Experimental and theoretical near edge x ray absorption fine structure studies of NO

Experimental near edge x ray absorption fine structure NEXAFS spectra of the nitrosonium NO ion are presented and theoretically analyzed. While neutral NO has an open shell, the cation is a closed shell species, which for NEXAFS leads to the simplicity of a closed shell spectrum. Compared to neutral NO, the electrons in the cation experience a stronger Coulomb potential, which introduces a shift of the ionization potential towards higher energies, a depletion of intensity in a large interval above the amp; 960; amp; 8727; resonance, and a shift of the amp; 963; amp; 8727; resonance from the continuum to below the ionization threshold. NEXAFS features at the nitrogen and oxygen K edges of NO are compared, as well as NEXAFS features at the nitrogen edges of the isoelectronic closed shell species NO , N2, and N2

Ultrafast modification of the electronic structure of a correlated insulator

A nontrivial balance between Coulomb repulsion and kinematic effects determines the electronic structure of correlated electron materials. The use of electromagnetic fields strong enough to rival these native microscopic interactions allows us to study the electronic response as well as the time scales and energies involved in using quantum effects for possible applications. We use element specific transient x ray absorption spectroscopy and high harmonic generation to measure the response to ultrashort off resonant optical fields in the prototypical correlated electron insulator NiO. Surprisingly, fields of up to 0.22 V lead to no detectable changes in the correlated Ni 3d orbitals contrary to previous predictions. A transient directional charge transfer is uncovered, a behavior that is captured by first principles theory. Our results highlight the importance of retardation effects in electronic screening and pinpoints a key challenge in functionalizing correlated materials for ultrafast device operatio

SOFT X-RAY ABSORPTION AND EMISSION SPECTRA AND THE ELECTRONIC STRUCTURE OF THE Ba2 YCu3 O7-x SUPERCONDUCTOR

Nous présentons des spectres d'émission dans les X mous, du Ba2YCu3O7-x supraconducteur, excité par des faisceaux d'électrons, ainsi que les spectres du rendement total de photoélectrons du même materiau excité par des photons d'énergie comprise entre 20 et 600 eV. Nous confirmons, par la mesure de ce rendement, que le cuivre a une valence +2 dans ce composé. L'émission de rayons X mous fourni, par l'étude du spectre N4,5 du barium, du spectre M4,5 de l'yttrium et du spectre K de l'oxygène, une mesure de la densité d'états partielle de type p (p-PDOS) localisée sur chacun des sites atomiques respectifs. Dans chaque cas cette densité d'états est très petite à l'énergie de Fermi, et a un premier pic situé entre 3.5 et 4 eV en dessous du niveau de Fermi. L'étude du spectre K de l'oxygène confirme l'interprétation selon laquelle les structures observées dans les mesures de photoémission sont associées aux orbitales 2p de l'oxygène. Enfin nous n'avons observé aucun changement entre les spectres enregistré au dessus ou au dessous de la température critique Tc.We present e-beam excited soft x-ray emission spectra and total photoelectron yield spectra in the 20-600 eV photon energy range for the Ba2YCu3O7-x superconductor. We confirm the 2+ valency of Cu in the compound by total yield measurements. In soft x-ray emission, the N4.5 spectrum of Ba, the M4,5 spectrum of Y, and the K spectrum of O provide measures of the p-type partial density of states (p-PDOS) localized on the respective atomic sites. In each case the p-PDOS is very small at the Fermi energy with the first peak in the p-PDOS lying 3.5 to 4 eV below the Fermi energy. The K spectra of O confîrm the interpretation that the structure observed in the photoemission measurements are associated with the O 2p orbitals. Finally no changes are observed between spectra taken above and below Tc

Insulating surface layer on single crystal K 3C60\mathsf{_{3}}\mathsf{C}\mathsf{_{60}}

Using angle-dependent photoemission spectra of core and valence levels we show that metallic, single crystal K 3C 60 is terminated by an insulating or weakly-conducting surface layer. We attribute this to the effects of strong intermolecular correlations combined with the average surface charge state. Several controversies on the electronic structure are thereby resolved. Copyright Springer-Verlag Berlin/Heidelberg 2004