John D. Bozek

Surface relaxation in liquid water and methanol studied by x-ray absorption spectroscopy

X-ray absorption spectroscopy is a powerful probe of local electronic structure in disordered media. By employing extended x-ray absorption fine structure spectroscopy of liquid microjets, the intermolecular O–O distance has been observed to undergo a 5.9% expansion at the liquid water interface, in contrast to liquid methanol for which there is a 4.6% surface contraction. Despite the similar properties of liquid water and methanol (e.g., abnormal heats of vaporization, boiling points, dipole moments, etc.), this result implies dramatic differences in the surface hydrogen bond structure, which is evidenced by the difference in surface tension of these liquids. This result is consistent with surface vibrational spectroscopy, which indicates both stronger hydrogen bonding and polar ordering at the methanol surface as a consequence of “hydrophobic packing” of the methyl group.

Adiabatic and vertical carbon 1s ionization energies in representative small molecules

Abstract Adiabatic and vertical carbon 1s ionization energies are reported for methane (CH 4 ), ethane (CH 3 CH 3 ), ethene (CH 2 CH 2 ), ethyne (HCCH), carbon monoxide (CO), carbon dioxide (CO 2 ), fluoromethane (CH 3 F), trifluoromethane (CHF 3 ), and tetrafluoromethane (CF 4 ) with an absolute accuracy of about 0.03 eV. The results are in good agreement with earlier values but are measured with higher resolution and accuracy than has previously been available.

Characterization of hydrogen bond acceptor molecules at the water surface using near-edge x-ray absorption fine-structure spectroscopy and density functional theory

We present a combined experimental/computational study of the near-edge x-ray absorption fine structure of the liquid water surface which indicates that molecules with acceptor-only hydrogen bonding configurations constitute an important and previously unidentified component of the liquid/vapour interface.

Carbon 1s photoelectron spectroscopy of CF4 and CO: Search for chemical effects on the carbon 1s hole-state lifetime

Carbon 1s photoelectron spectra for CF4 and CO have been measured at several photon energies near the carbon 1s threshold. The spectra have been analyzed in terms of the vibrational structure and the natural linewidth. For CO the vibrational structure shows evidence for anharmonicity in both the energy spacing and the intensity. Analysis of the results using an anharmonic model gives an equilibrium bond length for core-ionized CO that is 4.85 pm shorter than that of neutral CO. For CF4, the vibrational structure is very weak, and the analysis shows that the change in equilibrium CF bond length upon ionization is no more than 0.54 pm. Ab initio theoretical calculations give results in accord with these bond-length changes. The unusually small bond-length contraction in CF4 can be understood in terms of nonbonded fluorine–fluorine repulsion. The natural linewidth for core-ionized CO, 95±5 meV, is essentially the same as that of CH4. This result is in contrast with expectations based on the one-center model ...

Very high resolution electron spectroscopy with third generation synchrotron radiation sources: the resonant Auger decay spectrum of the Xe 4d5/2−16p state

Abstract We have recorded an Auger decay spectrum of the resonantly excited Xe 4d 5/2 −1 6p state between 36.4 and 37.3 eV kinetic energy. The spectrum was obtained under resonant Raman conditions, with a bandwidth of ≈6 meV for the exciting radiation, much smaller than the natural lifetime width of 110–120 meV. The full width at half maximum of the peaks in the spectrum was 10 meV, making it possible to completely resolve structure that overlapped in earlier studies.

Xenon N4, 500 Auger spectrum - a useful calibration source

Abstract In the xenon N 4,5 OO Auger spectrum there are 19 prominent lines ranging in kinetic energy from 8 to 36 eV that provide a convenient set of standards for calibrating electron spectrometers. Combining optical data with recent measurements of this spectrum gives energies for these lines that are absolutely accurate to 11 meV. For most lines the relative accuracy is better than 1 meV; for a few it is about 3 meV. The spin–orbit splitting of the xenon 4d lines is determined to be 1979.0±0.5 meV.

