Cross Sections for the Excitation of the Metastable 2s State of Atomic Hydrogen by Electron Collision
The function for excitation of the 2s state of atomic hydrogen by electron impact has been measured from threshold to 45 ev by an atomic beam method. The absolute value of the total cross section has been determined by two independent methods which are in agreement. In one method the excitation function was normalized to the Born approximation at the higher energies. The mechanism of cascade from higher p states was found to play a significant role in population of the metastable 2s level. The other method proceeded by determining the metastable detection efficiency in terms of the known efficiency for Lyman-α photons. The yield for ejection of electrons from an untreated platinum surface by H(2s) is 0.065+/-0.025. The total cross section reaches a maximum value of (0.35+/-0.05)πa02 at 11.7 ev. The exchange cross section was also measured by the atomic beam method. The incident atoms were polarized in a Stern-Gerlach experiment; the metastable atoms were analyzed by the selective quenching action of a magnetic field of 575 gauss. The ratio of the exchange to total cross section is 0.45+/-0.05 near threshold. At higher energies, this ratio approaches zero. The cross section for production of metastable atoms by direct bombardment of molecular hydrogen is 0.03πa02. This value is considered correct to within a factor of two.