Manfred Faubel

Photoelectron spectroscopy of liquid water, some alcohols, and pure nonane in free micro jets

The recently developed technique of accessing volatile liquids in a high vacuum environment by using a very thin liquid jet is implemented to carry out the first measurements of photoelectron spectra of pure liquid water, methanol, ethanol, 1-propanol, 1-butanol, and benzyl alcohol as well as of liquid n-nonane. The apparatus, which consists of a commercial hemispherical (10 cm mean radius) electron analyzer and a hollow cathode discharge He I light source is described in detail and the problems of the sampling of the photoelectrons in such an environment are discussed. For water and most of the alcohols up to six different electronic bands could be resolved. The spectra of 1-butanol and n-nonane show two weakly discernable peaks from which the threshold ionization potential could be determined. A deconvolution of the photoelectron spectra is used to extract ionization potentials of individual molecular bands of molecules near the surface of the liquid and shifts of the order of 1 eV compared to the gas p...

A molecular beam study of the evaporation of water from a liquid jet

A method to maintain a clean surface of a liquid in a high vacuum is described. Using a very thin and fast liquid jet it is not only possible to prevent freezing of the liquid but also to reduce the number of collisions between evaporating molecules to negligibly small values. Thus many of the standard, vacuum dependent, particle probing techniques for solid surfaces can be used for studies of rapidly vaporizing, high vapor pressure liquids. In a first molecular beam investigation we have used time-of-flight analysis to measure the velocity distribution of H2O molecules vaporizing from thin jets of pure liquid water. The experiments were carried out for liquid jet diameters between 50 and 5 µm. In this range the expanding vapor is observed to undergo the transition to the collision-free molecular flow regime. From the measured velocity distributions the local surface temperature is determined to be less than 210 K. This appears to be the lowest temperature ever reported for supercooled liquid water.

Interaction between liquid water and hydroxide revealed by core-hole de-excitation

The hydroxide ion plays an important role in many chemical and biochemical processes in aqueous solution. But our molecular-level understanding of its unusual and fast transport in water, and of the solvation patterns that allow fast transport, is far from complete. One proposal seeks to explain the properties and behaviour of the hydroxide ion by essentially regarding it as a water molecule that is missing a proton, and by inferring transport mechanisms and hydration structures from those of the excess proton. A competing proposal invokes instead unique and interchanging hydroxide hydration complexes, particularly the hypercoordinated OH-(H2O)4 species and tri-coordinated OH-(H2O)3 that can form a transient hydrogen bond between the H atom of the OH- and a neighbouring water molecule. Here we report measurements of core-level photoelectron emission and intermolecular Coulombic decay for an aqueous hydroxide solution, which show that the hydrated hydroxide ion is capable of transiently donating a hydrogen bond to surrounding water molecules. In agreement with recent experimental studies of hydroxide solutions, our finding thus supports the notion that the hydration structure of the hydroxide ion cannot be inferred from that of the hydrated excess proton.

Scattering Studies of Rotational and Vibrational Excitation of Molecules

Atom-molecule and ion-molecule inelastic scattering is reviewed. The basic theory is covered in detail with particular note of potential hypersurfaces. Various experimental techniques are discussed and results presented for scattering by H/sub 2/, N/sub 2/, CO, CO/sub 2/, N/sub 2/O and CsI.

Hydrogen bonds in liquid water studied by photoelectron spectroscopy

The authors report on photoelectron emission spectroscopy measurements of the oxygen 1s orbital of liquid water, using a liquid microjet in ultrahigh vacuum. By suitably changing the soft x-ray photon energy, within 600-1200 eV, the electron probing depth can be considerably altered as to either predominantly access the surface or predominantly bulk water molecules. The absolute probing depth in liquid water was inferred from the evolution of the O1s signal and from comparison with aqueous salt solution. The presence of two distinctive components in the core-level photoelectron spectrum, with significantly different binding energies, is revealed. The dominant contribution, at a vertical binding energy of 538.1 eV, was found in bulk and surface sensitive spectra. A weaker component at 536.6 eV binding energy appears to be present only in bulk water. Hartree-Fock calculations of O1s binding energies in different geometric arrangements of the water network are presented to rationalize the experimental distribution of O1s electron binding energies.

