Selective photoionization of lithium isotopes in a hollow cathode lamp: a feasibility study for a laser ion source...

Two-step selective photoionization of lithium isotopes is carried out in a hollow cathode (HC) discharge lamp using a nitrogen-laser-pumped homemade tunable dye laser (∼0.2  cm^-1, 7 ns, 640-680 nm). The HC lamp is developed in-house and used as a lithium ion source as well as a detector. The dark space region of HC discharge is illuminated by a dye laser exciting pulse followed by an ionizing radiation from the nitrogen laser. The high electric field (∼kV/cm) present in HC dark space is exploited for fast charge collection, produced by the simultaneous interaction of exciting and ionizing laser beams with sputtered lithium atoms, enabling the detection of photoionization signal. Photoionization signals with a high signal-to-noise ratio are obtained without using any amplifier. Effects of the ionizing laser pulse energy and the precise tuning of the dye laser on lithium photoionization signal, across 670.780 nm (S_1/22→P_1/2,3/22) transition, are studied. The fine structure of lithium is clearly identified, and its two isotopes (Li7,6) are fairly resolved in the measured photoionization spectrum. Using a saturation technique, the photoionization cross section and the number density of lithium atoms for the Li7 (P_3/22) excited level are also measured as 18.5±2.4  Mb, N₀∼1.7×10⁹  (atoms/cm³). The results obtained infer that HC-lamp-based photoionization is an easier and cost-effective way of selective photoionization of lithium atoms, and it can be applied for other elements also.