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ABSTRACT: We introduce a model combining a density functional based tight-binding method with a configuration interaction like scheme
to treat charge delocalization in ionized molecular clusters. As an application, we determine the size-evolution of some properties
of stack coronene clusters, namely charge delocalization, binding energies and ionization potentials.
The European Physical Journal D 04/2012; 52(1):55-58. · 1.48 Impact Factor
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ABSTRACT: 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.
The Journal of Physical Chemistry A 05/2010; 114(18):5846-54. · 2.95 Impact Factor
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ABSTRACT: Extended Red Emission (ERE) was recently attributed to the photo-luminescence of either doubly ionized Polycyclic Aromatic Hydrocarbons (PAH$^{++}$), or charged PAH dimers. We analysed the visible and mid-infrared (mid-IR) dust emission in the North-West and South photo-dissociation regions of the reflection nebula NGC 7023.Using a blind signal separation method, we extracted the map of ERE from images obtained with the Hubble Space Telescope, and at the Canada France Hawaii Telescope. We compared the extracted ERE image to the distribution maps of the mid-IR emission of Very Small Grains (VSGs), neutral and ionized PAHs (PAH$^0$ and PAH$^+$) obtained with the Spitzer Space Telescope and the Infrared Space Observatory. ERE is dominant in transition regions where VSGs are being photo-evaporated to form free PAH molecules, and is not observed in regions dominated by PAH$^+$. Its carrier makes a minor contribution to the mid-IR emission spectrum. These results suggest that the ERE carrier is a transition species formed during the destruction of VSGs. Singly ionized PAH dimers appear as good candidates but PAH$^{++}$ molecules seem to be excluded.
02/2008;
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ABSTRACT: This work was conducted as part of the SPECPDR program, dedicated to the study of very small particles and astrochemistry, in Photo-Dissociation Regions (PDRs). We present the analysis of the mid-IR spectro-imagery observations of Ced 201, NCG 7023 East and North-West and rho-Ophiuchi West filament. Using the data from all four modules of the InfraRed Spectrograph onboard the Spitzer Space Telescope, we produced a spectral cube ranging from 5 to 35 um for each one of the observed PDRs. The resulting cubes were analysed using Blind Signal Separation methods (NMF and FastICA). For Ced 201, rho-Ophiuchi West filament and NGC 7023 East, we find that two signals can be extracted from the original data cubes, which are 5 to 35 um spectra. The main features of the first spectrum are a strong continuum emission at long wavelengths, and a broad 7.8 um band. On the contrary, the second spectrum exhibits the classical Aromatic Infrared Bands (AIBs) and no continuum. The reconstructed spatial distribution maps show that the latter spectrum is mainly present at the cloud surface, close to the star whereas the first one is located slightly deeper inside the PDR. The study of the spectral energy distribution of Ced 201 up to 100 um suggests that, in cool PDRs, the 5-25 um continuum is carried by Very Small Grains (VSGs). The AIB spectra in the observed objects can be interpreted as the contribution of neutral and positively-charged Polycyclic Aromatic Hydrocarbons (PAHs). We extracted the 5 to 25 um emission spectrum of VSGs in cool PDRs, these grains being most likely carbonaceous. We show that the variations of the mid-IR (5-35 um) spectra of PDRs can be explained by the photo-chemical processing of these VSGs and PAHs, VSGs being the progenitors of free PAH. Comment: 14 pages, 11 figures, to appear in A&A
03/2007;
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ABSTRACT: The competition between the formation and destruction of coronene clusters under interstellar conditions is investigated theoretically. The unimolecular nucleation of neutral clusters is simulated with an atomic model combining an explicit classical force field and a quantum tight-binding approach. Evaporation rates are calculated in the framework of the phase space theory and are inserted in an infrared emission model and compared with the growth rate constants. It is found that, in interstellar conditions, most collisions lead to cluster growth. The time evolution of small clusters (containing up to 312 carbon atoms) was specifically investigated under the physical conditions of the northern photodissociation region of NGC 7023. These clusters are found to be thermally photoevaporated much faster than they are reformed, thus providing an interpretation for the lowest limit of the interstellar cluster size distribution inferred from observations. The effects of ionizing the clusters and density heterogeneities are also considered. Based on our results, the possibility that PAH clusters could be formed in PDRs is critically discussed. Comment: 14 pages, 14 figures. Astronomy & Astrophysics, accepted for publication
09/2006;
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ABSTRACT: 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.
