Rovibrational molecular populations, atoms, and negative ions in H2 and D2 magnetic multicusp discharges

Abstract

Coherent anti‐Stokes Raman scattering is applied to the study of rovibrational populations in magnetic multicusp H₂ and D₂ discharges. This subject is of interest to negative hydrogen ion formation by volume plasma processes. The populations of high‐lying rotational states (J≳5) in the vibrational levels v=0, 1, and 2 are found to be significantly higher than expected from the Boltzmann law. In H₂ the net populations of the first four vibrational levels follow approximately the Boltzmann law, with the vibrational temperature of 2390 K (in a 90 V‐10 A discharge at 55 μbar). In similar discharge conditions, the population of the state v=3 in D₂ is higher than expected from the Boltzmann law. In the presence of the discharge a deficiency in H₂ and D₂ molecule density was observed and was attributed to the possible presence of H and D atoms. This was verified by an independent measurement of the atomic fraction and temperature. The density of negative ions, measured by the photodetachment technique, is also reported.