ZERO-KINETIC-ENERGY PHOTOELECTRON-SPECTROSCOPY FROM THE A(OVER-TILDE)(1)A(U) STATE OF ACETYLENE - RENNER-TELLER INTERACTIONS IN THE TRANS-BENDING VIBRATION OF C2H2-TILDE) (2)PI(U)( X(OVER)

Double-resonance excitation via the A 1A(u) state is used to record ze ro-kinetic-energy photoelectron spectra of acetylene. The analysis of these spectra leads to an improved value of 91 952 +/- 2 cm-1 for the adiabatic ionization potential to the C2H2+ X 2PI(u) ionic ground stat e. Because the A 1A(u) intermediate state has a trans-bent geometry, t ransitions from it readily populate the trans-bending vibration of the ground state ion, leading to new information about this mode and its Renner-Teller interactions. The relative intensities of the Renner-Tel ler components and of the rotational structure within each component a lso provide information on the dynamics of the photoionization process .