Conor Hogan

Ab initio electronic and optical spectra of free-base porphyrins: The role of electronic correlation

We present a theoretical investigation of electronic and optical properties of free-base porphyrins based on density functional theory and many-body perturbation theory. The electronic levels of free-base porphine (H(2)P) and its phenyl derivative, free-base tetraphenylporphyrin (H(2)TPP) are calculated using the ab initio GW approximation for the self-energy. The approach is found to yield results that compare favorably with the available photoemission spectra. The excitonic nature of the optical peaks is revealed by solving the Bethe-Salpeter equation, which provides an accurate description of the experimental absorption spectra. The lowest triplet transition energies are in good agreement with the measured values.

Correlation effects in the optical spectra of porphyrin oligomer chains: Exciton confinement and length dependence

Excited states of ethylene-linked free-base porphyrin oligomers and polymer are studied using many-body perturbation theory (MBPT) within the GW approximation and the Bethe-Salpeter equation. Trends in the electronic levels with oligomer length are analysed and the correct long-range behaviour in the band gap is obtained. High polarizabilities and strong redshifts in the optical absorption peaks are predicted in agreement with observations on other strongly conjugated oligoporphyrins. We explain these trends by means of spatial and spectral analyses of the exciton character. Although Wannier-Mott and charge-transfer excitons are identified in the optical spectra, the strongest polarizabilities are actually associated with small, tightly bound excitons (Frenkel-like), in contrast to expectations. Furthermore, the common procedure of extrapolating polymer properties from oligomer calculations is examined from a MBPT perspective.

Colour degradation of artworks: an ab initio approach to X-ray, electronic and optical spectroscopy analyses of vermilion photodarkening

Light-induced photodarkening of the pigment vermilion (α-HgS, cinnabar), observed in historical museum paintings and in murals at archaelogical sites worldwide, is an intriguing phenomenon that has triggered intense study using microscopy and spectroscopy techniques. However, the origins of the degradation and the nature of the concomitant physical, chemical and structural transformations are not yet completely clear. We present a first-principles study based on state-of-the-art electronic structure methods which sheds light on this darkening phenomenon. The presence of secondary compounds proposed to form during the transformation of vermilion (Hg3S2Cl2 polymorphs, Hg2Cl2, and HgCl2) is confirmed using X-ray spectroscopy simulations, and their structural, electronic, and optical properties are analysed using different levels of theory and compared with experimental observations. A scheme for growth of α-Hg3S2Cl2 on α-HgS is proposed, and possible formation and decomposition paths for the mercury chlorides are discussed. Approximations used in computing band gaps and band edges are examined in detail. This work highlights the key role that first-principles methods can play in the application of materials science to art conservation.

Theory for Modeling the Optical Properties of Surfaces

We summarize the theoretical framework and some of the computational methods currently used for the calculation of the optical properties of surfaces within the “ab initio” scheme. Applications and examples are given for calculations including self-energy, excitonic, and local-field effects, with an emphasis on convergence problems and on the approximations related to the pseudopotential approach.

Si Nanoribbons on Ag(110) Studied by Grazing-Incidence X-Ray Diffraction, Scanning Tunneling Microscopy, and Density-Functional Theory: Evidence of a Pentamer Chain Structure

We report a combined grazing incidence x-ray diffraction (GIXD), scanning tunneling microscopy (STM), and density-functional theory (DFT) study which clearly elucidates the atomic structure of the Si nanoribbons grown on the missing-row reconstructed Ag(110) surface. Our study allows us to discriminate between the theoretical models published in the literature, including the most stable atomic configurations and those based on a missing-row reconstructed Ag(110) surface. GIXD measurements unambiguously validate the pentamer model grown on the reconstructed surface, obtained from DFT. This pentamer atomistic model accurately matches the high-resolution STM images of the Si nanoribbons adsorbed on Ag(110). Our study closes the long-debated atomic structure of the Si nanoribbons grown on Ag(110) and definitively excludes a honeycomb structure similar to that of freestanding silicene.

Electronic structure calculations of mercury mobilization from mineral phases and photocatalytic removal from water and the atmosphere

Mercury is a hazardous environmental pollutant mobilized from natural sources, and anthropogenically contaminated and disturbed areas. Current methods to assess mobility and environmental impact are mainly based on field measurements, soil monitoring, and kinetic modelling. In order to understand in detail the extent to which different mineral sources can give rise to mercury release it is necessary to investigate the complexity at the microscopic level and the possible degradation/dissolution processes. In this work, we investigated the potential for mobilization of mercury structurally trapped in three relevant minerals occurring in hot spring environments and mining areas, namely, cinnabar (α-HgS), corderoite (α-Hg3S2Cl2), and mercuric chloride (HgCl2). Quantum chemical methods based on density functional theory as well as more sophisticated approaches are used to assess the possibility of a) direct photoreduction and formation of elemental Hg at the surface of the minerals, providing a path for ready release in the environment; and b) reductive dissolution of the minerals in the presence of solutions containing halogens. Furthermore, we study the use of TiO2 as a potential photocatalyst for decontamination of polluted waters (mainly Hg(2+)-containing species) and air (atmospheric Hg(0)). Our results partially explain the observed pathways of Hg mobilization from relevant minerals and the microscopic mechanisms behind photocatalytic removal of Hg-based pollutants. Possible sources of disagreement with observations are discussed and further improvements to our approach are suggested.

Publisher’s Note: Si Nanoribbons on Ag(110) Studied by Grazing-Incidence X-Ray Diffraction, Scanning Tunneling Microscopy, and Density-Functional Theory: Evidence of a Pentamer Chain Structure [Phys. Rev. Lett. 117 , 276102 (2016)]

This corrects the article DOI: 10.1103/PhysRevLett.117.276102.