Abstract
Inner-shell photoionization of In+ ground state ions by a laser-produced-plasma broad-band soft X-ray pump source has created a large population inversion on the 185 nanometer 4d9 5s2 2D5/2 → 4d10 5p 2P3/2 transition in In++. A small signal intensity gain of exp(0.5) was measured in a path length of ~4 millimeters, yielding a gain coefficient of a ≈ 1.25 cm-1. The initial population of ~1016 cm-3 In+ ground state ions is prepared by ablating a liquid indium target with a 10 nanosecond, 100 millijoule, 532 nanometer laser pulse yielding a highly ionized indium plasma. The expanding plasma cools and recombines to give the required In+ 4d10 5s2 population, at which time a 100 picosecond, 40 millijoule, 1.06 mm laser pulse, incident on a nearby tantalum target, generates a hot plasma radiating broad-band soft X-rays. The soft X-rays photoionize inner shell 4d electrons from the In+ ions, populating the 4d9 5s2 In++ state, which is inverted with respect to the nearly empty 4d10 5p state.
© 1986 Optical Society of America
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