Observation of Accurate Ion Dissociation Thresholds in Pulsed Field Ionization-Photoelectron Studies

We report the first observation, together with a mechanism for such an observation, of a steplike feature in the pulsed field ionization photoelectron measurement of CH₄(C₂H₂), marking the 0 K dissociation threshold for the formation of CH₃⁺+H(C₂H⁺+H) from CH₄(C₂H₂). The nonexistence of a step in the spectrum for C₂H₄ at its dissociation threshold for C₂H₂⁺ formation provides strong support for the proposed mechanism. This experiment shows that, for a range of molecules, where the ion dissociation lifetimes near the dissociation thresholds are <10^-70ex0exs, pulsed field ionization photoelectron measurements will yield not only highly accurate ionization energies, but also 0 K dissociation thresholds.