Electric field reversals in dc negative glow discharges

Electric field reversals play an important role in maintaining glow discharges by determining the partitioning of ion flux between cathode and anode. For years, the existence of field reversals in the negative glow has been the subject of controversy. For a range of pressures, gas compositions, and electrode spacings, we observe field reversals using laser optogalvanic (LOG) spectroscopy: The discharge current is increased by excitation of molecular ions to a state with larger mobility. The sign of the LOG signal is a direct measure of the sign of the electric field. A single-beam electron model, where it is assumed that the glow is sustained by a monoenergetic, unidirectional beam of electrons emanating from the cathode, is in good agreement with most experimental observations. The model accurately predicts the existence of field reversals and the spatial dependence of both LOG and laser-induced fluorescence signals. By including momentum and energy dispersion in the beam, the model could be further improved.