Electronic State Distributions of He(ns, np, nd) Rydberg States Produced from Three-Body Collisional Radiative Recombination of He+ in a Helium Flowing Afterglow

Abstract

Singlet and triplet ns, np, and nd Rydberg states of He* in the 22.72–24.53 eV range have been produced in a helium flowing afterglow. The highest excited levels in each Rydberg state increased from n=6 to 16 with an increase in the degeneracy of the upper state. The dependence of the emission intensity of He* on the He pressure and the electron density indicated that these He* states were formed by the He+/2e- three-body collisional radiative recombination reaction. The electronic state distributions, which were independent of He pressure in the range of 1.0–2.7 Torr, increased with decreasing excitation energy of He*. They were expressed by double Boltzmann distributions with effective electronic temperatures of 0.46 eV in the 22.7–24.4 eV range and 0.089 eV in the 24.4–24.5 eV range. The observed distributions were discussed in terms of the Saha equilibrium between He* and free electrons for high-energy states and the non-Saha equilibrium for low-energy states due to the collisional excitation to a neighboring state and low reionization rates. The total steady-state distributions of ns 1S, np 1P, nd 1D, ns 3S, np 3P, and nd 3D states were 0.048, 0.053, 0.18, 0.24, 0.13, and 0.35, respectively.