A plasma source based on the Penning ionization of caesium by photoexcited mercury

A high-pressure plasma source based on the Penning ionization of caesium by photoexcited mercury is studied by a kinetic model. Mercury vapour is photoexcited by resonant excitation at 253.7 nm to the ³P₁ level, then quenched to the metastable ³P₀ level by a small admixture of nitrogen. Helium is added to enhance the electron collision frequency, but as a result also enhances the decay of electron density by three-body recombination processes. The results show that almost complete ionization of caesium to densities of order 10¹² ions cm^-3 can be achieved. Radiation transport in this medium is also studied, and the effects of pressure broadening and hyperfine splitting are explored. Using excitation sources and plasma sources enriched with different isotopes of mercury, hyperfine splitting can be exploited to increase the depth of penetration of the resonant excitation radiation into the plasma source volume and therefore increase the volume over which strong ionization can occur.