Anomalous optogalvanic line shapes of argon in a hollow cathode discharge

We report the observation of anomalous line shapes in the optogalvanic spec trum, which are different from those of absorption, for transitions origina ting from the 3P(2) (1s(5) in Paschen notation) metastable level of argon. The reason for these shapes is interpreted as being due to two competing at omic processes, which are the depletion and the collisional ionization of m etastable atoms. We show the former process is mainly responsible for the l ine shape at low currents (2.5 similar to 3 mA). When the competition is al most comparable at high currents (similar to 15 mA), the differences in the polarity and the magnitude of the optogalvanic signal between the center w avelength and the line wing of the transition result in the anomalous line shape.