Comparative Kr(L) spectra obtained under three different modalities of excitation are shown

<p><strong>Figure 12.</strong> Comparative Kr(L) spectra obtained under three different modalities of excitation are shown. (a) Axially recorded Kr(L) spectrum produced with self-trapped 248 nm channel (this work, file 677/18 May 2012). The principal feature is the <sup>1</sup>P<sub>1</sub> → <sup>1</sup>S<sub>0</sub> Kr<sup>26+</sup> 3s → 2p transition at λ 7.504 Å. (b) Kr(L) emission spectrum recorded with 248 nm excitation without channelled propagation. The emission profile is dominated by a broad feature involving several transitions in the 6.8–7.2 Å range. For details, see [<a href="http://iopscience.iop.org/0953-4075/46/15/155601/article#jpb468344bib1" target="_blank">1</a>]. (c) Kr(L) spectrum obtained with excitation at ~1 µm without channelled propagation. See [<a href="http://iopscience.iop.org/0953-4075/46/15/155601/article#jpb468344bib16" target="_blank">16</a>] for additional details.</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>