(3+1) -resonantly enhanced multiphoton ionization-photoelectron spectroscopy of the (3d-4s) supercomplex of acetylene: The geometry of the E state revisited through experiment and theory

Abstract

(3+1) -resonance enhanced multiphoton ionization-photoelectron spectroscopy (REMPI-PES) has been carried out via various low vibrational levels of the D, F, and E states belonging to the (3d-4s) supercomplex of acetylene. The photoelectron analysis takes into account the Renner–Teller coupling occurring in the ion ground state. In the 74 500–76 500 cm−1 energy range, the coupling between the F Rydberg state and E valence state is strongly revealed through the photoelectron spectra. Moreover, the vibrational analysis of the REMPI-PES spectra enlighten the controversial geometry of the E valence state. They strongly indicate a planar trans-bent geometry of the E valence state, quantitatively confirmed by an ab initio study. This study confirms the tentative frequencies for the ν1 stretching mode (3307 cm−1 for C2H2+, 2572 cm−1 for C2D2+), as well as the frequencies for the bending mode: trans-ν4 (694 cm−1 for C2H2+, 586 cm−1 for C2D2+) and cis-ν5 (775 cm−1 for C2H2+, 569 cm−1 for C2D2+) of the cations.