with the AIE of the first excited state (A²A″), rather than the lower appearance energies predicted from ground-state potential energy surfaces. In addition, the dissociation mechanisms to produce CF₃⁺ and SO₂⁺ fragment ions were discussed in detail. For CF₃⁺, photoexcitation populates the bound A²A″ state, which then crosses via a conical intersection to the repulsive X²A' state, leading to direct dissociation into CF₃⁺ + FSO₂. This predissociation pathway governs the experimental onset of the m/z = 69 signal. The m/z = 64 signal, corresponding to the SO₂⁺ fragment, is less intense than the m/z = 69 signal because its formation involves a more complex, multistep pathway. This pathway needs isomerization and crossing another conical intersection, making it less favorable than the direct fragmentation that produces the CF₃⁺ fragment.