Quantum-mechanical and experimental study of the excitation of the 2 P1 state of He by electron impact at 29-40 eV

The close-coupling approximation (c.c.), the Vainshtein-Presnyakov-Sobelman approximation (VPS), and various first-order approximations, including the first Born approximation (B), are compared to accurate normalized experimental differential cross sections (estimated error less than 20%) for excitation of the 2 P1 state of helium for impact energies 29.6, 34, and 40.1 eV, and scattering angles 3°-138°. The most accurate new measurements are those at 29.6 and 40.1 eV. These measurements were normalized to the experimental integral cross sections of Donaldson et al. The experimental differential cross sections of Hall et al. are in good agreement with these present measurements. All the calculations except B include electron exchange. The VPS has the most accurate magnitude of the present calculations at small scattering angles but it is still too large (by a factor which at 45°is about 2.5 at 29.6 and 34 eV and is about 1.5 at 40.1 eV). The c.c. has the most accurate magnitude at large scattering angles and the most accurate angular dependence at all scattering angles, but it is not in good agreement with the measurements at the largest scattering angles.