Hypervirial Theorem, Screening Parameters and Electric Quadrupole Oscillator Strengths in the Sodium Sequence

In view of the lack of experimental and theoretical values of electric quadrupole oscillator strengths for sodium-like ions, we have proceeded to calculate Hartree-Fock gf -values along the sequence (11 ⩽ Z ⩽ 26; nl 2 L-n'l' 2 L' transitions with (3 ⩽ n , n' ⩽ 8; 0 ⩽ l , l' ⩽ 3). All the calculations were performed using the first two forms of the off-diagonal hypervirial theorem. The agreement between the two forms is very good for all the transitions considered. The behaviour of the oscillator strengths was studied along the sequence by the Z-expansion method as for electric dipole transitions. As an extension of the "continued screening approximation" developed by Froese Fischer (1976) in the dipole case, it is shown that the transition integral can be represented accurately by the expression I 0 being the hydrogenic value and s i ( i = 1,...,4), being parameters.