Autoionization of acetylene studied by two-dimensional photoelectron spectroscopy

A two-dimensional (2D) photoelectron spectrum of acetylene is measured by using monochromatized synchrotron radiation. Vibrational excitation in the C-H stretching mode v(1) of C2H2+(X(2) Pi(u)) is observed over a photon energy region from 13.0 to 14.1 eV, which is due to autoionization of a valence state (3 sigma(g))(-1)(3 sigma(u))(1). Constant ionic state spectra for the (v(1)', v(2)')) = (3,0), (3,1), (4,0), and (4,1) vibrational levels of C2H2+(X(2) Pi(u)) show two progressions originating from the v(1) motion in the (3 sigma(g))(-1)(3 sigma(u))(1) state. It is likely that an autoionization lifetime of the valence state is comparable to vibrational periods of the v(1) and v(2) modes.