Spin–Orbit and Vibronic Coupling in the Ionic Ground State of Iodoacetylene from a Rotationally Resolved Photoelectron Spectrum

The X^+ 2Π ← X ¹Σ⁺ 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 ²Π 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^+ 2Π ground state of HC₂I⁺ were determined to be E_I(HC₂I)/hc = 78296.5(2) cm^–1 and Δν̃_SO = 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 (5¹) of the HCC-I bending mode ν₅. The interaction of ∼2 cm^–1 observed between the 5¹ levels of ²Σ_1/2 and ²Δ_5/2 symmetry is attributed to a vibronic interaction with the B ²Σ⁺ 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.