Excitation of forbidden transition in helium by electron impact at kinetic energies of 300–500 eV. Singlet-triplet transitions at high kinetic energy

Abstract

The transition $2^3{\text{S← 1}}^{1}S$ in helium has been studied by electron impact over the kinetic energy range 300–500 eV. The ratio ${\sigma }_T{\mathrm{/\sigma }}_S$ of scattered current for this transition to that for $2^1{\text{S← 1}}^{1}S$ is a maximum at scattering angle θ of 0°. The intensity ratio decreases with increasing θ to a minimum (which is below background) and then increases again to barely observable levels at $\text{θ ≥ 12°}$ . At $\text{θ = 0°}$ the ratio exceeds all presently developed theories by at least two orders of magnitude. At $\text{θ ≥ 12°}$ the ratio agrees approximately with the Born‐Oppenheimer formula for exchange scattering. The ratio of cross sections for the transitions $2^1{\text{S← 1}}^{1}S$ and $2^1{\text{P← 1}}^{1}S$ has been determined over the range $\text{θ = 5°\hspace{0.17em}}\mathrm{to}\text{\hspace{0.17em}20°}$ . Up to 15°, agreement with the Born approximation is good. Agreement is poor (25%) at $\text{θ = 20°}$ .