Atomic hydrogen remained for ten minutes after the termination of an atmospheric-pressure hydrogen-argon plasma jet

Since atomic hydrogen (H) plays an important role in syntheses of methane and ammonia, the measurement of the absolute H density is needed for the design of an efficient plasma source. For this reason, we measured the absolute H density in the spatial afterglow of an atmospheric-pressure hydrogen-argon plasma jet by two-photon absorption laser-induced fluorescence spectroscopy. In the experiment, we noticed that H existed in the stainless-steel chamber at 10 min after the termination of the discharge. The H density at 15 s and 10 min after the termination of the discharge were 4x10¹⁴ and 1x10¹⁴ cm^-3, respectively. The temporal decay of the H density was approximately exponential, and the decay time constant was 8-9 min. It is well known that a major loss process of H is the self-association reaction (H + H + M → H₂ + M), which has a two-body rate coefficient of 8.4x10^-14 cm³s^-1 at the atmospheric pressure. This rate coefficient corresponds to a lifetime of 50-60 ms at the H density in the present experiment. Hence, the experimental result suggests the continuous production of H even after the termination of the discharge. We will discuss possible production mechanisms at the conference.