The Conversion of Metastable Helium from the Singlet to the Triplet State by Electron Collision

The zero-order partial cross section has been calculated for the conversion of He * (2 1 S) metastable atoms to He * (2 3 S) by superlastic collision with thermal electrons. A numerical method has been used, which retains all the direct-coupling terms between the initial and final atomic states. Full allowance has been made for electron exchange. A cross section of 5 × 10 -15 cm 2 was obtained at a temperature of 300°K, which is rather too small to account for the experimental value of 3 × 10 -14 cm 2 observed by Phelps.