Mode-structure influences on the laser-induced optogalvanic effect in pre-breakdown discharges

The technique of quantitative laser-induced optogalvanic perturbation of pre-breakdown discharges, established in previous work, is developed through an investigation of the effect of the mode structure of the laser radiation. Results are presented using three different mode structures: single mode and two qualitatively different multimode structures. These show that one of the multimode structures, readily generated by a ring dye laser, is unsuitable for quantitative optogalvanic investigations. The other controlled multimode structure gives results broadly similar to those from single-mode perturbation. However, optogalvanic signals are harder to locate with single-mode perturbation and metastable-particle-density decay rates are higher, for a given laser intensity, thus making analysis more difficult. Also, effects possibly due to saturation become apparent at relatively low laser intensities with single-mode perturbation. Both multimode and single-mode perturbation show effects with variation of laser intensity that are not in accordance with existing theoretical expectations. These effects might be due to a subset of the 1s 5 metastable population being unable to absorb the laser radiation, because of its polarization state. This implies that the magnetic substates of the 1s 5 state are surprisingly resilient to collisional redistribution of population among them.