Problems associated with the measurement of coherence parameters: Superelastic electron scattering by laser-excitedBa138(...6s6pP11) atoms

Measurements of superelastic scattering of electrons by laser-excited ${}_{}{}^{138}{}_{}{}^{}\mathrm{Ba}$(. . .6s6p ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$) atoms were carried out. An asymmetry observed by us and previously by Register et al. [Phys. Rev. A 28, 151 (1983)] has been explained using a model of scattering from a target with finite dimensions. This model employed coherence parameters which we calculated in the distorted-wave approximation. The results indicated that the interpretation of coherence experiments in terms of scattering from a pointlike target can lead to serious errors in the deduction of coherence parameters at low scattering angles.