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Preparation of aqueous solutions for 6 µm diam liquid jet formation using HPLC pump (ca. 40 bar) and 10 mm glass capillary.
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Jet apparatus conntected to U41 beamline at BESSY (left). LN2 dump (front); Analyzer (top).
Glass capillary and 6 µm liquid jet at 4°C. Free vacuum jet travels at 120 m/s.
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Photoemission detection position: skimmer, liquid jet, capillary. Ca. 2x10-4 mbar working pressure. Light intersects perpendicular to view plane, with polarization vector parallel to jet propagation.
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Water jet is undercooled, and freezes after about 50 cm travel.
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Electronic structure of the liquid water surface: Valence orbitals and Auger electrons
The valence photoemission (PE) spectrum of pure liquid water is characterized by orbital peak shifts to about 1.5 eV lower binding energy as compared to gas-phase water. The top of the 1b 1 valence band is at 10.0 eV which agrees with early photoelectron threshold measurements (P. Delahay, Chem. Phys. Lett. 1981, 83, 250). Whereas the main effect of the energy shift is attributed to the electronic polarization of the solvent molecules around ionized water, the smaller differential gas-to-liquid shift reflects the specific participation of a given molecular orbital in the hydrogen bond in liquid water. This can be also inferred from the relative photoionization cross sections.
The off resonance O1s Auger spectrum of liquid water exhibit extra emission features at high kinetic energies when compared to gas-phase water. This is very similar to water clusters, and has been attributed to final states deolocalized over the clusters (G. Öhrwall et al., J. Chem. Phys. 2005, 123, 054310). The increase in kinetic energy of the liquid water Auger spectrum, when tuning into the liquid water absorption (XAS) preedge might be interpreted in terms of very fast delocalization of the excited electron (normal as opposed to spectator Auger). Details are being currently investigated.
Reference:
Winter, B.; Weber, R.; Widdra, W.; Dittmar, M.; Faubel, M.; Hertel, I.V., “Full Valence Band Photoemission from Liquid Water Using EUV Synchrotron Radiation”, J. Phys. Chem. A 2004, 108, 2625
Aqueous alkali and halide ions: Electron binding enrgies and CTTS transitions
Lowest vertical electron binding energies of the prototype aqueous alkali cations and halide anions were measured at 100 eV photon energy. Electron binding energies of a given aqueous ion are found to be independent of the counter ion and the salt concentration.
Anionic (aqueous) photoionization is more difficult to be treated accurately in the calculations (ab initio models and continuum model) due to the stronger reorganization of solvent water molecules (charged initial vs neutral final state).
The PE/Auger spectrum of aquoeus LiCl demonstrates that one can efficiently excite CTTS (Charge Transfer To Solvent) transitions from inner shells (Cl-(3p)aq to CTTS). The CTTS state is probed through Auger decay of the Cl- 3p hole upon filling by a transient CTTS electron. Details are being currently investigated.
References:
Weber, R.; Winter, B.; Schmidt, P.M.; Widdra, W.; Hertel, I.V.; Dittmar, M.; Faubel, M., “Photoemission from Aqueous Alkali-Metal – Iodide Salt Solutions Using EUV Synchrotron Radiation”, J. Phys. Chem. B 2004, 108, 4729
Winter, B.; Weber, R.; Hertel, I.V.; Faubel, M.; Jungwirth, P.; Brown, E.C.; Bradforth, S.E., “Electron Binding Energies of Aqueous Alkali and Halide Ions: EUV Photoelectron Spectroscopy of Liquid Solutions and Combinde Ab Initio and Molecular Dynamics Calculations”, J. Am. Chem. Soc. 2005, 127, 7203
Aqueous surfactant interface
PE spectra of aqueous solution of tetrabutylammonium iodide (TBAI) exhibit enormous iodide signal enhancement as compared to aqueous NaI of comparable concentration. This is a direct consequence of the hydrophobic hydrocarbon chains. Detailed analysis reveals in fact that this hydrophobic interaction even enhances the probability of the anion to exist at the solution interface. The data are consistent with a negative adsorption free energy within a simple Langmuir adsorption isotherm model. The experiments indicate that the TBAI is largely orientated within the surface, with no dipole moment perpendicular to the surface.
The competition between the larger iodide and the smaller bromide in occupying surface sites (and on the consequences for the structure of the segregation surface layer) can be inferred from a comparative study of TBAI vs TBABr.
References:
Winter, B.; Weber, R.; Schmidt, P.M.; Hertel, I.V.; Faubel, M.; Vrbka, L.; Jungwirth, P., “Molecular Structure of Surface-Active Salt Solutions: Photoelectron Spectroscopy and Molecular Dynamics Simulations of Aqueous Tetrabutylammonium Iodide”, J. Chem. B 2004, 108, 14558
Winter, B.; Weber, R.; Hertel, I.V.; Faubel, M.; Vrbka, L.; Jungwirth; P., “Effect of Bromide on the Interfacial Structure of Aqueous Tetrabutylammonium Iodide: Photoelectron Spectroscopy and Molecular Dynamics Simulations”, Chem. Phys. Lett. 2005, 410, 222
Winter, B. & Faubel, M., “Photoemission from Liquid Aqueous Solutions”, Chem. Rev. (soon)
Other topics of current focus
- Electronic finger print of the hydrated (Eigen) proton; PE studies of aqueous acids and bases
- A comparison of hydrogen bonding in liquid H2O2 vs H2O
- C- and N- chemical shifts in aqueous amino acids
Dr. Manfred Faubel, Max-Planck-Institut für Dynamik und Selbstorganisation, Göttingen
Prof. Pavel Jungwirth, Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Prague
Prof. Stephen Bradforth, USC, Los Angeles
Dr. Christian Pettenkofer, Hahn-Meitner-Institut, Berlin
Manfred, Martin Mucha (from Prague)
Steve, Bernd, Pavel
Manfred, Christian
Ramona Weber, PhD Thesis, FU Berlin, 2003
“Photoelectron Spectroscopy of Liquid Water and Aqueous Solutions in Free Microjets Using Synchrotron Radiation”
Philipp M. Schmidt, Diploma Thesis, FU Berlin, 2005
“Photoelektronenspektroskopie wässriger Triiodid-Lösungen“
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