Use of partial-wave decomposition to identify resonant interference effects in the photoionization?excitation of argon

We have studied simultaneous photoionization and excitation of Ar in the range of incident photon energies between 36.00 and 36.36 eV, where the resonant production of doubly excited neutral Ar states imbedded in the ionization continuum is dominant. By measuring the relative Stokes parameters of the fluorescence from residual Ar +∗ (3p 4 [ 3 P] 4p) ions ( 2 P1/2, 465.8 nm transition; 2 P3/2, 476.5 nm; 2 D3/2, 472.7 nm; 2 D5/2, 488.0 nm; 4 P5/2, 480.6 nm; 4 D5/2, 514.5 nm) we demonstrate a technique for determining individual partial-wave cross sections in photoionizing collisions. This procedure is shown to be important in sorting out competing dynamical ionization mechanisms, particularly with regard to resonant production of intermediate doubly excited autoionizing states. Comparison with theoretical photoionization cross sections demonstrates that spin–orbit coupling between different states of Ar II needs to be accounted for in the calculations. (Some figures in this article are in colour only in the electronic version)