Excitation ofn=2States in Helium by Electron Bombardment

We have measured the total cross section for excitation of the $2{}_{}{}^1{}_{}{}^{}S$ state in helium by electron bombardment, together with the sum of the $2{}_{}{}^3{}_{}{}^{}S$ and $2{}_{}{}^3{}_{}{}^{}P$ cross sections. Singlet metastable atoms were distinguished from triplet ones by quenching the singlets in an electric field. An estimate of the $2{}_{}{}^3{}_{}{}^{}P$ cross section is made. All measurements were relative to the peak of the $2{}_{}{}^3{}_{}{}^{}S$ cross section, and covered the energy range 19.8 to 23.2 eV. Assuming a value of (3.0±0.7) × ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ for this peak, the $2{}_{}{}^1{}_{}{}^{}S$ cross section was found to rise from threshold to a plateau at (1.0±0.3) × ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$. The $2{}_{}{}^3{}_{}{}^{}P$ cross section rises steadily from threshold to (3.0±1.0) × ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ at 23.2 eV. Measurements were also made of the relative populations of the magnetic substates of the $2{}_{}{}^3{}_{}{}^{}S$ state. These are related to the polarization of the collision light emitted in the $2{}_{}{}^3{}_{}{}^{}P\to 2{}_{}{}^3{}_{}{}^{}S$ decay and were motivated by the well-known polarization anomaly. For about 2 V above the $2{}_{}{}^3{}_{}{}^{}P$ threshold the measured tensor polarization has the constant value -0.06±0.06, which may be compared with a predicted threshold value of -0.55. This result is interpreted in terms of resonance structure.