High-resolution angle-resolved measurements in atoms and molecules using advanced photoelectron spectroscopy at the ALS

Abstract A two-dimensional photoelectron imaging technique using time-of-flight spectrometry and a high-resolution SES-200 electron energy analyzer have been used with photons from the atomic and molecular undulator beamline of the Advanced Light Source to study the structure and dynamics of atoms and molecules. The analyzers are rotatable around the direction of the photon beam, allowing angular-dependent studies to be performed. Results on the photoexcitation and decay mechanisms of doubly excited resonances in neon illustrate the importance of spin-orbit effects for such a light atom, since significant breakdown of LS-coupling was measured for such a light atom . Also, fully vibrationally resolved Auger electron spectra measured under Resonant Raman conditions following the C 1s→π* excitation in a diatomic molecule, CO, is presented which allows us to probe the core excited states, their excitation and de-excitation dynamics with unprecedented resolution.

Photo double-ionization of deuterium chloride studied by threshold photoelectrons coincidence spectroscopy

Abstract Photo-double ionization of DCl has been investigated in the 35.5–38 eV energy range using synchrotron radiation on beamline 10.0.1.2 of the Advanced Light Source by the threshold photoelectrons coincidence (TPEsCO) method. The TPEsCO spectrum of DCl, encompassing the formation of the ground (X 3 Σ − ) and first-excited (a 1 Δ ) states of DCl2+, was recorded at good resolution (∼13 meV) using a pair of penetrating-field electron spectrometers. Four vibrational bands were identified in both electronic-state systems. In addition, the threshold photoelectron (TPE) spectrum of DCl for the formation of singly-charged DCl was recorded simultaneously in the same energy range. Indirect resonance autoionization of Rydberg ion states appears to play a substantial role in the formation of the (X 3 Σ − ) state of DCl2+ while the (a 1 Δ ) state appears to be formed primarily by direct photo-double ionization.

ABSOLUTE PHOTOIONIZATION CROSS SECTION MEASUREMENTS OF O ii IONS FROM 29.7 TO 46.2 eV

Absolute photoionization cross sections have been measured for a mixture of ground-state and metastable O ii (O + ) ions at photon energies ranging from 29.9 to 46.0 eV (414.7 to 269.5 A ˚ ). All measurements were performed by merging an O + beam with synchrotron radiation from an undulator beam line at the Advanced Light Source (ALS). At a resolution of 17 meV, more than 70 spectral features have been resolved, most of them identified and characterized. These measurements are compared with two independent R-matrix calculations and the data in TOPbase. All three calculations agree within 25% on the direct photoionization cross section, and with the absolute measurements within 40%. Some differences are noted in the predicted resonance positions among the three close-coupling R-matrix calculations, the TOPbase data being the least accurate. The estimated total experimental uncertainty varies from 15% to 20%. Such measurements benchmark theoretical photoionization cross section calculations performed within the framework of the Opacity Project and the Iron Project. Subject headings: atomic data — atomic processes — methods: laboratory

Mode-specific photoelectron scattering effects on CO2+(C 2Σg+) vibrations

Using high-resolution photoelectron spectroscopy, we have determined the energy dependent vibrational branching ratios for the symmetric stretch [v+=(100)], bend [v+=(010)], and antisymmetric stretch [v+=(001)], as well as several overtones and combination bands in the 4σg−1 photoionization of CO2. Data were acquired over the range from 20–110 eV, and this wide spectral coverage highlighted that alternative vibrational modes exhibit contrasting behavior, even over a range usually considered to be dominated by atomic effects. Alternative vibrational modes exhibit qualitatively distinct energy dependences, and this contrasting mode-specific behavior underscores the point that vibrationally resolved measurements reflect the sensitivity of the electron scattering dynamics to well-defined changes in molecular geometry. In particular, such energy-dependent studies help to elucidate the mechanism(s) responsible for populating the symmetry forbidden vibrational levels [i.e., v+=(010), (001), (030), and (110)]. Th...