Reverberation Mapping and the Physics of Active Galactic Nuclei

Reverberation-mapping campaigns have revolutionized our understanding of AGN. They have allowed the direct determination of the broad-line region size, enabled mapping of the gas distribution around the central black hole, and are starting to resolve the continuum source structure. This review describes the recent and successful campaigns of the International AGN Watch consortium, outlines the theoretical background of reverberation mapping and the calculation of transfer functions, and addresses the fundamental difficulties of such experiments. It shows that such large-scale experiments have resulted in a ``new BLR'' which is considerably different from the one we knew just ten years ago. We discuss in some detail the more important new results, including the luminosity-size-mass relationship for AGN, and suggest ways to proceed in the near future.Comment: Review article to appear in Astronomical Time Series, Proceedings of the Wise Observatory 25th Ann. Symposium. 24 pages including 7 figure

Boosting hot electron flux and catalytic activity at metal-oxide interfaces of PtCo bimetallic nanoparticles

Despite numerous studies, the origin of the enhanced catalytic performance of bimetallic nanoparticles (NPs) remains elusive because of the ever-changing surface structures, compositions, and oxidation states of NPs under reaction conditions. An effective strategy for obtaining critical clues for the phenomenon is real-time quantitative detection of hot electrons induced by a chemical reaction on the catalysts. Here, we investigate hot electrons excited on PtCo bimetallic NPs during H-2 oxidation by measuring the chemicurrent on a catalytic nanodiode while changing the Pt composition of the NPs. We reveal that the presence of a CoO/Pt interface enables efficient transport of electrons and higher catalytic activity for PtCo NPs. These results are consistent with theoretical calculations suggesting that lower activation energy and higher exothermicity are required for the reaction at the CoO/Pt interface

Estimating soft-mode frequencies of surface overlayers by means of photoelectron diffraction: The (2x2) surface-V2O3/Pd(111)

The (2x2) surface-V2O3 layer, an interface-mediated vanadium oxide phase observed on Pd(111) in the submonolayer coverage range, has been investigated by means of angle scanned x-ray photoelectron diffraction (XPD), which gives a direct experimental confirmation of the model derived by scanning tunneling microscopy (STM) and density functional calculations (DFT), with a quantitative determination of the V-O interlayer spacing. In addition, XPD measurements compared to single scattering cluster\u2013spherical wave (SSC-SW) simulations revealed a peculiar broadening of V-O forward scattering (FS) maxima that is limited to azimuthal scans and that cannot be accounted for by isotropic Debye-Waller attenuation of the diffraction features. However, the existence of a soft phonon mode in the overlayer, associated with substantial in-plane displacements from equilibrium of O scatterers with respect to V emitters, could explain the experimental observation. The existence of such a soft mode has been confirmed by DFT calculations. It consists of an in-plane quasi rotation around the V emitter of the three nearest-neighbor O atoms, and the estimated DFT frequency amounts to 15 cm-1. The XPD data have been analyzed by means of SSC-SW simulations wherein a harmonic oscillator model has been employed to approximate the effect of the soft phonon mode on XPD curves. As a result, an experimental determination of the frequency of the mode has been obtained (40\ub125 cm-1), which is of the same order of magnitude as the DFT predicted frequency. Moreover, the sensitivity of XPD scans to the correlation of soft-mode atomic displacements has been studied, leading to the estimate of a \u2018\u2018soft-mode XPD coherence length\u2019\u2019 for the system under investigation. This work therefore explores an application of XPD as a surface spectroscopy sensitive to vibrational soft modes

Density functional study of the polar MnO(111) surface

By application of a density functional approach within the PBE and PBE+U approximations we investigate the ground state terminations of the polar MnO(111) surface being in thermodynamic equilibrium with an oxygen reservoir. In the allowed range of the oxygen chemical potential and for realistic oxygen partial pressures the surface is found to undergo different structural transitions. In the oxygen-poor regime the most stable phases are the O- and Mn-terminated octopolar structures, which are almost degenerate in energy. For oxygen-rich conditions we observe a competition between the O-terminated unreconstructed bulk face and a stripes structure. We show that the stabilization of the polar surface in the thermodynamic equilibrium with the oxygen environment is due to remarkable changes of the geometrical structure (i.e., reconstruction and relaxation) and of the electronic structure (i.e., metallization)

Tailor-made ultrathin manganese oxide nanostripes: 'magic widths' on Pd(1 1 N) terraces

The growth of ultrathin two-dimensional manganese oxide nanostripes on vicinal Pd(1 1 N) surfaces leads to particular stable configurations for certain combinations of oxide stripe and substrate terrace widths. Scanning tunneling microscopy and high-resolution low-energy electron diffraction measurements reveal highly ordered nanostructured surfaces with excellent local and long-range order. Density functional theory calculations provide the physical origin of the stabilization mechanism of 'magic width' stripes in terms of a finite-size effect, caused by the significant relaxations observed at the stripe boundaries

Density functional study of the polar MnO(111) surface

By application of a density functional approach within the PBE and PBE+U approximations we investigate the ground state terminations of the polar MnO(111) surface being in thermodynamic equilibrium with an oxygen reservoir. In the allowed range of the oxygen chemical potential and for realistic oxygen partial pressures the surface is found to undergo different structural transitions. In the oxygen-poor regime the most stable phases are the O- and Mn-terminated octopolar structures, which are almost degenerate in energy. For oxygen-rich conditions we observe a competition between the O-terminated unreconstructed bulk face and a stripes structure. We show that the stabilization of the polar surface in the thermodynamic equilibrium with the oxygen environment is due to remarkable changes of the geometrical structure (i.e., reconstruction and relaxation) and of the electronic structure (i.e., metallization)

Vanadium oxide nanostructures on Rh(111): Promotion effect of CO adsorption and oxidation

The adsorption of CO and the reaction of CO with pre-adsorbed oxygen at room temperature has been studied on the (2 x 1)O-Rh(1 1 1) surface and on vanadium oxide-Rh(1 1 1) "inverse model catalyst" surfaces using scanning tunnelling microscopy (STM) and core-level photoemission with synchrotron radiation. Two types of structurally well-defined model catalyst V3O9-Rh(1 1 1) surfaces have been prepared, which consist of large (mean size of similar to 50 nm, type I model catalyst) and small (mean size < 15 nm, type II model catalyst) two-dimensional oxide islands and bare Rh areas in between; the latter are covered by chemisorbed oxygen. Adsorption of CO on the oxygen pre-covered (2 x 1)O-Rh(1 1 1) surface leads to fast CO uptake in on-top sites and to the removal of half (0.25 ML) of the initial oxygen coverage by an oxidation clean-off reaction and as a result to the formation of a coadsorbed (2 x 2)-O + CO phase. Further removal of the adsorbed O with CO is kinetically hindered at room temperature. A similar kinetic behaviour has been found also for the CO adsorption and oxidation reaction on the type I "inverse model catalyst" surface. In contrast, on the type II inverse catalyst surface, containing small V-oxide islands, the rate of removal of the chemisorbed oxygen is significantly enhanced. In addition, a reduction of the V-oxide islands at their perimeter by CO has been observed, which is suggested to be the reason for the promotion of the CO oxidation reaction near the metal-oxide phase boundary