170,153 research outputs found
Search for far-IR PAH bands with Herschel: Modelling and observational approaches
No full textHerschel opens the possibility to detect the low-frequency vibrational bands of individual polycyclic aromatic hydrocarbon (PAH) molecules and therefore to progress in our understanding of the nature of these species and the properties of the environments from which they emit. However, unless one individual molecule dominates the PAH family, this detection will not be straightforward and it is necessary to optimise the observational search with an educated guess of the band profiles and intensities. Such educated guess can be obtained from models that include a detailed description of the molecular properties (anharmonicity, rotation...) in the modelling of the cooling cascade of the emitting species. First results are expected soon from the observation of the Orion Bar as part of the HEXOS Herschel key program
Theoretical evaluation of PAH dication properties
No full textAims.We present a systematic, theoretical study of 40 polycyclic aromatic
hydrocarbon dications (PAHs++) containing up to 66 carbon atoms.
Methods.We performed our calculations using well-established quantum-chemical
techniques in the framework of (i) the density functional theory (DFT) to
obtain the electronic ground-state properties and of (ii) the
time-dependent DFT (TD-DFT) to evaluate the excited-state properties.
Results.For each PAH++ considered, we computed the absolute visible-UV
photo-absorption cross-section up to about 30 eV. We also evaluated their
vibrational properties and compared them to those of the corresponding
neutral and singly-ionised species. We estimated the adiabatic and
vertical second ionisation energy through total energy differences.
Conclusions.The values obtained fall in the energy range 8-13 eV, confirming that
PAHs could reach the doubly-ionised state in HI regions.
The total integrated IR absorption cross-sections show a marked increase upon
ionisation, being on the average about two and five times larger for PAHs++
than for PAHs+ and PAHs, respectively.
The visible-UV photo-absorption cross-sections for the 0, +1 and +2
charge-states show comparable features but PAHs++ are found to absorb
slightly less than their parent neutral and singly ionised species between
~7 and ~12 eV. Combining these pieces of information, we found that
PAHs++ should actually be stabler against photodissociation than PAHs and
PAHs+, if dissociation thresholds are nearly unchanged by
ionisation
Electronic absorption spectra of PAHs up to vacuum UV Towards a detailed model of interstellar PAH photophysics
No full textWe computed the absolute photo-absorption cross-sections up to the vacuum ultaviolet (VUV) of a total of 20 Polycyclic Aromatic Hydrocarbons
(PAHs) and their respective cations, ranging in size from naphthalene
(C10H8) to dicoronylene (C48H20). We used an implementation
in real time and real space of the Time-Dependent Density Functional Theory
(TD-DFT), an approach which was proven to yield accurate results for
conjugated molecules such as benzene. Concerning the low-lying excited states
of character occurring in the near-IR, visible and
near-UV spectral range, the computed spectra are in good agreement
with the available experimental data, predicting vertical excitation energies
precise to within a few tenths of eV, and the corresponding oscillator
strengths to within experimental errors, which are indeed the typical
accuracies currently achievable by TD-DFT.
We find that PAH cations, like their parent molecules,
display strong electronic transitions in the UV, a piece of
information which is particularly useful since a limited amount of laboratory
data is available concerning the absorption properties of PAH ions in this
wavelength range. Moreover, a detailed discussion of the UV–VUV properties
of both neutral and cation species is presented. Concerning neutrals, the
agreement with existing laboratory data is very good, the specific
TD-DFT implementation used in this work apparently being
able to reproduce the overall far-UV behaviour, including the broad
absorption peak dominated by transitions, which matches well both in
position and width. The implications of these results are discussed in
conjunction with the contribution PAH-like molecules are expected to give
to the interstellar extinction curve
Polycyclic aromatic hydrocarbons and the extinction curve
No full textAromatic carbon, in some form, has been an essential ingredient by and large in all models of the extinction curve, since the original proposal to attribute the bump at 217.5 nm to "astronomical graphite". This aromatic carbon is most naturally identified, in up to date models, with a population of Polycyclic Aromatic Hydrocarbons (PAHs), free and/or clustered. In all models, this PAH population accounts for the far-UV nonlinear rise in the extinction curve, contributes to the bump and possibly part of the large set of unidentified, discrete absorption features in the visible (the Diffuse Interstellar Bands). We review the current state of our understanding of the contribution of PAHs to interstellar extinction, and what constraints can be imposed on the PAH population by fitting extinction models to observations
A general model for the identification of specific PAHs in the far-IR
No full textContext.In the framework of the interstellar PAH hypothesis, far-IR
skeletal bands are expected to be a fingerprint of single species in
this class.