Ultrashort 1-kHz laser plasma hard x-ray source

We achieved a continuous, stable, ultrashort pulse hard x-ray point source by focusing 1.8-W, 1-kHz, 50-fs laser pulses onto a novel, 30-microm -diameter, high-velocity, liquid-metal gallium jet. This target geometry avoids most of the debris problems of solid targets and provides nearly 4pi illumination. Photon fluxes of 5x10(8) photons/s are generated in a two-component spectrum consisting of a broad continuum from 4 to 14 keV and strong K(alpha) and K(beta) emission lines at 9.25 and 10.26 keV. This source will find wide use in time-resolved x-ray diffraction studies and other applications.

Ionization Energies of Aqueous Nucleic Acids: Photoelectron Spectroscopy of Pyrimidine Nucleosides and ab Initio Calculations

Vertical ionization energies of the nucleosides cytidine and deoxythymidine in water, the lowest ones amounting in both cases to 8.3 eV, are obtained from photoelectron spectroscopy measurements in aqueous microjets. Ab initio calculations employing a nonequilibrium polarizable continuum model quantitatively reproduce the experimental spectra and provide molecular interpretation of the individual peaks of the photoelectron spectrum, showing also that lowest ionization originates from the base. Comparison of calculated vertical ionization potentials of pyrimidine bases, nucleosides, and nucleotides in water and in the gas phase underlines the dramatic effect of bulk hydration on the electronic structure. In the gas phase, the presence of sugar and, in particular, of phosphate has a strong effect on the energetics of ionization of the base. Upon bulk hydration, the ionization potential of the base in contrast becomes rather insensitive to the presence of the sugar and phosphate, which indicates a remarkable screening ability of the aqueous solvent. Accurate aqueous-phase vertical ionization potentials provide a significant improvement to the corrected gas-phase values used in the literature and represent important information in assessing the threshold energies for photooxidation and oxidation free energies of solvent-exposed DNA components. Likewise, such energetic data should allow improved assessment of delocalization and charge-hopping mechanisms in DNA ionized by radiation.

Vibrational and Rotational Excitation in Molecular Collisions

Publisher Summary This chapter focuses on the vibrational and rotational excitation in molecular collisions. The dynamics of a molecular collision is governed by the interaction forces between and within the colliding molecules. For given molecular electronic states, these can be summarized in potential energy functions of the respective internuclear distances and, when excluding electronic transitions, only one potential energy surface is relevant for the molecular collision. Differential vibrationally and rotationally inelastic scattering experiments measure the angular dependence of fully resolved state-to-state cross sections or, when internal state resolution cannot be completely achieved, inelastic excitation cross sections for energetically close groups of states. The joint experimental and theoretical studies of microscopic vibration–rotation inelastic cross sections are also well suited for the understanding and quantitative prediction of macroscopic phenomena such as gas-phase transport properties and internal-state relaxation processes of non-spherical molecules onto firmer microscopic grounds.

The He–O2 anisotropic van der Waals potential from the interplay of theory and scattering experiments

The interaction of helium atoms with ground state O2 molecules is examined over a wide range of internuclear distances that fully sample the extension of the shallow well region. A previous multiproperty study of its spherical part is extended to obtain the shape of its anisotropic behavior accurately by a best‐fit procedure which involves very precise state‐to‐state measurements of rotationally inelastic cross sections and IOSA calculations of total and partial differential cross sections. The final form of the interaction is discussed and analyzed with respect to previous findings for similar systems.

A high resolution crossed molecular beam investigation of the absolute cross sections and product rotational states for the reaction F+D2 (vi=0; ji=0,1)→DF(vf;jf)+D

High resolution time of flight spectra of DF products have been measured for 12 different center‐of‐mass angles in the range θc.m.=114° to 180° for the reaction F+D2→DF+D at a center‐of‐mass collision energy of Ec.m.=82.5±2.6 meV. The resolution is sufficient to clearly resolve the different final product vibrational states and to extract rotational product distributions for each of the vibrational states. Absolute reactive cross sections for the final vibrational states vf=1, 2, 3, and 4 were determined from a careful calibration of the beam source intensities and detector sensitivity. For all final vibrational states, nearly the same large rotational surprisal values of ΘR=5.3 were found. From the rotational distributions, it has also been possible to estimate opacity functions for these final vf states via the method of Elsum and Gordon [J. Chem. Phys. 76, 3009 (1982)]. The angular distributions for different vf states are compared to recent infinite order sudden approximation (IOSA) and classical tra...