Differential cross sections for electron impact excitation of then= 2 states of helium at intermediate energies (80, 100 and 120 eV) measured across the complete angular scattering range (0–180°)

Differential cross sections (DCS) for inelastic electron scattering to the n = 2 states in helium
have been measured at incident energies of 80, 100 and 120 eV. These DCS have been
determined across the complete angular scattering range (0–180◦) using a magnetic angle
changer (MAC) with a soft-iron core. The convergent close-coupling (CCC), R-matrix with
pseudostates (RMPS), and B-spline R-matrix (BSR) methods have been used to calculate these
DCS. Agreement between the experimental data and the predictions from these highly
sophisticated theoretical methods is generally good. The remaining discrepancies mainly
occur at small and large angles for the triplet states 23S and 23P, whereas excellent agreement
is found between 30◦ and 150◦. The small-angle differences are likely due to contamination of
the observed experimental signal from the neighbouring 21S and 21P states. The present
results demonstrate the effective use of a soft-iron core MAC for DCS measurements at
intermediate energies, extending the operational energy range of such devices by a factor of
approximately 25.