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 elect ron 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 measur ed for excitation out of these metastable levels to a variety of higher-lyi ng levels at electron-impact energies of 10, 20 and 36.7 eV. A comparison o f experimental data with the results of theoretical approaches using the un itarized distorted-wave approximation and the convergent close-coupling sch eme is given.