494 research outputs found
Dehydrogenated polycyclic aromatic hydrocarbons and UV bump
Recent calculations have shown that the UV bump at about 217.5 nm in the
extinction curve can be explained by a complex mixture of PAHs in several
charge states. Other studies proposed that the carriers are a restricted
population made of neutral and singly-ionised dehydrogenated coronene molecules
(C24Hn, n less than 3), in line with models of the hydrogenation state of
interstellar PAHs predicting that medium-sized species are highly
dehydrogenated. To assess the observational consequences of the latter
hypothesis we have undertaken a systematic study of the electronic spectra of
dehydrogenated PAHs. We use our first results to see whether such spectra show
strong general trends upon dehydrogenation. We used state-of-the-art techniques
in the framework of the density functional theory (DFT) to obtain the
electronic ground-state geometries, and of the time- dependent DFT to evaluate
the electronic excited-state properties. We computed the absorption
cross-section of the species C24Hn (n=12,10,8,6,4,2,0) in their neutral and
cationic charge-states. Similar calculations were performed for other PAHs and
their fullydehydrogenated counterparts. pi electron energies are always found
to be strongly affected by dehydrogenation. In all cases we examined,
progressive dehydrogenation translates into a correspondingly progressive blue
shift of the main electronic transitions. In particular, the pi-pi* collective
resonance becomes broader and bluer with dehydrogenation. Its calculated energy
position is therefore predicted to fall in the gap between the UV bump and the
far-UV rise of the extinction curve. Since this effect appears to be
systematic, it poses a tight observational limit on the column density of
strongly dehydrogenated medium-sized PAHs.Comment: 5 pages, 7 figures, Astronomy & Astrophysics, in pres
Large prebiotic molecules in space: photo-physics of acetic acid and its isomers
An increasing number of large molecules have been positively identified in
space. Many of these molecules are of biological interest and thus provide
insight into prebiotic organic chemistry in the protoplanetary nebula. Among
these molecules, acetic acid is of particular importance due to its structural
proximity to glycine, the simplest amino acid. We compute electronic and
vibrational properties of acetic acid and its isomers, methyl formate and
glycolaldehyde, using density functional theory. From computed photo-absorption
cross-sections, we obtain the corresponding photo-absorption rates for solar
radiation at 1 AU and find them in good agreement with previous estimates. We
also discuss glycolaldehyde diffuse emission in Sgr B2(N), as opposite to
emissions from methyl formate and acetic acid that appear to be concentrate in
the compact region Sgr B2(N-LMH).Comment: 8 pages, 5 figure
Estimated IR and phosphorescence emission fluxes for specific Polycyclic Aromatic Hydrocarbons in the Red Rectangle
Following the tentative identification of the blue luminescence in the Red
Rectangle by Vijh et al. (2005), we compute absolute fluxes for the vibrational
IR emission and phosphorescence bands of three small polycyclic aromatic
hydrocarbons. The calculated IR spectra are compared with available ISO
observations. A subset of the emission bands are predicted to be observable
using presently available facilities, and can be used for an immediate,
independent, discriminating test on their alleged presence in this well-known
astronomical object.Comment: accepted for publication on A&
The role of the charge state of PAHs in ultraviolet extinction
Aims: We explore the relation between charge state of polycyclic aromatic
hydrocarbons (PAHs) and extinction curve morphology. Methods: We fit extinction
curves with a dust model including core-mantle spherical particles of mixed
chemical composition (silicate core, and carbonaceous layers),
and an additional molecular component. We use exact methods to calculate the
extinction due to classical particles and accurate computed absorption spectra
of PAHs in different charge states, for the contribution due to the molecular
component, along a sample of five rather different lines of sight. Results: A
combination of classical dust particles and mixtures of real PAHs
satisfactorily matches the observed interstellar extinction curves. Variations
of the spectral properties of PAHs in different charge states produce changes
consistent with the varying relative strengths of the bump and non-linear
far-UV rise.Comment: 5 pages, 3 figures, Astronomy & Astrophysics Letters, in pres
Diagnostics for specific PAHs in the far-IR: searching neutral naphthalene and anthracene in the Red Rectangle
Context. In the framework of the interstellar polycyclic aromatic
hydrocarbons (PAHs) hypothesis, far-IR skeletal bands are expected to be a
fingerprint of single species in this class. Aims. We address the question of
detectability of low energy PAH vibrational bands, with respect to spectral
contrast and intensity ratio with ``classical'' Aromatic Infrared Bands (AIBs).
