  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.

Chemical Physics