Observation of hyperfine splitting on an x-ray laser transition

We report the first observation of hyperfine splitting on an x-ray laser transition. Hyperfine splitting can have a major impact on laser gain and help explain anomalous behavior and poor performance of x-ray lasers with odd [ital Z]. In our experiments we measure the line shape of the [ital J]=0[r arrow]1, 3[ital p][r arrow]3[ital s] transition in Ne-like Nb and Zr and observe a 28 mA splitting between the two largest hyperfine components in the Nb ([ital Z]=41) line at 145.9 A, in good agreement with theory. As predicted, no splitting is observed in Zr ([ital Z]=40).