Formation and Destruction of Para and Ortho H + in Discharge and Afterglow. Experiments with Para-Hydrogen

Dissociative recombination of H + ions with electrons at temperatures lower than 100 K is being studied intensively recently to confirm the theoretical prediction of a big dierence between recombination rate coecients of these ions in two lowest rotational levels (belonging to para and ortho nuclear spin states). In the present work, we investigated the time evolution of the aforementioned ion's concentration in para and ortho states separately in a pulsed microwave discharge (in a He/Ar/H2 gas mixture) and in afterglow plasma. For this purpose, a near infra-red cavity ring-down spectrometer (CRDS) was used. In the experiments, we observed conversion between both nuclear spin states and production of H + in afterglow. We constructed the model of chemical kinetics for discharge and afterglow to explain such production process. The anomaly in plasma decay was found to be possibly caused by reaction chain initiated by excited argon molecules and ArH + ions remaining in the beginning of the afterglow. The hydrogen gas concentration seems to be crucial for the speed of establishment of a steady state ratio between para-H + and ortho-H + in the afterglow.