High-resolution pulsed field ionization photoelectron-photoion coincidence study of C_2H_2: Accurate 0 K dissociation threshold for C_2H~+

摘要

By employing the newly developed pulsed field ionization photoelectron-photoion coincidence (PFI-PEPICO) apparatus of the Chemical Dynamics Beamline at the Advanced Light Source, we have examined the formation of ethynyl ion (C_2H~+) from acetylene (C_2H_2) at high resolution. The PFI-PEPICO time-of-flight spectra reveal that fragmentation of C_2H_2 in high-n Rydberg states occurs at energies above the dissociation threshold prior to pulsed field ionization. This study shows that for a prompt dissociation process, the disappearance energy of the parent molecule determined in PFI-PEPICO measurements provides an unambiguous measure of the 0 K ion dissociation threshold. For the formation of C_2H~+ from C_2H_2 this is found to be 17.3576 +- 0.0010 eV.