Absolute Electron Excitation Cross Sections of Helium

The absolute apparent electron excitation functions of helium have been measured for the 3¹S, 4¹S, 5¹S, 6¹S, 3¹P, 4¹P, 3¹D, 4¹D, 5¹D, 6¹D, 3³S, 4³S, 5³S, 3³P, 3³D, 4³D, 5³D, and 6³D states at pressures sufficiently low so that the effects of radiation imprisonment and collisional excitation transfer can be neglected. Corrections due to polarization of the radiation and the cascading from the upper excited states have been applied to the experimental data and the true excitation functions obtained. The peak values and the shape of these excitation functions are compared with the results reported from other laboratories. Generally good agreement is found with the previous works where the collisional excitation transfer was properly reduced and allowance was made for the cascading effect. The experimental cross sections show satisfactory agreement with the theoretical values calculated by the Born approximation for the ¹S and ¹P states at high-electron energies, but are about four times larger than the theoretical values for ¹D states. In the case of the triplet series, the experimental cross sections exceed the calculated values by factors of ten to one hundred or more. It is concluded that the population of the triplet states is produced mainly by processes other than direct excitation.