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The behaviour of cross sections in the neighbourhood of the 2s excitation threshold is investigated using M matrix theory. Calculations allowing for 1s-2s strong coupling are carried out for zero angular momentum in singlet, triplet and no-exchange cases. Threshold effects in all cases have the form of a cusp. A narrow resonance has been found below the threshold in the singlet case.
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Damburg et al. (1962) studied this question.
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