Observations of long-lived H−2 and D−2 ions from non-thermal plasmas

Abstract

Strong mass signals of H−2 and D−2 ions have been observed from low-pressure dielectric barrier discharge hydrogen and deuterium plasmas via molecular beam mass spectrometry. The observed H−2/H− and D−2/D− ratios (∼0.35–0.4) are over five orders of magnitude higher than those observed by other techniques. The kinetic energy of H−2 and D−2 ions sampled from the plasmas was determined to be widely distributed, from a few eV to >100 eV, giving lifetimes greater than ∼40 µs for H−2 and ∼55 µs for D−2. The highest vib-rotational excitation of neutral H2 species in the plasma was determined to be about J = 0, v = 5 or J = 19, v = 0 via threshold ionization mass spectrometry. The possible pumping mechanisms for generating H−2 with further high J, required by the current high-rotation model, have been proposed. Similar to the lifetime of D−2 determined recently by another group, the H−2 lifetime observed in this work is about two orders of magnitude longer than that predicted by the current theoretical model. To explain these experimental observations regarding the meta-stability of long-lived H−2 and D−2 ions, the improved current high-rotation model or other new models, including the possible existence of some long-lived electronically excited states of H−2/D−2, need to be developed.