The Journal of Physical Chemistry A 04/2005; 109(11):2487-97. · 2.95 Impact Factor
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M. Rapacioli
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ABSTRACT: 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.
02/2005;
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ABSTRACT: Polycyclic Aromatic Hydrocarbons (PAHs) have been proposed as the
carriers of the aromatic infrared bands observed in many places of the
interstellar medium. However the main difficulty faced by the PAH model
is the identification of individual species. We have analyzed
spectro-imagery data obtained with the camera CAM on board of the
Infrared Space Observatory, applying the singular value decomposition
method. This work (Rapacioli et al. (2005)) strongly suggests that PAHs
are produced by photoevaporation of very small carbonaceous grains which
could be PAH clusters. This result motivated the following theoretical
work. We have performed a parallel tempering Monte Carlo search of the
structures with lowest energy of PAH clusters ranging from two to thirty
molecules (Rapacioli et al. (2005b)). We found evidence that stacking
the PAH molecules generally yields the most stable structures. After
some critical size, one-dimensional pattern is replaced by two and
three-dimensional growth, again with small stacks as the building block.
The collision of a PAH molecule with a PAH cluster has been studied.
The resulting excited cluster can loose one or more molecules or relax
through infrared emission. The competition between the IR emission rate
and the evaporation rate calculated with a statistical method allows to
characterize the astrophysical environments that are favourable to the
PAH clusters growth.
Proceedings of the International Astronomical Union 235:200P.
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C. Joblin,
A. Abergel,
J.-P. Bernard,
O. Berne,
F. Boulanger,
J. Cernicharo,
M. Compiègne,
Y. Deville,
M. Gerin,
J. Goicoechea,
E. Habart,
J. Le Bourlot,
J.-P. Maillard, M. Rapacioli,
W. Reach,
A. Simon,
J.-D. Smith,
D. Teyssier,
L. Verstraete
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ABSTRACT: Since the IRAS mission, it is well known that there exists a population
of very small particles which are transiently heated by the absorption
of single UV photons and emit in the mid-IR range. Polycyclic aromatic
hydrocarbons (PAHs) were proposed to account for the well-known Aromatic
IR Bands (AIBs) between 3.3 and 12.7 μm. Another component, called
very small grains (VSGs; Désert et al. (1990)), was introduced as
the carrier of the emission in the 25 μm IRAS band. The very small
particles (PAHs and VSGs) are now considered to play a major role in the
physics and chemistry of the interstellar medium. They dominate the
heating of interstellar gas through photoelectric effect. They are also
suspected to play a major role in the formation of H2, in
particular in Photo-Dissociation Regions (PDRs) where UV photons heat
the dust making inefficient the physisorption and diffusion of H atoms
at the surface of grains, the processes which are usually invoked to
form H2 (Habart et al. (2004)). In PDRs, the UV field varies
as a function of depth within the cloud, and this gives a unique
opportunity to study how the dust populations and the molecular content
evolve with the excitation and physical conditions. Spectro-imagery in
the mid-IR (5-16 μm) was possible with the camera CAM on board the
Infrared Space Observatory (ISO). A striking result from these
observations is the presence of continuum emission in PDRs far from
exciting sources (Abergel et al. (2002)). This reveals the presence of
VSGs transiently heated by single UV photons. The continuum relative to
the bands is highly variable, showing that VSGs are a dust component
different from PAHs. Further analysis of the data (Rapacioli et al.
