The 1 s -2 s excitation of hydrogen atoms by proton impact

Abstract
It is shown that the cross-sections given by the customary second Born approximation are not reliable because of the neglect of a large term of the fourth order in the interaction energy. The expression for the cross-section obtained by including only terms to the third order in the interaction energy is employed to calculate the cross-sections for the proton impact excitation of the 2 s state of atomic hydrogen, allowance being made for distortion and polarization due to the 1 s , 2 s and 2 p 0.±1 intermediate states. When polarization is ignored by neglecting all terms involving the 2 p states, excellent agreement is found with the distortion approximation calculation of Bates.

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