Energy-transfer processes in decaying helium-cadmium plasmas

Light absorption and emission spectroscopy were used in studies of the time dependences of the CdII spectral emission and He^m(2³S) number density in the stationary afterglow of helium-cadmium mixtures. Transitions from 12 CdII energy levels were monitored in the afterglow. The five CdII states below 20 eV are produced, either directly or via cascading, by the Penning ionization of Cd by He^m(2³S). The remaining CdII states studied are produced by the chargeexchange excitation of Cd by He⁺. No evidence was found for the production of CdII energy levels by charge transfer between Cd and He ₂ ⁺ or by Penning ionization of Cd by He ₂ ^m (2³ _u ⁺ ). A value of (5.4±0.3)×10^–10 cm³ sec^–1 was measured for the reaction rate coefficient for the Penning ionization of Cd by He^m(2³S), and a value of (2.1±0.2)×10^–10 cm³ sec^–1 was measured for the reaction rate coefficient for charge exchange between He⁺ and Cd at a gas temperature of 200°C.