Electric field induced energy shift in F-centre emission

For the Relaxed Excited States of the F-centre, the simplest vibronic model involves only the Gamma(4)(-) mode in the electron-lattice interaction H-cL. In analyzing the perturbation correction due to H-cL, Ham and Grevsmuhl kept only the linear terms in the coupling constant G and ended up with a positive value for the field-induced shift in the energy of the parallel component in emission Delta E(parallel to) - in disagreement with the experiments. In this paper, it is shown that when the perturbation correction terms are kept up to G(2), Delta E(parallel to) is negative.