“…In order to proceed with the calculation, we introduce (similarly as in [7,10,21]) the simplifying approximarion ~d 5 G W(E, G)P(co, E, G)= W(E)P(co, E, <G>E), (15) which is exact to linear order in <G)E rio]. Making use of the tracelessness and the symmetry of tensors E u and Qu, in the coordinate system xyz with Eg0z, the term describing the emitter-ion quadrupole interaction Hq in the Hamiltonian (1) can be expressed in terms of the mean field gradients…”