The calculations of the whole energy spectrum of ZnSe:Ni2+ at normal pressure and pressure-induced shifts of its levels have been carried out on the basis of the theory of pressure-induced shifts and the diagonalization of the complete d8 energy matrix in a regular tetrahedral field. The calculated results are in very good agreement with experimental data at normal and various pressures. By taking into account the pressure-induced shifts of spectral bands, the new assignments of bands are given. The comparison between the results of ZnSe:Ni2+ and MgO:Ni2+ shows that the covalency of the bonding between Ni2+ and ligands (Se2-) in ZnSe:Ni2+ is obviously stronger than the one of the bonding between Ni2+ and ligands (O2-) in MgO:Ni2+; the expansion of electron wavefunctions of Ni2+ in ZnSe:Ni2+ under pressure is obviously larger than that of Ni2+ in MgO:Ni2+ under pressure. On the basis of these results, the physical origins of the remarkable difference between the pressure-induced shifts of levels of ZnSe:Ni2+ and those of MgO:Ni2+ are explained. It is found that the large expansion of electron wavefunctions and a phase transition of ZnSe:Ni2+ make the pressure-induced shifts of levels of ZnSe:Ni2+ strongly nonlinear.
A great improvement on a previous work (Phys. Rev. B48 (1993) 14067) has been made. By taking into account all the irreducible representations and their components in the electron-phonon interaction (EPI) as well as all the levels and the admixtures of basic wavefunctions within d3 electronic configuration, the values of all the parameters in the expressions of thermal shift (TS) and thermal broadening (TB) from EPI for the ground level, R level and R line of MgO:V2+ have microscopically been evaluated; and then, both the TS and TB of R line and various contributions to them have uniformly been calculated. The results are in very good agreement with the experimental data. It is found that all the three terms of TS from EPI are red shifts; the term of the contribution to TS from thermal expansion is blue shift. The Raman term is the largest, and the other terms are also important for TS. The R-line TS of MgO:V2+ comes from the first-order term of EPI. The elastic Raman scattering of acoustic phonons plays a dominant role in R-line TB of MgO:V2+. For calculations of both the TS and TB, it is very important to take into account all the admixtures of wavefunctions.
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