Structures and stabilities of PAH clusters solvated by water aggregates: The case of the pyrene dimer.

Although clusters made of polycyclic aromatic hydrocarbon and water monomers are relevant objects in both atmospheric and astrophysical science, little is known about their energetic and structural properties. In this work, we perform global explorations of the potential energy landscapes of neutral clusters made of two pyrene units and one to ten water molecules using a density-functional-based tight-binding (DFTB) potential followed by local optimizations at the density-functional theory level. We discuss the binding energies with respect to various dissociation channels. It shows that cohesion energies of the water clusters interacting with a pyrene dimer are larger than those of the pure water clusters, reaching for the largest clusters an asymptotic limit similar to that of pure water clusters and that, although the hexamer and octamer can be considered magic numbers for isolated water clusters, it is not the case anymore when they are interacting with a pyrene dimer. Ionization potentials are also computed by making use of the configuration interaction extension of DFTB, and we show that in cations, the charge is mostly carried by the pyrene molecules.

Radiative relaxation in isolated large carbon clusters: Vibrational emission versus recurrent fluorescence.

Recurrent fluorescence (RF) from isolated carbon clusters containing between 24 and 60 atoms is theoretically investigated as a function of internal energy, cluster size, and structural features. The vibrational relaxation kinetics and the associated IR emission spectra are determined by means of a Monte Carlo approach with vibrational density of states computed in the harmonic approximation. RF is generally found to be highly competitive with vibrational emission. The behaviors predicted for clusters of various sizes and archetypal structures indicate that the IR emission spectra are strongly influenced by RF, an energy gap law being obtained for the evolution of the RF rate constant depending on the electronic excitation state. The present results are relevant to the photophysics of the interstellar medium and could contribute to elucidating the carriers of the extended red emission bands and the continuum emission lying below the aromatic infrared bands believed to originate from mixed aromatic-aliphatic compounds.

Experimental and theoretical study of photo-dissociation spectroscopy of pyrene dimer radical cations stored in a compact electrostatic ion storage ring.

In this paper, we present an experimental and theoretical study of the photo-dissociation of free-flying dimer radical cations of pyrene (C16H10)2+. Experimentally, the dimers were produced in the plasma of an electron cyclotron resonance ion source and stored in an electrostatic ion storage ring, the Mini-Ring for times up to 10 ms and the photo-dissociation spectrum was recorded in the 400 to 2000 nm range. Two broad absorption bands were observed at 550 (2.25 eV) and 1560 nm (0.79 eV), respectively. Theoretical simulations of the absorption spectrum as a function of the temperature were performed using the Density Functional based Tight Binding approach within the Extended Configuration Interaction scheme (DFTB-EXCI) to determine the electronic structure. The simulation involved all excited electronic states correlated asymptotically with the five lowest excited states D1-D5 of the monomer cation and a Monte Carlo exploration of the electronic ground state potential energy surface. The simulations exhibit three major bands at 1.0, 2.1 and 2.8 eV respectively. They allow assigning the experimental band at 1560 nm to absorption by the charge resonance (CR) excited state correlated with the ground state of the monomer D0. The band at 550 nm is tentatively attributed to dimer states correlated with excited states D2-D4, in the monomer cation. Simulations also show that the CR band broadens and shifts towards longer wavelength with increasing temperature. It results from the dependence on the geometry of the energy gap between the ground state and the lowest excited state. The comparison of the experimental spectrum with theoretical spectra at various temperatures allows us to estimate the temperature of the stored (C16H10)2+ in the 300-400 K range, which is also in line with the expected temperatures of the ions deduced from the analysis of the natural decay curve.

Molecular dynamics simulations of anharmonic infrared spectra of [SiPAH](+) pi-complexes.

This paper presents an investigation of anharmonic effects in the IR spectra of [SiPAH](+) complexes by using Born-Oppenheimer molecular dynamics for a variety of PAHs ranging from naphthalene (C(10)H(8)) to ovalene (C(32)H(14)). The potential energy surfaces are calculated with the self-consistent charge density functional-based tight binding approach (DFTB). The DFTB parameters are modified to reproduce potential energy surfaces and the harmonic infrared spectra of the studied complexes with respect to DFT calculations. For bare PAHs, we find that the evolution of the vibrational frequencies of the C-H out-of-plane bending and C-C stretching modes as a function of temperature follows a linear law in quantitative agreement with experimental data. For cationic PAHs, the anharmonicity of the bands in terms of position shifts is found to be enhanced with respect to that of neutrals. As compared with bare cationic PAHs, the coordination of Si induces (i) larger broadenings, (ii) a slightly larger shift of the C-C stretching mode, and (iii) a smaller shift of the C-H out-of-plane bending modes. We discuss the implications of the work and the spectroscopic constraints for the detection of [SiPAH](+) in the interstellar medium.

Dissociation of polycyclic aromatic hydrocarbons: molecular dynamics studies.

