State-selective single-photon double ionization of Ne indicating singlet-triplet mixing of doubly excited LS-states

The state-selective double-photoionization cross sections of the Ne III 2s( 2)2p(4) [P-3, D-1, S-1], 2s(1)2p(5) [P-3, P-1] and 2s(0)2p(6) [S-1] states in the exciting-photon energy region from 60 up to 200 eV are calculated wi th the use of V(N-1) and V(N-2) potentials. To our knowledge the basis of w avefunctions obtained with the use of the V(N-2) potential is employed for the first time for the calculation of the total DPI cross section of a many -electron atom. The lowest-nonvanishing-order perturbation theory V(N-2) cr oss sections are compared with V(N-1) ones and with theoretical results of other authors. The obtained results show that the V(N-1) potential overesti mates and the V(N-2) potential underestimates the experimental total DPI cr oss section of the L shell in Ne. The best agreement with experiment is ach ieved for the sum of the 2s(1)2p(5) and 2s(0)2p(6) state-selective V(N-2) v elocity-form cross sections. The necessity of including the higher-order co rrelations, especially those responsible for singlet-triplet mixing of the final doubly excited states, is demonstrated and discussed.