Time evolution of EUV fluorescence following single-photon absorption of harmonic FEL radiation (25.5 nm or 17 nm)

<p><strong>Figure 1.</strong> Time evolution of EUV fluorescence following single-photon absorption of harmonic FEL radiation (25.5 nm or 17 nm). The first-order photon intensity was suppressed to 3% using an Ar gas attenuator. (b) EUV fluorescence spectrum obtained by projecting the fluorescence intensity in the 2D map onto the horizontal axis. Some excited states of Ne<sup>+</sup> are shown. Note that only the 2s<sup>−1</sup> state can be reached at 25.5 nm. (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>