We present dynamical studies of the dissociation of polycyclic aromatic hydrocarbon (PAH) radical cations in their ground electronic states with significant internal energy. Molecular dynamics simulations are performed, the electronic structure being described on-the-fly at the self-consistent-charge density functional-based tight binding (SCC-DFTB) level of theory. The SCC-DFTB approach is first benchmarked against DFT results. Extensive simulations are achieved for naphthalene [Formula: see text], pyrene [Formula: see text] and coronene [Formula: see text] at several energies. Such studies enable one to derive significant trends on branching ratios, kinetics, structures and hints on the formation mechanism of the ejected neutral fragments. In particular, dependence of branching ratios on PAH size and energy were retrieved. The losses of H and C2H2 (recognized as the ethyne molecule) were identified as major dissociation channels. The H/C2H2 ratio was found to increase with PAH size and to decrease with energy. For [Formula: see text], which is the most interesting PAH from the astrophysical point of view, the loss of H was found as the quasi-only channel for an internal energy of 30 eV. Overall, in line with experimental trends, decreasing the internal energy or increasing the PAH size will favour the hydrogen loss channels with respect to carbonaceous fragments.This article is part of the themed issue 'Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces'.

Size Effect in the Ionization Energy of PAH Clusters.

We report the first experimental measurement of the near-threshold photoionization spectra of polycyclic aromatic hydrocarbon clusters made of pyrene C16H10 and coronene C24H12, obtained using imaging photoelectron-photoion coincidence spectrometry with a VUV synchrotron beamline. The experimental results of the ionization energy are compared to calculated ones obtained from simulations using dedicated electronic structure treatment for large ionized molecular clusters. Experiment and theory consistently find a decrease of the ionization energy with cluster size. The inclusion of temperature effects in the simulations leads to a lowering of this energy and to quantitative agreement with the experiment. In the case of pyrene, both theory and experiment show a discontinuity in the IE trend for the hexamer. This work demonstrates the ability of the models to describe the electronic structure of PAH clusters and suggests that these species are ionized in astronomical environments where they are thought to be present.

Stacked clusters of polycyclic aromatic hydrocarbon molecules.

Clusters of polycyclic aromatic hydrocarbon (PAH) molecules are modeled using explicit all-atom potentials using a rigid-body approximation. The considered range of PAHs goes from pyrene (C10H8) to circumcoronene (C54H18) and clusters containing between 2 and 32 molecules are investigated. In addition to the usual repulsion-dispersion interactions, electrostatic point-charge interactions are incorporated, as obtained from density functional theory calculations. The general electrostatic distribution in neutral or singly charged PAHs is reproduced well using a fluctuating-charges analysis, which provides an adequate description of the multipolar distribution. Global optimization is performed using a variety of methods, including basin-hopping and parallel tempering Monte Carlo. We find evidence that stacking the PAH molecules generally yields the most stable motif. A structural transition between one-dimensional stacks and three-dimensional shapes built from multiple stacks is observed at larger sizes, and the threshold for this transition increases with the size of the monomer. Larger aggregates seem to evolve toward the packing observed for benzene in bulk. Difficulties met in optimizing these clusters are analyzed in terms of the strong anisotropy of the molecules. We also discuss segregation in heterogeneous clusters and vibrational properties in the context of astrophysical observations.

Nonlinear analyses of cell proliferation in the central nervous system reveal stochastic and deterministic components.

This paper analyzes the dynamics of cell proliferation in the developing central nervous system. Three different algorithms, Fano factor, Allan factor and detrended fluctuations analysis, are used to estimate de scaling exponent of space numerical series obtained by recording the number and position of proliferating cells along the cephalic-caudal axis of the system. It can be concluded that the dynamics of proliferation involves two component: (a) a random noncorrelated stochastic component representing a basal proliferating activity uniformly distributed along the cephalic-caudal axis and (b) a deterministic nonstationary component that imposes a defined global trend to the process. The deterministic nonstationary trend can be interpreted as the effect of a controlling influence operating along the cephalic-caudal axis. This result indicates that the proliferative activity is spatially organized along the cephalic-caudal axis of the system.

Caracterización de la complejidad de la profileración celular durante la corticogénesis en el SNC / Characterization of the complexity of cell proliferation during hain corticogenesis

This report analyzes the dynamics temporo-spacial of the cellular proliferation during brain corticogenesis. It is an attempt of the authors to find operative rules of the proliferation that can explain the morpho and histogenetic effects of this activity.

Dinámica de la córticoangiogénesis en el sistema nervioso central / Dinamics of the cortico-angiogenesis in the central nervous system

This study analyzes the temporal-spatial correlation between angio-and corticogenesis in the developing chick optic tectum (OT) by means of NADPH-diaphorase and immunolabeling methods. Qualitative and quantitative parameters were used to analyze a new vessels formation and growth of preexisting ones as a function of time and space...