Reinvestigation on Bond Cleavage Mechanisms of Fluoromethane Cations in Low-Lying Electronic States Using TPEPICO Velocity Imaging and DFT Calculations

This study re-examines the bond cleavage mechanisms of fluoromethane cations (CH₃F⁺) in their low-lying electronic states (X²E, A²A₁, B²E) using a combination of threshold photoelectron photoion coincidence (TPEPICO) velocity map imaging and quantum chemical calculations. The C-H bond cleavage from the X²E state is confirmed to proceed via a statistical, thermodynamically controlled mechanism, as evidenced by a Boltzmann kinetic energy release distribution (KERD). In contrast, the C-F bond cleavage from the excited A²A₁ and B²E states exhibits nonstatistical dynamics. For the A²A₁ state, the dissociation is direct and rapid, characterized by a Gaussian-type KERD and a negative anisotropy parameter (β ≈ -0.5). Crucially, for the B²E state, our results contradict the previously proposed mechanism of internal conversion to the X²E state followed by statistical dissociation. Instead, we provide compelling evidence that CH₃F⁺(B²E) undergoes internal conversion to the A²A₁ state, which then dissociates directly. This revised pathway is supported by RRKM calculations, the near absence of CH₂F⁺ fragments, and the similarities in KERD and β parameters between the B²E and A²A₁ states.