Photoelectron spectroscopy from the A 1Au state of acetylene: The bending vibrations of C2H+2 X 2Πu

Photoelectron spectra are reported for one-photon resonant, two-photon ionization of acetylene via a number of vibronic levels of the A 1Au state. Because the A 1Au state is trans bent and the X 2Πu ground state of the ion is linear, ionization from the A 1Au levels produces a long progression in the trans-bending vibration of the ion. The X 2Πu state is split by the Renner--Teller interaction along the trans-bending coordinate; the analysis of the photoelectron spectra allows the determination of the Renner--Teller parameters and leads to reassignments of the cis and trans-bending vibrations in earlier photoelectron studies