Photoelectron spectroscopy via the 1 1Δu state of diacetylene

Abstract

Photoelectron spectra are reported for one-photon resonant, two-photon ionization of jet-cooled diacetylene via a number of vibronic levels of the 1 1Δu state. An improved value for the adiabatic ionization threshold is found to be 82 064±30 cm−1 (10.175±0.004 eV), in good agreement with the earlier result. The photoelectron spectra of different vibronic bands of the 1 1Δu state nearly all show long progressions in what appear to be low frequency bending vibrations. At energies just above the ionization threshold, the observed progressions can be understood in terms of excitation of a single Renner-Teller active mode in the ion, with Renner-Teller parameters similar to those of the ν4+ trans-bending mode in the ground state acetylene cation.