Third-order interelectronic-interaction correction to the 2p(1/2)-2s transition energy in lithiumlike ions

Third-order interelectronic-interaction correction to the energies of (1s)( 2)2s and (1s)(2)2p(1/2) states of high-Z lithiumlike ions is evaluated with in the Breit approximation in the range 20 less than or equal to Z less tha n or equal to 100. The calculation is carried out using both the relativist ic configuration-interaction method and perturbation theory. The correction is shown to be important for the comparison of theory and experiment. (C) 2000 Elsevier Science B.V. All rights reserved.