Demonstration of soft x-ray amplification in nickel-like ions

We report the first observation of amplification of spontaneous emission at soft x-ray wavelengths by ions in the nickel-like sequence. The ions are created by high-intensity laser irradiation of a thin foil. Gains of order 1 cm^-1 are observed on J=0-1, 4d-4p transitions in Eu³⁵⁺ at 65.83 and 71.00 A{}. There is some evidence for amplification on the analogous lines at 50.26 and 56.09 A{} in Yb⁴²⁺. The pumping mechanism is believed to be electron collisional excitation from the ground state. The scheme should be readily scalable to produce amplification in W⁴⁶⁺ at 43.15 A{}.