High-resolution HeI and HeII photoelectron spectra of the Zn 3d and Cd 4d orbitals in the zinc and cadmium dihalides: Ligand field splittings and intensity variations

Abstract

We have recorded the high-resolution HeI and HeII photoelectron spectra of the Zn 3d and Cd 4d levels in gas-phase MX 2 molecules (M = Zn, Cd; X = Cl, Br, I). The d level spectra split into five peaks due to the combined effect of spin-orbit splitting and ligand field splitting on the d 9 hole state, and the spectra have been fitted to a crystal field hamiltonian involving the cubic ( C 4 0) and non-cubic ( C 2 0) parts of the field from the halide ligands. Additional peaks in some spectra are due to vibrational splitting and configuration interaction. The ¦ C 2 0¦ value increases substantially from the chloride to the iodide for both Zn and Cd. Calculations of both the crystal field ( C 2 CF 0) and valence ( C 2 val 0) parts of C 2 0 show that the increase in observed C 2 0 is due to the C 2 val 0 term attributed to the increase in covalency from the chlorides to the iodides. Shifts in the peak position due to the 2Σ 1 2 g and 1Π 3 2 g states from those expected on the ligand field basis. are attributed to slight bonding effects. These effecs cause a large discrepancy between calculated and observed C 4 0 values. The intensities of the five Zn 3d peaks change markedly from HeI- to HeII-excited spectra. The Xα SW method has been used to calculate the intensities of the σ, π and δ 3d orbitals as a function of photon energy. These calculations show dramatic changes in intensity due. for example. to shape resonances. There is usually qualitative agreement between calculated and observed intensities.