Since the turn of the century, up to 200 diffuse interstellar bands (DIBs) have now been observed (Jenniskens & Désert 1994) in absorption against many lines of sight and the question of their identification is still opened (Herbig 1995). There is now strong evidence that the carriers are large, carbon-bearing species absorbing in the visible/near-IR range (Scarrott et al. 1992, Herbig 1995, Sarre et al. 1995). In addition, in the conditions prevailing in the interstellar medium (ISM), DIB carriers are cold and isolated (i.e., gas phase species). A firm DIB assignment can therefore only be achieved from comparison of astronomical and laboratory data when the latter have been obtained in (or can safely be extrapolated to) conditions close to interstellar.
Some gas-phase experiments have been done on species produced by vaporization of a solid sample (Flinckinger et al. 1991, Kurtz 1992) but the molecules so produced are hot and suffer mutual collisions. To date, the technique simulating best the interstellar environment locks up the sample molecule in a rare-gas matrix whereby it is cold (a few K) and isolated from its companions (Salama and Allamandola 1992, Léger et al. 1995). However, the sample molecule always interacts, although very weakly (as in neon matrices), with the rare-gas substrate and absorption lines are broadened and displaced towards the red (by a few tens of Å). Unfortunately, the effect of the matrix depends specifically on the substrate/sample molecule interaction and no quantitative extrapolation can be drawn from the results on the few species for which both matrix and gas-phase spectra exist (Léger et al. 1995). This effect is even larger in the case of ions. Therefore, only rough guesses can be performed which do not lead to positive DIB identifications. In addition, in contrast with the ISM, the matrix blocks any rotation motion of the molecule: this quenching may substantially change the absorption profiles peculiarly for large molecules. Hence, the match (or mismatch) of a matrix spectrum with astronomical data may be indicative but inconclusive.
We have designed an experiment aimed at collecting absorption spectra of cold molecules in the gas phase using the laser induced fluorescence (LIF) signal. This technique requires the molecule under study to be fluorescent. Our approach has been stimulated by recent observations of red emission counterparts to 7 DIBs (Rao & Lambert 1993, Scarrott et al. 1992) demonstrating that some DIB carriers are indeed fluorescent species.
We set out to study a polycyclic aromatic hydrocarbon (PAH) named perylene in its cationic form (C20 H ). This choice has been motivated by a recent laboratory study in rare-gas matrix bearing out evidence of the fluorescence of the perylene cation (Joblin et al. 1995).
Although the perylene cation has not been claimed to be at the origin of any DIB, PAH ions and radicals are amongst good candidates for DIB carriers (Léger 1994, Herbig 1995). Since the original proposal that PAH cations may produce some DIBs (van der Zwet & Allamandola 1985, Léger & d'Hendecourt 1985, Crawford et al. 1985), a large number of absorption studies of small (containing up to 32 carbon atoms) PAHs cations in rare-gas matrices has been accumulated (Salama & Allamandola 1992, 1993; Ehrenfreund et al. 1992). Undeniably, PAH cations absorption profiles could match the strongest DIBs (Léger, 1994). The presumption in favor of PAH cations has been reinforced by recent transition energy quantum chemistry calculations (Parisel et al. 1992). Encouraging DIB assignments with the pyrene cation and its derivatives (Salama & Allamandola 1993; Léger et al. 1995) have been recently proposed on the basis of matrix spectra. In a similar context, two near-IR DIBs have been attributed to C (Foing & Ehrenfreund 1994).
To confirm these attributions, gas-phase spectra of cold molecules are needed. It must be stressed, however, that rare-gas matrices experiments, routinely conducted over a wide spectral range, are essential to usefully guide LIF studies.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998