Excitation of metastable argon and helium atoms by electron impact

The excitation of argon and helium metastables by electron impact was studied in the energy range from threshold to about 50 eV with a time-of-flight method. The metastables were detected by Auger ejection of secondary electrons from a CuBeO surface. The argon cross section was obtained in two different ways yielding peak cross sections of 3.4 ×{} 10^-17 and 3.7 ×{} 10^-17 cm² at 22 eV, respectively. The estimated uncertainty for both values is a factor of 2. The helium cross section was measured with a trapped-electron method near threshold and found to have a peak value of (6.2 ±{} 2.0) ×{} 10^-18 cm² at the 25-eV maximum. The secondaryelectron yields of the metastable detector used in this work are discussed in detail. The effect of metastable recoil is also discussed. Argon metastables suffer only negligible recoil because of their large atomic weight, but this is not the case for the lighter helium metastables.