A study of the photoionisation dynamics of the cyanogen halides

Abstract

The valence shell photoelectron spectra of BrCN and ICN have been studied using synchrotron radiation. In addition to the main bands associated with the single-hole states, complex satellite structure due to many-electron effects has been observed in the inner valence region. The Green’s function method has been employed to evaluate the ionisation energies and pole strengths of all valence states and the results have facilitated an interpretation of the experimental spectra. Photoelectron angular distributions and branching ratios for the valence shell, the Br 3d level in BrCN and the I 4d level in ICN have been measured in the photon energy range 14–120 eV. The continuum multiple scattering (CMS-Xα) approach has been used to calculate photoelectron asymmetry parameters and branching ratios for the valence orbitals of ClCN, BrCN and ICN. These have been used to assess the influence of Cooper minima and shape resonances on the photoionisation dynamics. In ICN, the experimental and theoretical results provide evidence of core-valence shell coupling at energies above the I 4d threshold. The experimentally determined asymmetry parameters and branching ratios for the spin–orbit split Br 3d5/2,3/2 and I 4d5/2,3/2 components have been compared to those for Kr and Xe, and a strong atomic-like behaviour is evident. The calculated spin–orbit averaged β-parameters for the Br 3d and I 4d levels confirm this interpretation. Hartree–Fock (HF) calculations have been carried out to study the evolution of the valence orbital characteristics through the series of halogen-substituted XCN (X=H, Cl, Br, I) molecules.