Issue |
A&A
Volume 426, Number 1, October IV 2004
|
|
---|---|---|
Page(s) | 105 - 117 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20040541 | |
Published online | 05 October 2004 |
Electronic absorption spectra of PAHs up to vacuum UV
Towards a detailed model of interstellar PAH photophysics
1
INAF - Osservatorio Astronomico di Cagliari - Astrochemistry Group, Strada n.54, Loc. Poggio dei Pini, 09012 Capoterra (CA), Italy e-mail: [gmalloci;gmulas]@ca.astro.it
2
Astrochemical Research in Space Network,
3
Centre d'Étude Spatiale des Rayonnements, CNRS et Université Paul Sabatier, 9 avenue du colonel Roche, 31028 Toulouse, France e-mail: christine.joblin@cesr.fr
Received:
29
March
2004
Accepted:
17
June
2004
We 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.
Key words: astrochemistry / molecular data / molecular processes / ISM: molecules / ultraviolet: ISM / methods: numerical
© ESO, 2004
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