Spatial Dependence of the Strong Optogalvanic Effects Due to Metastable Quenching in a DC Helium Discharge

The optogalvanic effects of the He 2.058-¿m line (21P-21S) from He lamps illuminating a weak dc He discharge (current density ¿10 ¿A/cm2, field/pressure ~25 V/cm - torr) are reported. For illumination at the positive column, we have made quantitative measurements of the decreases in the discharge current, electron density, and metastable densities, as weli as the increase in the electric field in the positive column, as the intensity of illumination increases. We have also observed that for sufficiently strong illumination (using He lamps only), the optogalvanic effect is catastrophic, i.e., the discharge is switched off; this clearly shows that a sufficiently large metastable density (which is reduced by the illumination) is necessary to maintain a weak He discharge. For illumination at the cathode regions, the optogalvanic effects are "anomalous": the discharge current is strongly suppressed by illumination at the cathode dark space next to the cathode, but is enhanced by iumination at the adjacent negtive glow region.