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Astron. Astrophys. 319, 331-339 (1997)
6. Conclusions
We have reported first results on LIF spectroscopy of jet-cooled
species with the aim of identifying DIB carriers.u hey show that our
experimental set-up produces cold, gas-phase and isolated ions or
radicals and thus represents the best simulation to date of
interstellar conditions. This technique relies on the detection of
fluorescence photons. It is encouraged by the fact that recent work
points out that fluorescent species could be the carriers of, at
least, some DIBs. LIF spectroscopy appears thus a powerful tool to
assert specific DIB assignments.
The fluorescence of a PAH cation, perylene, has recently been
observed in condensed phase (rare-gas matrix) motivating us to study
perylene. We failed to reproduce the matrix results. The reason may be
related either to the species formed, or to the nature and/or
efficiency of its luminescence. In spite of this, we recorded the
absorption spectrum of a large perylene fragment, detected from the
fluorescence of the photo-ejected C2 -molecule in its Swan
bands.
Although this large radical has a near-infrared/visible absorption
spectrum showing no coincidence with any DIB and is far too unstable
to stand in the interstellar radiation field, this study leads to
interesting avenues for the future of the DIB carriers searching game.
It shows that, not only cations (which have long been suggested and
more thoroughly studied in condensed phase), but also large PAH
neutral radicals can be good candidates for DIB carrier
identification.
Our results also illustrate the behaviour of PAHs in the interstellar medium: we observed a process in which perylene sequentially fragments while absorbing photons in a scheme similar to interstellar conditions to finally form the C2 -molecule. Other processes involving perylene or another PAH may however lead to a stable fragment possibly being a DIB carrier. Moreover, the spectral pattern we found in the present case may be common to many other PAH radicals, especially the electronic fine structure of the vibronic bands. This sheds new light on the DIB problem, in that we can interpret some broad DIBs or sequences of narrow DIBs as rotationally broadened electronic bands. Rotational temperatures of a few tens of K are sufficient to reproduce some common DIB series. With higher temperatures (a few 100 K) the fine structure is smeared out and a quasi-gaussian profile results, close to that of some broad DIBs.
Extension of this kind of studies to other DIB carrier candidates
is presently in progress. The use of a mass spectrometer will allow to
look specifically at cationaic species.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998
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