Schematic of the proposed two-photon resonant, three-photon ionization process

<p><strong>Figure 3.</strong> Schematic of the proposed two-photon resonant, three-photon ionization process. Two-photon absorption at 51 nm (red arrow) is resonantly enhanced by the Ne** (2p<sup>−2</sup>(<sup>3</sup>P<sup>e</sup>)3p<sup>2</sup>(<sup>1</sup>S<sup>e</sup>)) doubly excited state (horizontal red line). The absorption of an additional photon leads to ionization to the singly charged Ne<sup>+</sup>* (2p<sup>−2</sup>(<sup>3</sup>P<sup>e</sup>)3p(<sup>2</sup>P<sup>o</sup>)) state with the emission of a photoelectron (blue arrow). This state decays to the Ne<sup>+</sup>* (2p<sup>−2</sup>(<sup>3</sup>P<sup>e</sup>)3s(<sup>2</sup>P<sup>e</sup>)) state by emitting 331 nm fluorescence (short yellow arrow). Finally, EUV fluorescence at 44.6 nm is emitted, corresponding to the transition from the Ne<sup>+</sup>* (2p<sup>−2</sup>(<sup>3</sup>P<sup>e</sup>)3s(<sup>2</sup>P<sup>e</sup>)) state to the Ne<sup>+</sup> (2p<sup>−1</sup>(<sup>2</sup>P<sup>o</sup>)) ground state (long yellow arrow).</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>