High-resolution energy-selected study of the reaction CH3X+→CH3++X: Accurate thermochemistry for the CH3X/CH3X+ (X=Br, I) system

Using the high-resolution pulsed field ionization-photoelectron (PFI-PE) and PFI-PE-photoion coincidence (PFI-PEPICO) techniques, we have examined the formation of methyl cation (CH3+) from the dissociation of energy-selected CH3X+ (X=Br and I) near their dissociation thresholds. The breakdown diagrams for CH3X thus obtained yield values of 12.834±0.002 eV and 12.269±0.003 eV for the 0 K dissociative threshold or appearance energy (AE) for CH3+ from CH3Br and CH3I, respectively. Similar to the observation in PFI-PE studies of CH4, C2H2, and NH3, the PFI-PE spectrum for CH3Br exhibits a step at the 0 K AE for CH3+, indicating that the dissociation of excited CH3Br in high-n (⩾100) Rydberg states at energies slightly above the dissociation threshold occurs in a time scale of ⩽10−7 s. The observed step is a confirmation of the 0 K AE(CH3+) from CH3Br determined in the PFI-PEPICO study. The adiabatic ionization energies (IEs) for the CH3Br+(X 2E3/2,1/2) spin–orbit states were determined by PFI-PE measurement...