Calculation of phonon sidebands in emission spectra of V2+and Ni2+in MgO

Calculations of the phonon sidebands associated with the 2 E g to 4 A 2g transition in MgO:V 2+ and the Gamma 3g to Gamma 5g transition in MgO:Ni 2+ are presented. The problem can be separated into two parts: (i) the coupling of various symmetry adapted displacements of the crystal ions to the transition; (ii) the response of the lattice to symmetry adapted applied forces. It is shown that the results required from (i) are a limited number of coupling parameters. These are treated here as adjustable constants, as are also the changes in force constants in the vicinity of the defect. The calculation of the second part is made using Green function methods. Satisfactory agreement with experiment is obtained by assuming that only displacements of the impurity ion and its nearest neighbours are coupled to the transitions.