The absorption of radiation near 600 Å by helium

Abstract
Free-bound and free-free radiative absorptions by a pair of atoms are discussed and explicit formulas are derived for the free-bound and free-free absorption cross sections. The theory is applied to the interpretation of helium absorption near 600 å and it is argued that the first two bands observed arise from transitions into quasibound resonating vibrational levels of the A 1Σ u + state. The shape of the A 1Σ u + potential energy curve is derived for internuclear distances between 2 and 3 å. The absorption data analysis lead to a potential barrier of magnitude 0·049 ev. The effects on the absorption structure of replacing 4He by 3He are predicted. An iterative procedure is described that simultaneously locates the energy and evaluates the width of a quasibound-shape resonance state.

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