High-resolution pulsed field ionization photoelectron–photoion coincidence study of CH[sub 4]: Accurate 0 K dissociation threshold for CH[sub 3][sup +]

The formation of methyl cation (CH3+) from methane (CH4) has been investigated in high resolution using the newly perfected pulsed field ionization photoelectron–photoion coincidence (PFI-PEPICO) scheme. The PFI-PEPICO data reveal that fragmentation of CH4 in high-n Rydberg states occurs at energies above the dissociation threshold prior to pulsed field ionization. The crossover point of the breakdown curves is found to depend strongly on the Stark field in the ion source and thus traditional simulation procedures based on such a feature for ion dissociation energy determination are not appropriate in PFI-PEPICO studies. We show that for a prompt dissociation process, the disappearance energy of the parent molecule provides an accurate measure of the 0 K ion dissociation threshold, as that for CH3+ from CH4 is 14.323±0.001 eV.