title: Spin-orbit and vibronic coupling in the ionic ground state of iodoacetylene from a rotationally resolved photoelectron spectrum.
abstract: The X(+ 2)Π ← X (1)Σ(+) photoionizing transition of iodoacetylene (HC2I) has been investigated by pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy. The resolution of the rotational structure of the spectra and its analysis provided information on the structure of the HC2I(+) cation and the photoionization dynamics of HC2I. In the ground electronic (2)Π state, the HC2I(+) cation is found to be linear and subject to a strong spin-orbit coupling. The first adiabatic ionization energy of HC2I and the spin-orbit splitting of the X(+ 2)Π ground state of HC2I(+) were determined to be EI(HC2I)/hc = 78296.5(2) cm(-1) and Δν̃SO = 3257(1) cm(-1), respectively. The large spin-orbit interaction almost entirely masks the Renner-Teller effect, which is only detectable through the observation of the nominally forbidden transition to the first excited level (5(1)) of the HCC-I bending mode ν5. The interaction of ∼2 cm(-1) observed between the 5(1) levels of (2)Σ1/2 and (2)Δ5/2 symmetry is attributed to a vibronic interaction with the B (2)Σ(+) electronic state of HC2I(+). The spin-orbit energy level structure of tri- and tetra-atomic molecules subject to the Renner-Teller effect and spin-orbit coupling is discussed for the two limiting cases where the spin-orbit-coupling constant is much smaller and much larger than the bending-mode frequencies.
authors.affiliation: Laboratorium für Physikalische Chemie, ETH Zürich , CH-8093 Zürich, Switzerland.
authors.forename: Bérenger
authors.initials: B
authors.lastname: Gans
authors.forename: Guido
authors.initials: G
authors.lastname: Grassi
authors.forename: Frédéric
authors.initials: F
authors.lastname: Merkt