Differential cross sections for electron-impact excitation out of the metastable levels of the barium atom

We report an experimental and theoretical investigation of low-energy electron scattering from Ba-138 atoms prepared in the metastable D-3(2) and D-1(2) levels by laser-optical pumping. Differential cross sections were measured for excitation out of these metastable levels to a variety of higher-lying levels at electron-impact energies of 10, 20 and 36.7 eV. A comparison of experimental data with the results of theoretical approaches using the unitarized distorted-wave approximation and the convergent close-coupling scheme is given.