Ph. Wernet

Spectroscopic probing of local hydrogen-bonding structures in liquid water

We have studied the electronic structure of liquid water using x-ray absorption spectroscopy at the oxygen K edge. Since the x-ray absorption process takes less than a femtosecond, it allows probing of the molecular orbital structure of frozen, local geometries of water molecules at a timescale that has not previously been accessible. Our results indicate that the electronic structure of liquid water is significantly different from that of the solid and gaseous forms, resulting in a pronounced pre-edge feature below the main absorption edge in the spectrum. Theoretical calculations of these spectra suggest that this feature originates from specific configurations of water, for which the H-bond is broken on the H-donating site of the water molecule. This study provides a fingerprint for identifying broken donating H-bonds in the liquid and shows that an unsaturated H-bonding environment exists for a dominating fraction of the water molecules.

Spectroscopic characterization of microscopic hydrogen-bonding disparities in supercritical water

The local hydrogen-bonding environment in supercritical water (380 degrees C, 300 bars, density 0.54 gcm3) was studied by x-ray Raman scattering at the oxygen K edge. The spectra are compared to those of the gas phase, liquid surface, bulk liquid, and bulk ice, as well as to calculated spectra. The experimental model systems are used to assign spectral features and to quantify specific local hydrogen-bonding situations in supercritical water. The first coordination shell of the molecules is characterized in more detail with the aid of the calculations. Our analysis suggests that approximately 65% of the molecules in supercritical water are hydrogen bonded in configurations that are distinctly different from those in liquid water and ice. In contrast to liquid water the bonded molecules in supercritical water have four intact hydrogen bonds and in contrast to ice large variations of bond angles and distances are observed. The remaining approximately 35% of the molecules exhibit two free O-H bonds and are thus either not involved in hydrogen bonding at all or have one or two hydrogen bonds on the oxygen side. We determine an average O-O distance of 3.1+/-0.1 A in supercritical water for the H bonded molecules at the conditions studied here. This and the corresponding hydrogen bond lengths are shown to agree with neutron- and x-ray-diffraction data at similar conditions. Our results on the local hydrogen-bonding environment with mainly two disparate hydrogen-bonding configurations are consistent with an extended structural model of supercritical water as a heterogeneous system with small patches of bonded molecules in various tetrahedral configurations and surrounding nonbonded gas-phase-like molecules.

The hydrogen bond in ice probed by soft x-ray spectroscopy and density functional theory

We combine photoelectron and x-ray absorption spectroscopy with density functional theory to derive a molecular orbital picture of the hydrogen bond in ice. We find that the hydrogen bond involves donation and back-donation of charge between the oxygen lone pair and the O-H antibonding orbitals on neighboring molecules. Together with internal s-p rehybridization this minimizes the repulsive charge overlap of the connecting oxygen and hydrogen atoms, which is essential for a strong attractive electrostatic interaction. Our joint experimental and theoretical results demonstrate that an electrostatic model based on only charge induction from the surrounding medium fails to properly describe the internal charge redistributions upon hydrogen bonding.

Metastable state contributions to the measured 3p photoabsorption spectrum of Cr+ ions in a laser-produced plasma

Recently, the spectrum of singly ionized chromium has come under theoretical scrutiny by Dolmatov (1996 J. Phys. B: At. Mol. Opt. Phys. 29 L673) and also by Donnelly et al (1997 J. Phys. B: At. Mol. Opt. Phys. 30 L285). Both of these theoretical investigations, employing the spin-polarized RPAE (SPRPAE) and R-matrix techniques, respectively, concerned the 3p photoabsorption cross section from the 3d5 6S5/2 ground state of Cr+. The two calculations are consistent with each other and agree with the previously reported experimental data for the 3p4s resonances and the onset of the 3p3d giant resonance (Costello 1991 Phys. Rev. A 43 1441). However, beyond the peak of the giant resonance, both calculations result in cross sections which differ considerably from the experimental data. From further dual-laser plasma experiments carried out under different spatio-temporal conditions and atomic structure calculations on metastable states this discrepancy can be ascribed to the presence of metastable excited singly ionized chromium within the plasma.

Term-dependent lifetime broadening effect on the 4d photoelectron spectrum of atomic thulium

The 4d photoelectron spectrum of free thulium atoms is measured and calculated in the single-configuration intermediate-coupling approximation considering multiplet splitting and lifetime broadening of the spectral components due to radiationless transitions following the 4d ionization. The individual natural widths of the 4d94f13 multiplet components are found to differ considerably, and the experimental profile of the spectrum can be reproduced theoretically only if this effect is taken into account.

Resonant 3p photoelectron spectroscopy of free Cu atoms

Excitation and decay of the Cu 3p core resonances have been studied by resonant photoelectron spectroscopy. In comparison with theoretical predictions the importance of electron correlation for the resonance energies and line shapes is verified.

Valence satellite and 3p photoelectron spectra of atomic Fe and Cu

The 3p photoelectron spectra of atomic Fe and Cu, and the Fe outer-shell satellites have been investigated. The complex multiline structure of the Fe spectra has been resolved using high-resolution electron spectroscopy. The thresholds of the direct 3p photoionization have been determined and are compared with experimental results from photoabsorption measurements and Auger spectra, and with the multiplet structures calculated taking into account intershell and configuration interaction. The stepwise decay of highly excited Fe states into has been verified by the detection of satellite lines with binding energies above the lowest ionization threshold.

Linear dichroism in the 4d photoionization of atomic europium

The dichroism in the 4d photoelectron spectra of laser polarized Eu atoms has been determined with linearly polarized soft x-ray radiation. The results are discussed in comparison with the predictions by the LS-coupling model and by configuration interaction Hartree-Fock calculations, thereby demonstrating the strong influence of valence electron correlations. The interpretation of the dichroism in the 4d photoelectron spectra of solid Gd in terms of an ionic model is corroborated by the close similarity of the Gd solid and the Eu atom spectra.

Breakdown of the three-parameter model for the dichroism in the 4f photoelectron spectrum of laser-aligned Eu atoms

The dichroism in the 4f photoelectron spectra of laser-aligned Eu atoms has been determined with linearly polarized vacuum ultraviolet radiation. The experimental results disagree with the prediction of the three-parameter model based on the absolute magnitude and relative phase of the 4f d, g transition matrix elements. It is suggested that strong configuration interactions, confirmed in high-resolution photoelectron spectra, are the cause of the breakdown of the three-parameter model.

Communication: Direct evidence for sequential dissociation of gas-phase Fe(CO)5 via a singlet pathway upon excitation at 266 nm

We prove the hitherto hypothesized sequential dissociation of Fe(CO)5 in the gas phase upon photoexcitation at 266 nm via a singlet pathway with time-resolved valence and core-level photoelectron spectroscopy with an x-ray free-electron laser. Valence photoelectron spectra are used to identify free CO molecules and to determine the time constants of stepwise dissociation to Fe(CO)4 within the temporal resolution of the experiment and further to Fe(CO)3 within 3 ps. Fe 3p core-level photoelectron spectra directly reflect the singlet spin state of the Fe center in Fe(CO)5, Fe(CO)4, and Fe(CO)3 showing that the dissociation exclusively occurs along a singlet pathway without triplet-state contribution. Our results are important for assessing intra- and intermolecular relaxation processes in the photodissociation dynamics of the prototypical Fe(CO)5 complex in the gas phase and in solution, and they establish time-resolved core-level photoelectron spectroscopy as a powerful tool for determining the multiplicity of transition metals in photochemical reactions of coordination complexes.