Methods. We extend our extablished Monte-Carlo model of the photophysics of
specific PAHs in astronomical environments, to include rotational and
anharmonic band structure. The required molecular parameters were calculated in
the framework of the Density Functional Theory. Results. We calculate the
detailed spectral profiles of three low-energy vibrational bands of neutral
naphthalene, and four low-energy vibrational bands of neutral anthracene. They
are used to establish detectability constraints based on intensity ratios with
``classical'' AIBs. A general procedure is suggested to select promising
diagnostics, and tested on available Infrared Space Observatory data for the
Red Rectangle nebula. Conclusions. The search for single, specific PAHs in the
far-IR is a challenging, but promising task, especially in view of the
forthcoming launch of the Herschel Space Observatory.Comment: 13 pages, 13 figures, accepted for publication in A&
On-line database of the spectral properties of polycyclic aromatic hydrocarbons
We present an on-line database of computed molecular properties for a large
sample of polycyclic aromatic hydrocarbons (PAHs) in four charge states: -1, 0,
+1, and +2. At present our database includes 40 molecules ranging in size from
naphthalene and azulene (C10H8) up to circumovalene (C66H20). We performed our
calculations in the framework of the density functional theory (DFT) and the
time-dependent DFT to obtain the most relevant molecular parameters needed for
astrophysical applications. For each molecule in the sample, our database
presents in a uniform way the energetic, rotational, vibrational, and
electronic properties. It is freely accessible on the web at
http://astrochemistry.ca.astro.it/database/ and
http://www.cesr.fr/~joblin/database/.Comment: Accepted for pubblication in Chem. Phys. (01/01/07
Electronic and optical properties of families of polycyclic aromatic hydrocarbons: a systematic (time-dependent) density functional theory study
Homologous classes of Polycyclic Aromatic Hydrocarbons (PAHs) in their
crystalline state are among the most promising materials for organic
opto-electronics. Following previous works on oligoacenes we present a
systematic comparative study of the electronic, optical, and transport
properties of oligoacenes, phenacenes, circumacenes, and oligorylenes. Using
density functional theory (DFT) and time-dependent DFT we computed: (i)
electron affinities and first ionization energies; (ii) quasiparticle
correction to the highest occupied molecular orbital (HOMO)-lowest unoccupied
molecular orbital (LUMO) gap; (iii) molecular reorganization energies; (iv)
electronic absorption spectra of neutral and charged systems. The
excitonic effects are estimated by comparing the optical gap and the
quasiparticle corrected HOMO-LUMO energy gap. For each molecular property
computed, general trends as a function of molecular size and charge state are
discussed. Overall, we find that circumacenes have the best transport
properties, displaying a steeper decrease of the molecular reorganization
energy at increasing sizes, while oligorylenes are much more efficient in
absorbing low-energy photons in comparison to the other classes.Comment: 26 pages, 9 figures, 4 tables, accepted for pubblication in Chemical
Physics (14/04/2011
Time-Dependent Density Functional Theory Investigation on the Electronic and Optical Properties of Poly-C,Si,Ge-acenes
We report a comparative computational investigation on the first six members of linear poly-C,Si,Ge-acenes (X4n+2H2n+4, X = C,Si,Ge; n = 1, 2, 3, 4, 5, 6). We performed density functional theory (DFT) and time-dependent DFT calculations to compare morphological, electronic, and optical properties. While C-acenes are planar, Si-and Ge-acenes assume a buckled configuration. Electronic properties show similar trends as a function of size for all families. In particular, differently from C-based compounds, in the case of both Si-and Ge-acenes, the excitation energies of the strongest low-lying electronic transition (β peaks) span the visible region of the spectrum, demonstrating their size tunability. For all families, we assessed the plasmonic character of this transition and found a linear relationship for the wavelength-dependence of the β peaks as a function of the number of rings. A similar slope of about 56 nm is observed for Si-and Ge-acenes, although the peak positions of the former are located at lower wavelengths. Outcomes of this study are compared with existing theoretical results for 2D lattices and nanoribbons, and experiments where available
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