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Journal of physics / Condensed matter 31(1), 014003 (2019). doi:10.1088/1361-648X/aaedf3, With the intense and coherent x-ray pulses available from free-electron lasers, the possibility to transfer non-linear spectroscopic methods from the laser lab to the x-ray world arises. Advantages especially regarding selectivity and thus information content as well as an improvement of signal levels are expected. The use of coherences is especially fruitful and the example of coherent x-ray/optical sum-frequency generation is discussed. However, many non-linear x-ray methods still await discovery, partially due to the necessity for extremely adaptable and versatile instrumentation that can be brought to free-electron lasers for the analysis of the spectral content emitted from the sample into a continuous range of emission angles. Such an instrument (called MUSIX) is being developed and employed at FLASH, the free-electron laser in Hamburg and is described in this contribution together with first results., Published by IOP Publ., Bristol

The photolysis of the hydronium cation H3O+ has been studied at the extreme ultraviolet wavelengths of 35.56±0.24 nm (34.87±0.24 eV) and 21.85±0.17 nm (56.74±0.44 eV) using a crossed ion-photon beam setup at the free-electron laser FLASH. Coincidence photoelectron and photofragment spectroscopy was performed at 21.85 nm, where both inner and outer valence ionization are allowed, and revealed that the XUV photolysis of H3O+ is by far dominated by ionization of outer valence electrons forming the 1A1 and 2E states of the dication H3O2+. The dications were found to dissociate into the channels H2O++H+ (72±4%), OH0+2H+ (18±6%), and OH++H++H0 (10±1%). A kinematic analysis of the H2O++H+ channel after photoabsorption at 35.56 nm (where only outer valence ionization is possible) showed dissociation into excited states of the water radical ion, where the 1A1 state breaks up into the linear A˜2A1 state of H2O+ and the 2E state decays into the strongly bent B˜2B2 state. Finally, from the 2E state of H3O2+, dissociation into OH0(X2Π)+2H+ was identified to occur with a near linear dissociation geometry.

Physical review / A 87, 013402 (2013). doi:10.1103/PhysRevA.87.013402, The photofragmentation of the water cation $H_2O^+$ through dicationic states has been studied at 35.0 ± 0.2 nm (35.4 ± 0.3 eV) and 21.8 ± 0.2 nm (56.8 ± 0.5 eV) with a crossed ion-photon beams experiment at the free electron laser FLASH. The dissociation of the dications is found to be similar at the two wavelengths and to proceed into $O^0 + 2H^+$, $OH^+ + H^+$, and $O^+ + H_2^+$, with determined ratios $\sigma_{OH^+ + H^+}/\sigma_{O^+ + H_2^+}$= 4.2 ±0.3 and $\sigma_{OH^+ + H^+}/\sigma_{O^+ + 2H^+}$>0.7. The measured kinetic-energy releases for these processes are consistent with three-body breakup ($O^0+2H^+$) occurring mainly through the $2^3A''$ and $2^1A''$ states of $H_2O_2^+$ and two-body breakup ($OH^+ + H^+$) occurring through $X^3A''$, $1^3A′$, and $1^1A''$ states of $H_2O_2^+$, as predicted in a recent theoretical study [Gervais et al., J. Chem. Phys. 131, 024302 (2009)]. In addition to the kinetic-energy releases, we also report on fragment correlation in the three-body channel where the two protons carry the major part of the released momentum., Published by APS, College Park, Md.