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

Abstract

The photoelectron spectrum of the ${\text{X}}^{+}{}^2\Pi \leftarrow \text{X}{}^1{\Sigma }^{+}$ 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 $E_{\text{I}}^{\text{ad}}/(hc)=74470.7(2)$ cm${}^{-1}$ and $\Delta {\tilde{\nu }}_{\text{so}}=1916.7(4)$ 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.

13 pages, 5 figures