New Process of Excitation Transfer in Helium

Abstract

Precise observations have been made on light emission from low triplet levels of helium gas traversed by a monoenergetic electron beam, with varying helium pressure and electron energy. In the past it has been customary to assign this triplet excitation to direct transfer from $^{1}P$ states in flagrant violation of the Wigner rule and seemingly excessive cross sections for the transfer have been inferred.A new process of excitation transfer is proposed which minimizes the conflict with the Wigner rule by reducing the sizes of the cross sections required to values close to the gas-kinetic cross section. It is hypothesized that many $^{1}P$ states, including those with large quantum number $n$, transfer excitation energy to neighboring triplet states having closely corresponding principal quantum numbers. The triplet states thus formed in turn populate low-level triplet states by radiative transitions. It is found that states lying between $n=4$ and $n=15$ would play the dominant role in the transfer process.Satisfying qualitative explanations of several additional excitation-transfer-process phenomena are derived from this new multiple-state-transfer process.