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 (CH 3 F + ) in their low-lying electronic states (X 2 E, A 2 A 1, B 2 E) using a combination of threshold photoelectron photoion coincidence (TPEPICO) velocity map imaging and quantum chemical calculations. The C–H bond cleavage from the X 2 E 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 A 2 A 1 and B 2 E states exhibits nonstatistical dynamics. For the A 2 A 1 state, the dissociation is direct and rapid, characterized by a Gaussian-type KERD and a negative anisotropy parameter (β ≈ −0.5). Crucially, for the B 2 E state, our results contradict the previously proposed mechanism of internal conversion to the X 2 E state followed by statistical dissociation. Instead, we provide compelling evidence that CH 3 F + (B 2 E) undergoes internal conversion to the A 2 A 1 state, which then dissociates directly. This revised pathway is supported by RRKM calculations, the near absence of CH 2 F + fragments, and the similarities in KERD and β parameters between the B 2 E and A 2 A 1 states.