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

The photoelectron spectrum of the $\textrm{X}^{+}\,{}^{2}\Pi \leftarrow \textrm{X}\,{}^{1}\Sigma^{+}$ photoionising transition in iododiacetylene, HC$_4$I, 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 HC$_4$I and the spin-orbit splitting of the X$^{+}\,{}^{2}\Pi$ state of HC$_4$I$^+$ are determined as $E^{\textrm{ad}}_{\textrm{I}}/(hc) = 74470.7(2)$ cm$^{-1}$ and $\Delta\tilde{\nu}_{\textrm{so}} = 1916.7(4)$ cm$^{-1}$, respectively. Several vibrational levels of the X$^{+}\,{}^{2}\Pi$ electronic ground state of the HC$_4$I$^+$ 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 HC$_4$I$^+$ and of the change in bond lengths upon ionisation of HC$_4$I.