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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 1s5 metastable population being unable to absorb the laser radiation, because of its polarization state. This implies that the magnetic substates of the 1s5 state are surprisingly resilient to collisional redistribution of population among them.