(2005)) allowed to extract three spectra : two carrying only band
emission (attributed to PAH cations and neutrals) and one with continuum
emission (attributed to VSGs). The authors found a drop in the VSG
emission at the cloud edge which is correlated with the increase in the
PAH emission. They proposed that VSGs could be aggregates of PAHs which
photoevaporate at the surface of clouds. This work can now continue
thanks to the unique spectro-imagery capabilities of the Spitzer Space
Telescope. With the spectrometer IRS, spectro-imagery studies are
possible in the 5-35 μm range, giving the possibility to better
characterize the VSG emission and therefore the chemical identity of
this dust population. Second, the spectrometer IRS allows to access to
the full set of rotational lines of H2: 0-0 S(0), S(1), S(2)
and S(3) at 28.2, 17.0, 12.3 and 9.7 μm. The interpretation of the
line intensities with a PDR model allows to derive precise information
on the formation mechanism of H2, in particular the formation
rate and the ortho-to-para ratio (Le Bourlot (2000)). Finally, the
spectro-imagery data obtained with IRS will be soon combined to MIPS-SED
observations between 55 and 95 μm in order to follow the spatial
evolution of the abundance and size distribution of the different dust
populations including large grains. The Spitzer data will be presented.
Our sources were selected in order to span a broad range of physical
conditions (radiation field, geometry and density). Interpretation is in
progress using PDR, radiative transfer and dust models developped by our
team. The obtained data is clearly a benchmark for models describing the
nature and evolution of all dust populations as well as for the
excitation and formation of H2. Complementary work includes
ground-based observations, blind source separation analysis and
laboratory/theoretical work.
Proceedings of the International Astronomical Union 235:194P.
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ABSTRACT: We have coupled a singular value decomposition method with a Monte Carlo search algorithm to analyse the mid-infrared ISOCAM spectral maps of photodissociation regions (PDRs) in NGC 7023 and $\rho$ Oph-SR3.Three different spectra and their associated spatial distribution were extracted from this analysis. It is shown that they can be associated with polycyclic aromatic hydrocarbons (PAHs) in their cationic and neutral forms and a third population of carbonaceous very small grains (VSGs). The method allows for the first time (i) to separate the contribution of neutral PAHs to the interstellar emission spectrum from that of cationic PAHs; (ii) to show that the 7.8 $\mu$m component of the “7.7 $\mu$m” broad feature is carried by VSGs, whereas the 7.6 $\mu$m component is due to PAHs; and (iii) to give evidence that free-flying PAHs are produced in PDRs by photoevaporation of VSGs. It is proposed that these carbonaceous VSGs are indeed PAH clusters. We derived a minimal size of 400 carbon atoms per cluster and estimated a UV absorption power of the order of 10$^{-24}$ W C$^{-1}$ for their dissociation.
http://dx.doi.org/10.1051/0004-6361:20041247.
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ABSTRACT: Context.Extended Red Emission (ERE) was recently attributed to the photo-luminescence of either doubly ionized Polycyclic Aromatic Hydrocarbons (PAH$^{++}$), or charged PAH dimers ($[$PAH$_{2}$] $^+$).Aims.We analysed the visible and mid-infrared (mid-IR) dust emission in the North-West and South photo-dissociation regions of the reflection nebula NGC 7023.Methods.Using a blind signal separation method, we extracted the map of ERE from images obtained with the Hubble Space Telescope, and at the Canada France Hawaii Telescope. We compared the extracted ERE image to the distribution maps of the mid-IR emission of Very Small Grains (VSGs), neutral and ionized PAHs (PAH$^0$ and PAH$^+$) obtained with the Spitzer Space Telescope and the Infrared Space Observatory.Results.ERE is dominant in transition regions where VSGs are being photo-evaporated to form free PAH molecules, and is not observed in regions dominated by PAH$^+$. Its carrier makes a minor contribution to the mid-IR emission spectrum.Conclusions.These results suggest that the ERE carrier is a transition species formed during the destruction of VSGs. $[$PAH$_{2}$] $^+$ appear as good candidates but PAH$^{++}$ molecules seem to be excluded.
http://dx.doi.org/10.1051/0004-6361:20079158.
