Impact of observed hyperfine splitting on X‐ray laser gain

Line broadening mechanisms play an important role in determining the gain of X‐ray laser transitions. Typically Doppler broadening is the primary mechanism which determines the linewidth of these transitions. However, we will present cases where the hyperfine effect is the dominant line broadening mechanism. Studying laser lines, which tend to have gain narrowed linewidths, enables us to observe these dramatic hyperfine effects which would be difficult to observe in opacity or Stark broadened lines. In this work we report the observation of hyperfine splitting on an X‐ray laser transition and discuss how hyperfine splitting has a major impact on the laser gain. In our experiments we measure the lineshape of the 3p→3s, J=0→1 transition in neon‐like niobium and zirconium and observe a 28 mA splitting between the two largest hyperfine components in the niobium (Z=41) line at 145.9 A, in good agreement with theory. In zirconium (Z=40), no splitting is observed since the hyperfine effect is proportional to the...