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Astron. Astrophys. 319, 331-339 (1997)

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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.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998