Spectroscopic Study of Electron Recombination with Monatomic Ions in a Helium Plasma

Recent work on electron-ion recombination has shown that a model including recombination into highly excited bound levels through electron-electron-ion collisions and transitions between the bound levels through electron-atom collisions, as well as the usual radiative transitions, is in good agreement with experiments in which processes involving molecular ions, etc., are unimportant. By making absolute intensity measurements of the lines of helium emitted from the decaying plasma of an arc jet in the visible and near ultraviolet, the number densities of the excited states of helium have been calculated. From these measurements electron temperatures between 0.15 and 0.30 eV and electron densities between 4×${10}^{12}$ and 4×${10}^{14}$ ${\mathrm{cm}}^{-3\mathrm{}}$ are found, with a fractional ionization of the order of 1%. When interpreted by means of the collisional-radiative recombination model, these measurements also give values for $K_{n,n-1\mathrm{}}$, the collisional de-excitation rate constant of an excited helium atom from level $n$ to $n-1\mathrm{}$. The constancy of $K_{n,n-1\mathrm{}}$ over a range of electron density from 4×${10}^{12}$ to 4×${10}^{14}$ per ${\mathrm{cm}}^3$ indicates the validity of the model, and the values of $K_{n,n-1\mathrm{}}$ are in reasonable agreement with a recent theory of classical excitation cross sections. A square-root dependence of this cross section on the excess energy is indicated, since the values of $K_{n,n-1\mathrm{}}$ appear to be independent of electron temperature. It is then found that $Q_{2,3}=9.3\mathrm{}\times \surd E$, $Q_{3,4}=73\mathrm{}\times \surd E$, and $Q_{4,5}=380\mathrm{}\times \surd E$, where $Q_{n,n+1\mathrm{}}$ is the averaged excitation cross section from the levels with principal quantum number $n$ to the levels with principal quantum number $n+1\mathrm{}$ in units ${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$, and $E$ is the excess energy in eV. These cross sections have an estimated error of about 30% in the threshold energy range to about 0.3 eV.