Measurement of photoionization cross-section of Li (2p, 3d) excited states using thermionic diode detector

Abstract

Studies on photoionization cross-section measurement of lithium 2p and 3d excited states selectively populated by single and two photons are reported. The photoionization is carried out in a heat-pipe cum thermionic diode detector using nitrogen laser pumped pulsed dye laser. Rhodamine perchlorate (Rh-640) dye is used to generate the desired tunable laser output in the range of 635 to 675 nm. Both, the thermionic diode detector and dye laser are developed in-house. A negative space charge region formed around diode cathode filament is illuminated by dye laser exciting pulse followed by ionizing UV radiation from nitrogen laser. The effect of ionizing laser pulse energy and precise tuning of dye laser on photoionization signal, across 670.780 nm (2S1/2 → 2P1/2,3/2) single-photon transition and 639.145 nm (2S1/2 → 2D3/2,5/2) two-photon transition are studied. The results obtained are compared with model calculations of photoionization signal saturation curve. The ionization cross-section for both the states (2P1/2, 2D5/2) are determined as 15.40 ± 2.8 Mb and 8.04 ± 1.6 Mb, respectively and atomic number density N ∼ 5.6 × 108 (cm−3).