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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Few-cycle plasmon oscillations controlling photoemission from metal nanoparticles

Few-cycle optical excitation of nanosystems holds promise of fundamental discoveries and applications in ultrafast nanoscience, the development of nanostructured photocathodes, and many more. For these, surface plasmon generation on unprecedented timescales needs to be controlled. For this, few-cycle plasmon oscillations on a metal nanoparticle can be generated by keeping considerable electric field enhancement factors. As an initial application of such a high spatiotemporal localization of an ultrashort laser pulse, we numerically demonstrate the control of photoelectrons on a true sub-fs timescale in nanometric spatial domains. We show that it is only off-resonant nanoparticles that can provide few-cycle plasmons and electron control on this timescale.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Pseudogap behaviour in FeTe and FeSe probed by photoemission

This study reports the valence band photoelectron spectroscopic studies of FeTe and FeSe. A doping induced and temperature dependent pseudogap occur near the Fermi level. The spectral weight transfer occurs due to the change in chalcogen height due to lowering of temperature. This result is in analogy with the reduction in chalcogen height due to the replacement of Te by Se.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Interaction between adsorbed hydrogen and potassium on a carbon nanocone containing material as studied by photoemission

Hydrogen adsorption on a potassium doped carbon nanocone containing material was studied by photoelectron spectroscopy and work function measurement. The valence band spectra indicate that there is charge transfer from potassium to carbon. Upon deposition on carbon potassium is in its ionic state for lower doping and shows both ionic and metallic behavior at higher doping. Adsorption of hydrogen facilitates diffusion of potassium on the carbon material as seen by changes in the K{sub 2p} core level spectrum. Variations in the measured sample work function indicate that hydrogen initially adsorb on the K dopants and subsequently adsorb on the carbon cone containing material.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

A photoemission study of the effectiveness of nickel, manganese, and cobalt based corrosion barriers for silicon photo-anodes during water oxidation

Silicon is an attractive material for solar water splitting applications due to its abundance and its capacity to absorb a large fraction of incident solar radiation. However, it has not received as much attention as other materials due to its tendency to oxidize very quickly in aqueous environments, particularly when it is employed as the anode where it drives the oxygen evolution reaction. In recent years, several works have appeared in the literature examining the suitability of thin transition metal oxide films grown on top of the silicon to act as a corrosion barrier. The film should be transparent to solar radiation, allow hole transport from the silicon surface to the electrolyte, and stop the diffusion of oxygen from the electrolyte back to the silicon. In this work, we compare Mn-oxide, Co-oxide, and Ni-oxide thin films grown using physical vapor deposition in order to evaluate which material offers the best combination of photocurrent and corrosion protection. In addition to the electrochemical data, we also present a detailed before-and-after study of the surface chemistry of the films using x-ray photoelectron spectroscopy. This approach allows for a comprehensive analysis of the mechanisms by which the corrosion barriers protect the underlying silicon, and how they degrade during the water oxidation reaction.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Ab initio determination of satellite intensities in transition-metal photoemission spectroscopy using a multiconfiguration framework

Following atomic photoionization, the abrupt change in potential can lead to secondary ionization of an outer-shell electron in a phenomenon known as shake-off, a process which gives rise to the asymmetric K{alpha} profile and satellite lines. Investigation of chemical effects and relativistic quantum mechanics requires a theoretical determination of these satellite intensities; however, existing theoretical predictions are inconsistent with experimental results by up to an order of magnitude. Previously theoretical modeling required up to 12 fitting parameters to account for transition widths, energy corrections, spectator intensities, and spectator broadening. Using a multiconfiguration atomic model to account for electron-electron correlation, we provide here the first ab initio calculations of shake-off probabilities which are in agreement with experimental results (except for copper), an important step toward a complete theoretical profile.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2010

Two-particle photoemission from strongly correlated systems: A dynamical mean-field approach

We study theoretically the simultaneous photoinduced two-particle excitations of strongly correlated systems on the basis of the Hubbard model. Under certain conditions specified in this work, the corresponding transition probability is related to the two-particle spectral function which we calculate using three different methods: the dynamical mean-field theory combined with quantum Monte Carlo technique, the first-order perturbation theory and the ladder approximations. The results are analyzed and compared for systems at the verge of the metal-insulator transitions. The dependencies on the electronic correlation strength and on doping are explored. In addition, the account for the orbital degeneracy allows an insight into the influence of interband correlations on the two-particle excitations. A suitable experimental realization is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Photoemission core-level shifts reveal the thiolate-Au(111) interface

The nature of the thiolate/Au(111) interface is a long-standing puzzle. It has been suggested that thiolates drive surface reconstruction, however, a consensus regarding the adsorption configuration is missing. Herein, the density-functional theory is used to evaluate surface core-level shifts (SCLSs) for methyl thiolates on Au(111) assuming a representative set of different surface reconstructions. The SCLSs are found to provide sensitive fingerprints of the anchoring configuration, and it is only thiolate adsorption in the form of MeS-Au-SMe complexes that can be reconciled with experimental data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Disentanglement of electron dynamics and space-charge effects in time-resolved photoemission from h-BN/Ni(111)

We observe time-resolved and polarization dependent two-, three-, and four-photon transitions from a Ni(111) surface covered with a monolayer of hexagonal boron nitride. The spectra show features due to transitions involving two unoccupied intermediate states: the n=1 image potential state of Ni(111) and a boron nitride-related interface state. We use these transitions in order to track the effects of space-charge on the spectra in view of pump-probe experiments, at high excitations densities. A simple model is presented, which reproduces the measured energy shifts and broadenings in great detail, allowing the ultrafast dynamics and the space-charge effects to be disentangled. Moreover, owing to the high excitation densities an additional particular three-photon transition via both the interface state and a virtual intermediate state could be measured and identified. The polarization-dependent transient of this transition has successfully been simulated, allowing the lifetime of the involved intermediate state to be extracted from the data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Resonant photoemission and absorption spectroscopy of the Cu{sub x}TiSe{sub 2} compound

Single crystals of the Cu{sub x}TiSe{sub 2} compound with x = 0.05, 0.09, and 0.33 have been grown. Resonance photoelectron Cu 3p-3d and 2d-3d spectra of the valence bands, the spectra of the core levels, and the L absorption spectra for titanium and copper have been obtained. It is shown that the degree of oxidation of titanium atoms is +4 and the state of copper atoms is close to the state of free copper ions. It is found that the spectra of the valence bands obtained under the Cu 3p and 2p resonance conditions radically differ. For the spectra in the Cu 2p excitation regime, several bands corresponding to different decay channels of the excited state are observed. According to calculations of the density of states, the 3d states of copper are filled incompletely; the occupancy of the 3d band of copper is 9.5 electrons per atom.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1995

X-Ray Photoemission Measurements and Electronic Structure of HgBa2CaCu2O6+Delta

X-ray photoelectron spectroscopy data presented here are the first on an epitaxial thin film of a mercury cuprate superconductor, the first in which signals from the superconducting phase dominate all the core level spectra, and the first to exhibit a clear Fermi edge in the valence band region. Mercury is found to be in a single chemical state, contrary to reports based on sintered polycrystalline pellets.

x-ray photoemission photoelectron spectroscopy mer↗