Is Single Electron Excitation in Helium Now Fully Understood?

Abstract

Electron impact excitation of atomic and molecular systems has been a subject for both experimental and theoretical study since Franck and Hertz demonstrated the loss of energy of electrons passing through mercury vapour in 1914. Experimentally the great majority of studies on short-lived excited states have been carried out by observation of either the scattered electron or the photon from decay of the excited state. In principle these latter measurements can yield the energy dependence of the cross section for exciting a particular state of the atom. In practice there are substantial difficulties in extracting this information from the spectrum line intensity which is actually measured. Particular problems are associated with cascade from more highly excited states and with the polarisation of the radiation. Major reviews of this type of work have been given by Moisewitsch and Smith¹ and Heddle and Gallagher.² Interestingly the latter omits comparison with theory on the basis that ‘it does not appear to us that detailed calculations have much to offer at the present time’.