Elastic electron scattering by laser-excited Ba-138( ... 6s6p P-1(1)) atoms

The results of a joint experimental and theoretical study concerning elasti c electron scattering by laser-excited Ba-138(...6s6p P-1(1)) atoms are des cribed. These studies demonstrate several important aspects of elastic elec tron collisions with coherently excited atoms, and are the first such studi es. From the measurements, collision and coherence parameters, as well as c ross sections associated with an atomic ensemble prepared with an arbitrary in-plane laser geometry and linear polarization (with respect to the colli sion frame), or equivalently with any magnetic sublevel superposition, have been obtained at 20 eV impact energy and at 10 degrees, 15 degrees and 20 degrees scattering angles. The convergent close-coupling (CCC) method was u sed within the non-relativistic LS-coupling framework to calculate the magn etic sublevel scattering amplitudes. From these amplitudes all the paramete rs and cross sections at 20 eV impact energy were extracted in the full ang ular range in 1 degrees steps. The experimental and theoretical results wer e found to be in good agreement, indicating that the CCC method can be reli ably applied to elastic scattering by Ba-138(...6s6p P-1(1)) atoms, and pos sibly to other heavy elements when spin-orbit coupling effects are negligib le. Small but significant asymmetry was observed in the cross sections for scattering to the left and to the right. It was also found that elastic ele ctron scattering by the initially isotropic atomic ensemble resulted in the creation of significant alignment. As a byproduct of the present studies, elastic scattering cross sections for metastable Ba-138 atoms were also obt ained.