Fragment Energy Distributions in Dissociative Photoionization of SF₆ via the X²T_1g Ionic State Determined with Threshold Photoelectron–Photoion Coincidence Velocity Imaging and Molecular Dynamics Calculations

Dissociative photoionization of SF₆ in the photon energy range of 15.00-16.50 eV has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. Both the kinetic energy release distribution (KERD) and the angular distribution of the unique fragment ion, SF₅⁺, resulting from dissociation from the SF₆⁺(X²T_1g) ions, were obtained from the TPEPICO time-sliced images. The F-loss potential energy curve and ab initio classical trajectory calculations not only unravel its dissociation mechanism but also declare that the ν₆⁺ deformation vibration of the SF₅⁺(<i>D</i>_3h, X¹A₁) fragment is predominantly excited. By fitting the total KERD curves derived from the images, we identified the fragment energy distributions. Surprisingly, the average total kinetic energy released in dissociation remains nearly constant within the range of the X²T_1g state. To explain this unusual behavior in such a fast bond-cleavage process, an intramolecular vibrational energy redistribution mechanism is proposed. This mechanism accounts for the rapid energy transfer among vibrational modes prior to complete dissociation. In addition, an adiabatic appearance potential of AP₀(SF₅⁺/SF₆) is accurately determined to be 14.145 ± 0.01 eV, which is in excellent agreement with the high-accuracy ab initio calculation results.