779 research outputs found
Time and energy-resolved two photon-photoemission of the Cu(100) and Cu(111) metal surfaces
We present calculations on energy- and time-resolved two-photon photoemission
spectra of images states in Cu(100) and Cu(111) surfaces. The surface is
modeled by a 1D effective potential and the states are propagated within a
real-space, real-time method. To obtain the energy resolved spectra we employ a
geometrical approach based on a subdivision of space into two regions. We treat
electronic inelastic effects by taking into account the scattering rates
calculated within a GW scheme. To get further insight into the decaying
mechanism we have also studied the effect of the variation of the classical
Hartree potential during the excitation. This effect turns out to be small.Comment: 11 pages, 7 figure
Quantifying the Plasmonic Character of Optical Excitations in a Molecular J-Aggregate
The definition of plasmon at the microscopic scale is far from being
understood. Yet, it is very important to recognize plasmonic features in
optical excitations, as they can inspire new applications and trigger new
discoveries by analogy with the rich phenomenology of metal nanoparticle
plasmons. Recently, the concepts of plasmonicity index and the generalized
plasmonicity index (GPI) have been devised as computational tools to quantify
the plasmonic nature of optical excitations. The question may arise whether any
strong absorption band, possibly with some sort of collective character in its
microscopic origin, shares the status of plasmon. Here we demonstrate that this
is not always the case, by considering a well-known class of systems
represented by J-aggregates molecular crystals, characterized by the intense J
band of absorption. By means of first-principles simulations, based on a
many-body perturbation theory formalism, we investigate the optical properties
of a J-aggregate made of push-pull organic dyes. We show that the effect of
aggregation is to lower the GPI associated with the J-band with respect to the
isolated dye one, which corresponds to a nonplasmonic character of the
electronic excitations. In order to rationalize our finding, we then propose a
simplified one-dimensional theoretical model of the J-aggregate. A useful
microscopic picture of what discriminates a collective molecular crystal
excitation from a plasmon is eventually obtained.Comment: Published by ACS under ACS AuthorChoice licens
Bonds, lone pairs, and shells probed by means of on-top dynamical correlations
The Electron Localization Function (ELF) by Becke and Edgecombe [J. Chem.
Phys. {\bf 92}, 5397 (1990)] is routinely adopted as a descriptor of atomic
shells and covalent bonds. Since the ELF and its related quantities find useful
exploitation also in the construction of modern density functionals, the
interest in complementing the ELF is linked to both the quests of improving
electronic structure descriptors and density functional approximations. The ELF
uses information which is available by considering parallel-spin electron pairs
in single-reference many-body states. In this work, we complement this
construction with information obtained by considering antiparallel-spin pairs
whose short-range correlations are modeled by a density functional
approximation. As a result, the approach requires only a contained
computational effort. Applications to a variety of systems show that, in this
way, we gain a spatial description of the bond in H (which is not available
with the ELF) together with some trends not optimally captured by the ELF in
other prototypical situations
Interplay between Intra- and Intermolecular Charge Transfer in the Optical Excitations of J-Aggregates
In a first-principles study based on density functional theory and many-body
perturbation theory, we address the interplay between intra- and intermolecular
interactions in a J-aggregate formed by push-pull organic dyes by investigating
its electronic and optical properties. We find that the most intense excitation
dominating the spectral onset of the aggregate, i.e., the J-band, exhibits a
combination of intramolecular charge transfer, coming from the push-pull
character of the constituting dyes, and intermolecular charge transfer, due to
the dense molecular packing. We also show the presence of a pure intermolecular
charge-transfer excitation within the J-band, which is expected to play a
relevant role in the emission properties of the J-aggregate. Our results shed
light on the microscopic character of optical excitations of J-aggregates and
offer new perspectives to further understand the nature of collective
excitations in organic semiconductors.Comment: published under ACS Authorchoice licens
A TDDFT study of the excited states of DNA bases and their assemblies
We present a detailed study of the optical absorption spectra of DNA bases
and base pairs, carried out by means of time dependent density functional
theory. The spectra for the isolated bases are compared to available
theoretical and experimental data and used to assess the accuracy of the method
and the quality of the exchange-correlation functional: Our approach turns out
to be a reliable tool to describe the response of the nucleobases. Furthermore,
we analyze in detail the impact of hydrogen bonding and -stacking in the
calculated spectra for both Watson-Crick base pairs and Watson-Crick stacked
assemblies. We show that the reduction of the UV absorption intensity
(hypochromicity) for light polarized along the base-pair plane depends strongly
on the type of interaction. For light polarized perpendicular to the basal
plane, the hypochromicity effect is reduced, but another characteristic is
found, namely a blue shift of the optical spectrum of the base-assembly
compared to that of the isolated bases. The use of optical tools as
fingerprints for the characterization of the structure (and type of
