(a) Time evolution of EUV fluorescence following multi-photon absorption at the fundamental FEL wavelength of 51 nm

<p><strong>Figure 2.</strong> (a) Time evolution of EUV fluorescence following multi-photon absorption at the fundamental FEL wavelength of 51 nm. The contribution of the higher harmonics (figure <a href="http://iopscience.iop.org/0953-4075/46/16/164020/article#jpb465312f1" target="_blank">1</a>) has been subtracted. (b) EUV fluorescence spectrum obtained by projecting the fluorescence intensity in the 2D map onto the horizontal axis. (c) Time evolution of the fluorescence intensity obtained by projecting the fluorescence intensity in the 2D map onto the vertical axis. The red line shows a two-exponential fit to the data over the time range 0–2.8 ns.</p> <p><strong>Abstract</strong></p> <p>Time-resolved extreme-ultraviolet (EUV) fluorescence spectroscopy has been applied to study the multi-photon, single ionization of Ne irradiated by intense EUV-free-electron laser (FEL) pulses at a wavelength of 51 nm. A broad, intense peak at a wavelength of around 46 nm is observed, which is shorter than the incident FEL wavelength. The time dependence of the fluorescence reveals that the peak has two unresolved components, which we attribute to the decay of the excited ion states Ne<sup>+</sup> 2s<sup>−1</sup>(<sup>2</sup>S) and 2p<sup>−2</sup>(<sup>3</sup>P<sup>e</sup>)3s(<sup>2</sup>P<sup>e</sup>) at 46.0 and 44.6 nm. From the observed intensity ratios and fluorescence lifetimes we conclude that the Ne<sup>+</sup> 2p<sup>−2</sup>(<sup>3</sup>P<sup>e</sup>)3s(<sup>2</sup>P<sup>e</sup>) state is populated by two-photon resonance enhancement, via a 2p<sup>4</sup>3p<sup>2</sup> doubly excited state of Ne.</p>