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

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.