interaction) is extensively discussed.Comment: 31 pages, 8 figure
An exact Coulomb cutoff technique for supercell calculations
We present a new reciprocal space analytical method to cutoff the long range
interactions in supercell calculations for systems that are infinite and
periodic in 1 or 2 dimensions, extending previous works for finite systems. The
proposed cutoffs are functions in Fourier space, that are used as a
multiplicative factor to screen the bare Coulomb interaction. The functions are
analytic everywhere but in a sub-domain of the Fourier space that depends on
the periodic dimensionality. We show that the divergences that lead to the
non-analytical behaviour can be exactly cancelled when both the ionic and the
Hartree potential are properly screened. This technique is exact, fast, and
very easy to implement in already existing supercell codes. To illustrate the
performance of the new scheme, we apply it to the case of the Coulomb
interaction in systems with reduced periodicity (as one-dimensional chains and
layers). For those test cases we address the impact of the cutoff in different
relevant quantities for ground and excited state properties, namely: the
convergence of the ground state properties, the static polarisability of the
system, the quasiparticle corrections in the GW scheme and in the binding
energy of the excitonic states in the Bethe-Salpeter equation. The results are
very promising.Comment: Submitted to Physical Review B on Dec 23rd 200
Theoretical description of protein field effects on electronic excitations of biological chromophores
Photoinitiated phenomena play a crucial role in many living organisms. Plants, algae, and bacteria absorb sunlight to perform photosynthesis, and convert water and carbon dioxide into molecular oxygen and carbohydrates, thus forming the basis for life on Earth. The vision of vertebrates is accomplished in the eye by a protein called rhodopsin, which upon photon absorption performs an ultrafast isomerisation of the retinal chromophore, triggering the signal cascade. Many other biological functions start with the photoexcitation of a protein-embedded pigment, followed by complex processes comprising, for example, electron or excitation energy transfer in photosynthetic complexes. The optical properties of chromophores in living systems are strongly dependent on the interaction with the surrounding environment (nearby protein residues, membrane, water), and the complexity of such interplay is, in most cases, at the origin of the functional diversity of the photoactive proteins. The specific interactions with the environment often lead to a significant shift of the chromophore excitation energies, compared with their absorption in solution or gas phase. The investigation of the optical response of chromophores is generally not straightforward, from both experimental and theoretical standpoints; this is due to the difficulty in understanding diverse behaviours and effects, occurring at different scales, with a single technique. In particular, the role played by ab initio calculations in assisting and guiding experiments, as well as in understanding the physics of photoactive proteins, is fundamental. At the same time, owing to the large size of the systems, more approximate strategies which take into account the environmental effects on the absorption spectra are also of paramount importance. Here we review the recent advances in the first-principle description of electronic and optical properties of biological chromophores embedded in a protein environment. We show their applications on paradigmatic systems, such as the light-harvesting complexes, rhodopsin and green fluorescent protein, emphasising the theoretical frameworks which are of common use in solid state physics, and emerging as promising tools for biomolecular systems
Theoretical S1 \u2192S0 Absorption Energies of the Anionic Forms of Oxyluciferin by Variational Monte Carlo and Many-Body Green's Function Theory
The structures of three negatively charged forms (anionic keto-1 and enol-1 and dianionic enol-2) of oxyluciferin (OxyLuc), which are the most probable emitters responsible for the firefly bioluminescence, have been fully relaxed at the variational Monte Carlo (VMC) level. Absorption energies of the S1 \u2190 S0 vertical transition have been computed using different levels of theory, such as TDDFT, CC2, and many-body Green\u2019s function theory (MBGFT). The use of MBGFT, by means of the Bethe\u2013Salpeter (BS) formalism, on VMC structures provides results in excellent agreement with the value (2.26(8) eV) obtained by action spectroscopy experiments for the keto-1 form (2.32 eV). To unravel the role of the quality of the optimized ground-state geometry, BS excitation energies have also been computed on CASSCF geometries, inducing a non-negligible blue shift (0.08 and 0.07 eV for keto-1 and enol-1 forms, respectively) with respect to the VMC ones. Structural effects have been analyzed in terms of over- or undercorrelation along the conjugated bonds of OxyLuc by using different methods for the ground-state optimization. The relative stability of the S1 state for the keto-1 and enol-1 forms depends on the method chosen for the excited-state calculation, thus representing a fundamental caveat for any theoretical study on these systems. Finally, Kohn\u2013Sham HOMO and LUMO orbitals of enol-2 are (nearly) bound only when the dianion is embedded into a solvent (water and toluene in the present work); excited-state calculations are therefore meaningful only in the presence of a dielectric medium which localizes the electronic density. The combination of VMC for the ground-state geometry and BS formalism for the absorption spectra clearly outperforms standard TDDFT and quantum chemistry approaches
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