Electron-mediated vibration–electronic (V–E) energy transfer in optically pumped plasmas

Abstract

The paper discusses experiments on vibration-to-electronic energy transfer in CO laser pumped CO–Ar and CO–N 2 plasmas. Ionization in these strongly nonequilibrium plasmas occurs by an associative mechanism, in collisions of two highly vibrationally excited CO molecules. The experiments show that removal of the electrons from the optically pumped plasmas using a saturated Thomson discharge results in considerable reduction of the UV/visible radiation from the plasma (CO 4th positive bands, NO γ bands, CN violet bands, and C 2 Swan bands). At some conditions, the removal of electrons results in a nearly complete extinguishing of the UV/visible glow of the plasma. This effect occurs even though electron removal results in an increase of the high vibrational level populations of the ground electronic state CO(X 1 Σ, v∼15–35). On the other hand, deliberate electron density increase by adding small amounts of O 2 or NO to the optically pumped CO–Ar plasmas produced a substantial increase of the UV/visible radiation intensity, which strongly correlates with the electron density. The results of the present experiments indicate that the vibration-to-electronic (V–E) energy transfer process CO( X 1Σ→ A 1Π ), and, possibly, analogous processes populating radiating excited electronic states of NO, CN, and C 2, in optically pumped plasmas, may be mediated by the presence of electrons which are created in the absence of an electric field, with low initial energies. Most importantly, this effect occurs at ionization fractions as low as n e/ N∼10 −9–10 −7.