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Spin–orbit coupling and rovibrational structure in the iododiacetylene radical cation by PFI-ZEKE photoelectron spectroscopy

ABSTRACT The photoelectron spectrum of the X+ 2Π ← X 1Σ+ photoionising transition of iododiacetylene, HC4I, has been recorded using pulsed-field-ionisation zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy with partial resolution of the rotational structure. The first adiabatic ionisation energy of HC4I and the spin–orbit splitting of the X+ 2Π state of HC4I+ are determined as EadI/(hc) = 74470.7(2) cm−1 and cm−1, respectively. Several vibrational levels of the X+ 2Π electronic ground state of the HC4I+ cation have been observed. The experimental data are discussed in the realm of a simple three-state charge-transfer model without adjustable parameters which allows for a qualitative description of the electronic structure and spin–orbit coupling in HC4I+ and of the change in bond lengths upon ionisation of HC4I.