Showing papers by "John D. Bozek published in 2019"

Assessing the Surface Oxidation State of Free-Standing Gold Nanoparticles Produced by Laser Ablation

Abstract: The surface chemistry of gold nanoparticles produced by the pulsed laser ablation in liquids method is investigated by X-ray photoelectron spectroscopy (XPS). The presence of surface oxide expected...

Full-dimensional theoretical description of vibrationally resolved valence-shell photoionization of H2O

Abstract: We have performed a full-dimensional theoretical study of vibrationally resolved photoelectron emission from the valence shell of the water molecule by using an extension of the static-exchange density functional theory that accounts for ionization as well as for vibrational motion in the symmetric stretching, antisymmetric stretching, and bending modes. At variance with previous studies performed in centrosymmetric molecules, where vibrationally resolved spectra are mostly dominated by the symmetric stretching mode, in the present case, all three modes contribute to the calculated spectra, including intermode couplings. We have found that diffraction of the ejected electron by the various atomic centers is barely visible in the ratios between vibrationally resolved photoelectron spectra corresponding to different vibrational states of the remaining H2O+ cation (the so-called v-ratios), in contrast to the prominent oscillations observed in K-shell ionization of centrosymmetric molecules, including those that only contain hydrogen atoms around the central atoms, e.g., CH4. To validate the conclusions of our work, we have carried out synchrotron radiation experiments at the SOLEIL synchrotron and determined photoelectron spectra and v-ratios for H2O in a wide range of photon energies, from threshold up to 150 eV. The agreement with the theoretical predictions is good.

Sequential electron emission and nuclear dissociation after the O 1 s → ( Σ u − 4 ) 4 p σ excitation in O 2 molecules

Abstract: When an electron is resonantly excited from a core orbital to a Rydberg one in a molecule, complex multi-time-scale multi-step processes are triggered. By using a high-resolution energy-resolved coincidence between Auger electron and ions, we identified the sequences of processes following the spectator Auger decays induced by K-shell excitation in O-2. The first step of the spectator Auger decay can end up to a few metastable O-2(+) states which autoionize into stable O-2(2+). However, the majority of O-2(+) states are dissociative. They evolve into several dissociation limits with complex crossings between potential-energy curves. After the molecular dissociation is finished, the generated fragments can further decay either by fluorescence emission or by autoionization. Interestingly, some electrons are always emitted after molecular dissociation.

Resonant Photoelectron Confinement in the SF6 Molecule.

Abstract: Recent thorough experimental activity aiming to generate high harmonics in the SF6 molecules requires the knowledge of, on one hand, accurate valence-shell photoionization cross sections and phases, from a threshold up to a few tens of eV, where resonances are likely to appear, and, on the other hand, the effect of the nuclear vibrational dynamics on the process. In this work, we have experimentally determined and theoretically evaluated vibrationally resolved photoionization cross sections of SF6 up to 80 eV photon energies, with an emphasis on the E2T1u channel, for which vibrational progressions are fully resolved in the experiment. Our results reveal the presence of shape resonances due to excitation to SF6 virtual states lying just above the ionization threshold, in agreement with previous synchrotron radiation work and theoretical calculations. More interestingly, our calculations also disclose resonance features at photoelectron energies as high as 40-50 eV, which are due to the transient confinement of the ejected electron in the octahedral cage formed by the peripheral F atoms. In the vicinity of all resonances, including those due to confinement, the calculated ionization phases experience an excursion of about π or π/2 and significantly depend on the final vibrational state of the remaining cation. Both effects should be taken into account to correctly interpret ongoing high-harmonic generation work in SF6. A similar behavior is expected for other symmetric molecules containing a central atom, such as BF3, CF4, and the like.