Diffuse interstellar bands

A recent success in the detection of H species has been that of the molecular ion H3+. All of the models of ion-molecule chemistry in hydrogen-dominated regions are controlled by reactions of H3+ but until recently the H2+ molecular ion had not been detected. However, the modes of vibration of H3"1" provide for an allowed IR transition at 3.668 pin used for its detection. These ro-vibrational transitions have now been observed in a number of places, including the interstellar medium and in the aurorae of Jupiter. Not all astronomical detection and identification problems have been solved, however, and the most annoying and compelling of these is the problem of diffuse interstellar bands. 

One of the oldest problems in molecular astronomy concerns identification of the molecules responsible for diffuse interstellar bands (DIBs). Since their first observation in 1922 some 127 bands have been detected all over the electromagnetic spectrum, shown schematically in Figure 3.19, but the origins of the transitions, the so-called carriers of DIBs, have not been determined. 

Figure 3.19 Diffuse interstellar bands. (A colour reproduction of this figure can be seen in the colour section). (Reproduced by permission of Peter Jeniskins, SETI Institute)...

Diffuse interstellar bands (DIBs) Unassigned spectral features that are seen along many lines of sight, especially towards reddened stars. 

Fullerenes and othe carbon aggregates and the diffuse interstellar bands, p. 133. 1998. in Ref. [11]. 

Abstract We review the potential contribution of single fullerenes and buckyonions to interstellar extinction. Photoabsorption spectra of these molecules are compared with some of the most relevant features of interstellar extinction, the UV bump, the far UV rise and the diffuse interstellar bands. According to semiempirical models, photoabsorption by fullerenes (single and multishell) could explain the shape, width and peak energy of the most prominent feature of the interstellar absorption, the UV bump at 2,175 A. Other weaker transitions are predicted in the optical and near-infrared providing a potential explanation for diffuse interstellar bands. In particular, several fullerenes could contribute to the well known strong DIB at 4,430 A comparing cross sections and available data for this DIB and the UV bump we estimate a density of fullerenes in the diffuse interstellar medium of 0.1-0.2 ppm. These molecules could then be a major reservoir for interstellar carbon. 

In this chapter, we review the possible contribution of single fullerenes, fulleranes and buckyonions to the interstellar extinction and the anomalous microwave emission. We compare their photoabsorption spectra with some of the observational properties of interstellar extinction, in particular with the characteristics of the UV bump and with the distribution of diffuse interstellar bands (DIBs) in the optical and near infrared and discuss the potential of fulleranes as carriers of anomalous microwave emission. 

Soon after the discovery of the fullerenes it was suggested that these or particles of similar nature, could be related to one of the most intriguing problems of astrophysics the diffuse interstellar bands discovered more than eight decades ago, but not yet explained, and with the ultraviolet band centered in 2,175 A, which is the most intense band in the interstellar medium discovered more than 30 years ago. The origin of the UV bump is attributed to carbon particles of small size whose characteristics are not yet conclusively established. 

Hydrides of the fullerene C60 have also been investigated as potential DIB carriers (Webster 1992, 1993b). No specific identification has been suggested but it should be noted that the conjugated systems of -electrons are predicted to have transitions in the visible range. The predicted optical and near-infrared transitions of fullerene based molecules may offer a potential explanation for the long-standing problem of the diffuse interstellar bands and other interstellar and circumstellar features. 

Fullerenes, fulleranes and buckyonions are expected to present weaker transitions in the optical and near infrared with their number decreasing towards longer wavelengths. These transitions may be responsible for some of the known but unexplained diffuse interstellar bands. It would be very important to obtain high sensitivity, high resolution laboratory spectra of these molecules in the optical and near infrared for a more precise comparison with the very detailed observations of DIBs. 

The best evidence for a relation between carbon-particles and the diffuse interstellar bands comes from analysis of the Red Rectangle. The Red Rectangle is an usual mass-losing carbon star which is probably in transition into becoming a planetary nebula. Schmidt et al. (1980) using 6-20 A resolution discovered intense optical emission bands longward of 5400 A. With a higher spectral resolution of 1 A,... 

With current instruments it is possible to make spatial maps of the emission from different species in the Red Rectangle. These maps might provide valuable clues to the origin of different spectroscopic features. For example, in the spectrum of the Red Rectangle, the emission features which correspond to the diffuse interstellar bands are concentrated in what appears to be two hollow cones oriented perpendicular to the plane of this bipolar system (Schmidt Witt 1991). This hollow cone is similar to that proposed by Jura Kroto (1990) to explain the observed (Nguyen-Q-Rieu et al. 1986) HC,N emission (see around AFGL 2688, the Egg Nebula ), a very well studied carbon-rich object that appears to be in transition from a red giant to a planetary nebula. 

A. S. Webster (Royal Observatory, Edinburgh, U.K.). There is now one carbon star known, and in the outflow from that star, the diffuse interstellar bands are known. So there is a binary companion between the spectra. 

Returning to the question which motivated the experiments which in turn led to the discovery of the fullerenes do such carbon clusters exist in nature Astronomical searches for the distinctive fullerene signature of infrared absorption lines have been unsuccessful, and laboratory spectra of fullerenes have not shown any explicit connection to unsolved as-trophysical problems such as the so-called diffuse interstellar bands or other unidentified spectral features.[Ha92]... 

Abstract The study of the fate of electronically excited radical and radical cation of aromatic hydrocarbons is an emerging topic in modern chemical dynamics. Observations like low quantum yield of fluorescence and photostability are of immediate concern to unravel the mechanism of ultrafast nonradiative internal conversion dynamics in such systems. The radical cations of polycyclic aromatic hydrocarbons (PAHs) have received considerable attention in this context and invited critical measurements of their optical spectroscopy in a laboratory, in striving to understand the enigmatic diffuse interstellar bands (DIBs). 

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