We demonstrate the feasibility of soft X-ray absorption spectroscopy in the water window using a table-top laser-based approach with organic molecules and inorganic salts in aqueous solution. A high-order harmonic source delivers femtosecond pulses of short wavelength radiation in the photon energy range from 220 to 450 eV. We report static soft X-ray absorption measurements in transmission on the solvated compounds O=C(NH 2 ) 2 , CaCl 2 , and NaNO 3 using flatjet technology. We monitor the absorption of the molecular samples between the carbon (∼280 eV) and nitrogen (∼400 eV) K-edges and compare our results with previous measurements performed at the BESSYII facility. We discuss the roles of pulse stability and photon flux in the outcome of our experiments. Our work paves the way toward table-top femtosecond, solution-phase soft X-ray absorption spectroscopy in the water window.

We use N K-edge absorption spectroscopy to explore the electronic structure of the amine group, one of the most prototypical chemical functionalities playing a key role in acid-base chemistry, electron donor-acceptor interactions, and nucleophilic substitution reactions. In this study, we focus on aliphatic amines and make use of the nitrogen 1s core electron excitations to elucidate the roles of N-H σ* and N-C σ* contributions in the unoccupied orbitals. We have measured N K-edge absorption spectra of the ethylamine bases Et NH ( x = 0...3; Et- = CH-) and the conjugate positively charged ethylammonium cation acids Et NH ( y = 0...4; Et- = CH-) dissolved in the protic solvents ethanol and water. Upon consecutive exchange of N-H for ethyl-groups, we observe a spectral shift, a systematic decrease of the N K-edge pre-edge peak, and a major contribution in the post-edge region for the ethylamine series. Instead, for the ethylammonium ions, the consecutive exchange of N-H for ethyl groups leads to an apparent reduction of pre-edge and post-edge intensities relative to the main-edge band, without significant frequency shifts. Building on findings from our previously reported study on aqueous ammonia and ammonium ions, we can rationalize these observations by comparing calculated N K-edge absorption spectra of free and hydrogen-bonded clusters. Hydrogen bonding interactions lead only to minor spectral effects in the ethylamine series, but have a large impact in the ethylammonium ion series. Visualization of the unoccupied molecular orbitals shows the consecutive change in molecular orbital character from N-H σ* to N-C σ* in these alkylamine/alkylammonium ion series. This can act as a benchmark for future studies on chemically more involved amine compounds.

We report on an experimental study of dissociative photoionization and neutral dissociation of H 2 by two-photon excitation from single B 1 + u (v = 5) rotational states (photon energy ≈ 4 eV). The B-state rovibrational levels are excited in the same light pulse by threephoton absorption from X 1 + g (v = 0). Angle resolved photoelectron and ion kinetic energy distributions allow us to discriminate between the different decay channels. Neutral decay is detected by one-photon ionization of excited atomic hydrogen product states. The kinetic energy distribution of photoelectrons from dissociative photoionization shows an oscillatory structure which depends on the initial B-state rotational quantum number J . An indication of a corresponding structure in the ion kinetic energy distributions is only found for J = 0. Similar to results reported by Verschuur et al we find neutral decay preferentially into high principal quantum number atomic states. The results indicate that the H 2 Q 1 doubly excited states play a crucial role in the decay processes.

We present photoelectron spectra measured by 'short' pulse (τ p 0.7 ps) MPI of xenon at 526.5 nm excitation wavelength in the intensity range 10 13 to 3 × 10 14 W cm −2 . Ionization into Xe + 2 P 3/2 and 2 P 1/2 channels is observed and resonance structures are found in both ionization channels. They are induced by the AC Stark shift of bound Xe states within the light pulse which brings them into transient n-photon resonance with the Xe ground state. The resonant states are tentatively identified as d-and s-angular momentum states having AC Stark shifts which differ from the shift of the xenon ionization threshold. In the high-intensity limit, ionization into the Xe + 2 P 1/2 channel is completely suppressed.

Seemingly simple yet surprisingly difficult to probe, excess protons in water constitute complex quantum objects with strong interactions with the extended and dynamically changing hydrogen‐bonding network of the liquid. Proton hydration plays pivotal roles in energy transport in hydrogen fuel cells and signal transduction in transmembrane proteins. While geometries and stoichiometry have been widely addressed in both experiment and theory, the electronic structure of these specific hydrated proton complexes has remained elusive. Here we show, layer by layer, how utilizing novel flatjet technology for accurate x‐ray spectroscopic measurements and combining infrared spectral analysis and calculations, we find orbital‐specific markers that distinguish two main electronic‐structure effects: Local orbital interactions determine covalent bonding between the proton and neigbouring water molecules, while orbital‐energy shifts measure the strength of the extended electric field of the proton.