Electron impact coherence parameters for the (...6s5d(1)D(2)) to (...6s6p(1)P(1)) excitation in Ba-138 determined by superelastic scattering measurements

Superelastic electron scattering involving the de-excitation of laser-excited Ba-138 (...6s6p(1)P(1)) to the lower-lying (...6s5d(1)D(2)) metastable level has been investigated. These measurements have been used to extract the electron impact coherence parameters which describe excitation out of an isotropic (...6s5d(1)D(2)) level to the higher-lying (...6s6p(1)P(1)) state. Studies have been carried out for an electron impact energy of 20 eV and a range of scattering angles up to 45 degrees. This investigation represents a significant extension of coherence parameter measurements to inelastic transitions between excited atomic levels.