The energy level diagram of the Kr<sup>26+</sup> 3s → 2p transition array illustrating the two transitions at λ<sub>1</sub>  7.504 Å and λ<sub>2</sub>  7.265 Å

<p><strong>Figure 4.</strong> The energy level diagram of the Kr<sup>26+</sup> 3s → 2p transition array illustrating the two transitions at λ<sub>1</sub>  7.504 Å and λ<sub>2</sub>  7.265 Å. The corresponding spontaneous emission transition rates are also shown.</p> <p><strong>Abstract</strong></p> <p>Experimental evidence demonstrating amplification on the Kr<sup>26+</sup> 3s→2p transition at λ 7.5 Å (~1652 eV) generated from a (Kr)<em><sub>n</sub></em> cluster medium in a self-trapped plasma channel produced with 248 nm femtosecond pulses is presented. The x-ray beam produced had a spectral width of ~3 eV and a corresponding beam diameter of ~150 µm, properties that were simultaneously determined by a two-dimensional x-ray spectral image formed with an axially placed von Hámos spectrometer and a matching Thomson image of the spatial electron density generated by the x-ray propagation.</p>