Molecular symmetry in two-electron excited and ionized states

It is well established that lower-lying doubly excited two-electron resonan t states can be described by approximate collective quantum numbers. These quantum numbers appear naturally if the problem is treated in 'molecular' c oordinates using the interelectronic vector as an adiabatic variable. The a ppropriateness of these coordinates is demonstrated further using some rece nt numerical calculations on negative ion excitation. As the threshold of t he two-electron continuum is approached from below and different manifolds of resonances overlap, it is believed that these quantum numbers cease to b e applicable. However, one quantum number, the saddle-inversion quantum num ber, usually denoted by A = +/-1, is preserved through the threshold. A det ailed analysis of the T-matrix for photo-double-ionization is given with th e aim of emphasizing the role played by the saddle-inversion symmetry.