Collisional-radiative transfer between Rydberg states of helium and electronic recombination ofHe+

We report detailed spectroscopic measurements of light emission and excited level populations in a pure-helium afterglow at a pressure of 2.03 Torr. The free-electron temperature could be varied by microwave heating. Electron density $n_e$ and temperature $T_e$ were measured by precision microwave interferometry and they range from 5 × ${10}^9$ to 3 × ${10}^{11}$ ${\mathrm{cm}}^{-3\mathrm{}}$ for $n_e$ and from 300 to 600°K for $T_e$. Rydberg states of neutral helium near the bottleneck for collisional-radiative (CR) recombination were perturbed in a well-controlled manner by photoionization with 10.6-μm photons. Our experimental data are consistent with a CR recombination scheme with simultaneous stabilization by collisions with free electrons and with neutral helium atoms. They establish the fact that the Gryziński formalism for electron-impact-induced transitions between bound rydberg states of high principal quantum number seriously overestimates the probability of collisions with small energy transfers. This study gives also experimental indications of what appears to be neutral-stabilized CR recombination of an atomic ion, ${\mathrm{He}}^{+}$. The experimental recombination rate coefficient is well represented over the range of parameters covered at 2.03 Torr by $\alpha \left({\mathrm{cm}}^3 invsec\right)=3.5\mathrm{}\times {10}^{-5\mathrm{}}{T}_e^{-1.9\mathrm{}}+6.0\mathrm{}\times {10}^{-9\mathrm{}}{T}_e^{-2.18\mathrm{}}{n_e}^{0.37}+3.8\mathrm{}\times {10}^{-9\mathrm{}}{T}_e^{-4.5\mathrm{}}{n}_e$, in reasonable agreement with available theories.