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.

Relaxation behavior of rovibrationally excited H2 in a rarefied expansion

“…The rovibrational population leaving the plasma source 30,31 shows a remarkable similarity with rovibrational distributions measured in other plasma environments. 17,[23][24][25] The higher rotational levels (Jу5) in all of the aforementioned studies are overpopulated, usually even by orders of magnitude. Although the measurements are performed in different experiments, under very different experimental conditions ͑pressure, temperature, electron density, and electron temperature͒ the distributions are stunningly similar.…”

“…And this reaction becomes five orders of magnitude more efficient when the rovibrational energy of the hydrogen molecule is increased to around 2.5 eV. 21,22 Since in the plasmas used for the production of the H Ϫ ion, the high rotational levels are usually enormously overpopulated, 17,[23][24][25] these high rotational levels play an important role in the production of the negative hydrogen ion. Knowledge of the behavior of the high rotational levels is therefore required.…”

“…[17][18][19][20] In such sources rovibrationally excited hydrogen plays a key role, since the negative hydrogen ions is formed in the dissociative attachment ͑DA͒ reaction. And this reaction becomes five orders of magnitude more efficient when the rovibrational energy of the hydrogen molecule is increased to around 2.5 eV.…”

“…The most suited techniques are laser-based methods, because of their state-selective character. For hydrogen, proven techniques are coherent anti-Stokes Raman scattering ͑CARS͒, 17,30 and the laser Schlieren technique. 9,11,12 Due to the relatively high detection limits these techniques are limited to the lower vibrational states and to the lower rotational levels of these lower vibrational states.…”

Rotational and gas temperatures of molecular deuterium in a hollow cathode glow discharge

“…Unfortunately, the electronic-vibro-rotational (ro-vibrational) transitions of H 2 or D 2 ground state are in the medium infrared region which is inconvenient for application of standard emission and absorption techniques for T g measurements. To overcome this problem and to measure T g of such an important molecule, different laser techniques were developed: coherent anti-stokes Raman scattering (CARS) [5][6], laser-induced fluorescence (LIF) [7,8]. Nevertheless, for practical reasons, the search for a simple, reliable optical emission spectroscopic (OES) technique continued, see e.g.…”