A two-dimensional (2D) photoelectron spectrum of acetylene is measured by using monochromatized synchrotron radiation. Vibrational excitation in the C-H stretching mode ν1of C2H2+(X2Πu) is observed over a photon energy region from 13.0 to 14.1eV, which is due to autoionization of a valence state (3σg)-1(3σu)1. Constant ionic state spectra for the (ν1′, ν2′) = (3,0), (3,1), (4,0), and (4,1) vibrational levels of C2H2+(X2Πu) show two progressions originating from the ν1motion in the (3σg)-1(3σu)1state. It is likely that an autoionization lifetime of the valence state is comparable to vibrational periods of the ν1and ν2modes.
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