Symmetry- and vibrationally-resolved C K-shell photoionization studies of C2H2

The C K-shell photoelectron mainline of C2H2 molecules have been measured at photon energies 295-350 eV. The gerade and ungerade symmetries of the core-ionized states, 1σg and 1σu, are resolved. Resonance enhancement is found only in the 1σg photoionization cross-section and assigned to the σu∗ shape resonance. The photoelectron asymmetry parameter takes a minimum value at the shape resonance energy. The shape resonance energy decreases with increase in the vibrational quantum number. No correlational shape resonance appears in the 1σu photoionization channel, suggesting that the channel coupling is negligible.