Observation of accurate ion dissociation thresholds in pulsed field ionization-photoelectron studies

We report the first observation, together with a mechanism far such an obse rvation, of a steplike feature in the pulsed field ionization photoelectron measurement of CH4(C2H2), marking the 0 K dissociation threshold for the f ormation 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 pr ovides strong support for the proposed mechanism. This experiment shows tha t, for a range of molecules, where the ion dissociation lifetimes near the dissociation thresholds are <10(-7) s, pulsed field ionization photoelectro n measurements will yield not only highly accurate ionization energies, but also 0 K dissociation thresholds.