1,721,227 research outputs found
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
Electronic excitations of oligoacenes in four different charge states:-1,0,+1 and +2
No full textElectronic excitation of oligoacenes in four different charge states:-1,0,+1 and +
High-resolution infrared absorption spectroscopy of thermally excited naphthalene. Measurements and calculations of anharmonic parameters and vibrational interactions
No full textAn 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
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
Evolution of PAHs in Protoplanetary Disks
No full textDepending on whom you ask, PAHs are either the smallest dust particles or the largest gas-phase molecules in space. Whether referred to as gas or dust, these PAHs can contain up to 20% of the total cosmic carbon abundance and as such also play an important role in the carbon chemistry of protoplanetary disks. The interpretation of PAR bands is often a complex procedure involving not only gas physics to determine their ionization stage and temperature, but also radiative transfer effects that can bury these bands in a strong thermal continuum from a population of larger dust particles. PAHs are most readily seen in the spectral energy distributions (SEDs) of disks around Herbig AeBe stars where they are photoprocessed by the stellar radiation field. Resolved images taken in the PAH bands confirm their origin in the flaring surfaces of circumstellar disks: if the SED is consistent with a flat disk structure (less illuminated), there is little or no evidence of PAH emission. The very low detection rates in the disks around T Tauri stars often require an overall lower abundance of PAHs in these disk surface as compared to that in molecular clouds. In this review, I will adress three aspects of PAHs in protoplanetary disks: (1) Do PAHs form in protoplanetary disks or do they originate from the precursor molecular cloud? (2) Is the presence of PAH features in SEDs a consequence of the disk structure or do PAHs in fact shape the disk structure? (3) How can we use PAHs as tracers of processes in protoplanetary disks
Electronic excitations in homologous classes of PAH's for applications in photonics and electronics
No full textElectronic excitations in homologous classes of PAH's for applications in photonics and electronic
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
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