Electrostatic polarization in the exchange scattering of electrons of high kinetic energy. 11S → 23S transition in helium. Singlet–triplet excitation in CO

Abstract

Excitation in forward scattering by electron impact of the 2³S state in helium is studied in the kinetic energy range 100−300 eV. This extends to lower energies the investigation of Skerbele, Harshbarger, and Lassettre [J. Chem. Phys. 58, 4285 (1973)]. A minimum in the cross section is found for the first time at ∼225 eV, in approximate agreement with the calculations of Huo [J. Chem. Phys. 60, 3544 (1974)]. Abnormally high cross sections in forward scattering (ϑ = 0°) are expected (from Huo’s theory) for those singlet−triplet transitions for which the orbital term symbols (like S for 1¹S → 2³S) are the same in ground and excited states. A search for such transitions is made in CO at 200 and 300 eV incident energies and an example is found.