Differential cross sections and electron impact coherence parameters for elastic electron scattering from laser-excited 138Ba

We have measured and calculated differential cross sections and the P-3 collision Stokes parameter for elastic electron scattering from the laser-excited (... 6s6p P-1(1)) and cascade-populated (... 6s5d D-3(1,2)) levels of barium at 5, 10, and 20 eV impact energy. Measurements were performed using a crossed-beam apparatus, where excited-state atomic targets were prepared with resonant laser radiation incident on the electron-atom interaction region. Calculations were performed in the convergent close-coupling (CCC), relativistic CCC, and optical-model potential approximations. We present measured and calculated partial differential cross sections, which relate to differential cross sections and electron impact coherence parameters through the target state anisotropy introduced by the laser geometry.