Aims.A detailed model of the photo-physics of interstellar PAHs is required
for such single-molecule identification of their far-IR features in the
presently available Infrared Space Observatory data and in those of the
forthcoming Herschel Space Observatory mission.
Methods.We modelled the detailed photophysics of a vast sample of species in
different radiation fields, using a compendium of Monte-Carlo techniques
and quantum-chemical calculations. This enabled us to validate the use
of purely theoretical data and assess the expected accuracy and
reliability of the resulting synthetic far-IR emission spectra.
Results.We produce positions and intensities of the expected far-IR features
that ought to be emitted by each species in the sample in the considered
radiation fields. A composite emission spectrum for our sample is computed
for one of the most favourable sources for detection, namely the Red
Rectangle nebula. The resulting spectrum is compared with the estimated
dust emission in the same source to assess the dependence of detectability
on key molecular parameters.
Conclusions.Identifying specific PAHs from their far-IR features is going to be a
difficult feat in general; still, it may well be possible under
favourable conditions
Diagnostics for specific PAHs in the far-IR: Searching neutral naphthalene and anthracene in the Red Rectangle
No full textContext.In the framework of the interstellar polycyclic aromatic hydrocarbons
(PAHs) hypothesis, far-IR
skeletal bands are expected to be the fingerprints 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 established 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 Rectangl
An atlas of synthetic far-IR emission spectra of specific PAHs for comparison with herschel data
No full textIn the framework of the interstellar PAHs hypothesis, far-IR skeletal bands are expected to be a fingerprint of single species in this class. We developed a detailed model of the photophysics of interstellar PAHs which provides a powerful tool for single-molecule identification. We here present synthetic far-IR emission spectra predicted by the above general approach, based on quantum-chemical calculations, for 20 PAHs and their respective cations, ranging in size from naphthalene (C10H8) to dicoronylene (C 48H20), excited by a range of different radiation fields. Such spectra aim at a direct, quantitative comparison with present (ISO) and forthcoming (Herschel) observations. The accuracy of these spectra is essentially the same as that of the quantum-chemical vibrational analysis, and can be refined using laboratory data, when available
Dehydrogenated polycyclic aromatic hydrocarbons and UV bump
No full textContext. 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 polycyclic
aromatic hydrocarbons (PAHs) in several ionisation states.
Other studies proposed that the carriers are a restricted population made of
neutral and singly-ionised dehydrogenated coronene molecules
(C24Hn, ), in line with models of the hydrogenation state
of interstellar PAHs predicting that medium-sized species
are highly dehydrogenated.
Aims. To assess the observational consequences of the latter hypothesis
we have undertaken a systematic theoretical study of the electronic
spectra of dehydrogenated PAHs. We use our first results to see
whether such spectra show strong general trends upon dehydrogenation.
Methods. We performed calculations using
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.
Results. 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 fully
dehydrogenated counterparts.
Conclusions. π-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 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
Synthetic rotational profiles of emission and absorption bands of interstellar polycyclic aromatic hydrocarbons
No full textThe spectrometers on board Herschel are likely to be able to detect far-IR emission bands from interstellar PAHs, possibly resolving their rotational envelopes. For some specific PAHs we calculated synthetic rotational profiles of their low-energy vibrational modes and of their low-lying permitted electronic transitions, for comparison respectively with the forthcoming Herschel observations and high resolution spectra of Diffuse Interstellar Bands. Such profiles were obtained through a Monte-Carlo model of the photo-physics of isolated interstellar PAHs, whose molecular properties were calculated using state-of-the-art quantum-chemical techniques. Upon successful identification of specific PAHs, comparison with observations will provide both useful information on the physical environment in which the molecule is embedded and be an indirect measurement of molecular parameters which are very difficult to obtain either theoretically or in terrestrial laboratories
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