Photoionisation of nickel in ZnS and ZnSe

Abstract
The energy required to photoionise nickel d8 into d9 is found from absorption data to be 2.458 eV in ZnS and 1.849 eV in ZnSe at 6K. Fine structure near these energies is attributed to a hydrogenic hole state of binding energy 20 meV in ZnS and 27 meV in ZnSe, with optical phonon coupling characterised by Huang-Rhys factors of 3.5 and 4.9 respectively. In ZnSe the photoionisation of nickel d9 into d8 is found to have a threshold near 1.05 eV.