Photoionization of acetylene near the threshold

The photoionization of C 2H 2 has been studied by threshold photoelectron (TPE) spectroscopy and constant photoelectron energy (CPE) spectroscopy in the energy region between 11.3 and 20eV. TPE spectroscopy has enabled a detailed observation of the first three electronic states of the ion as well as the non-Franck-Condon region. Further investigation by CPE spectroscopy at photoelectron energies between 0.2 and 3.7eV has established that the main process occurring in this region is the autoionization of superexcited states of C 2H 2. A dynamic effect in the autoionization process has been proposed to explain the excitation of a quasicontinuum of vibrational levels of the ionic ground state in the region of 13eV.