Unidentified infrared bands

Infrared emission from many regions of the interstellar medium is dominated by what are often called the unidentified infrared bands (UIBs).As shown in Fig. 1.3, these broad features are observed at several wavelengths between 3 and 15 /rm [18]. They are attributed to vibrational transitions associated with the C—H and C—C stretching vibrations (at ca. 3.3 and 6-8 pm, respectively) and CH in-plane and CH out-of-plane bending modes (at ca. 8.5 and 10-15 pm, respectively) in large... 

The term unidentified infrared emission is used to refer to the long-known emission features of interstellar dusts in the spectral region from just over 3,000 cm-1 to below 800 cm-1 (Gillett et al. 1973). These features comprise sharp IR bands at 2,920,1,610, and 880 cm-1, as well as a broader envelope near 1,300 cm-1. In addition, a recurrent mode at 3,050 cm-1, a weak mode near 1,450 cm-1, and a shoulder near 1,150 cm-1 are observed. These spectral features can all be attributed to vibrational modes of hydrogenated carbon species, as summarized in Table 2.1. The chemical structure of these species remains the subject of debate. Furthermore, a number of carbon-rich astronomical objects reveal an emission feature in the far-IR at 490 cm-1, of unclear attribution (Kwok et al. 1989). 

Calculated spectra for hydrogenated fullerenes have been published in comparison with the unidentified infrared emission bands (Webster 1991 Stoldt et al. 2001). The infrared spectrum of C60H36 has been compared also with the infrared features of other astrophysical objects like the proto-planetary nebulae (Cataldo 2003a, b). An inventory about fullerenes and hydrogenated derivatives in the interstellar... 

The normal-mode vibrational frequencies of a molecule correspond, with qualifications, to the bands seen in the infrared (IR) spectrum of the substance. Discrepancies may arise from overtone and combination bands in the experimental IR, and from problems in accurate calculation of relative intensities (less so, probably, from problems in calculation of frequency positions). Thus the IR spectrum of a substance that has never been made can be calculated to serve as a guide for the experimentalist. Unidentified IR bands observed in an experiment can sometimes be assigned to a particular substance on the basis of the calculated spectrum of a suspect if the spectra of the usual suspects are not available from experiment (they might be extremely reactive, transient species), we can calculate them. 

Whereas on Earth, PAHs and related species such as (benzo[a]pyrene) are considered as highly carcinogenic, mutagenic, and teratogenic, and therefore resemble unwanted by-product in combustion processes, the situation in quite reversed in hydrocarbon-rich atmospheres of planets and their moons, as well as in the interstellar medium. Here, PAH-like species are thought to contribute to the unidentified infrared emission bands (UIBs) observed between 3 and 15 )o,m. " Itis also estimated that PAHs and related molecules such as their radicals, ionized PAHs, and heteroaromatic PAH make up to 20% of the total cosmic carbon budget. Also, the role of PAHs in astrobiology should be noted. ... 

Many celestial objects show a distinctive set of emission features in the infrared, known as the unidentified infrared emission bands (UIR bands) [21,22]. Since 1981 when Duley and Williams [23] pointed out that a few of the bands fell at the frequencies characteristic of polycyclic aromatic hydrocarbon molecules (PAHs) and suggested that these bands were produced by aromatic units in thermally excited dust grains, a number of arguments coming from observations, experiments and calculations, converge towards the hypothesis that the features eventually arise from free molecular PAHs laSier than dust grain [24-27]. Each band of this set, i.e. 3.3, 6.2, 7.7, 8.6 and 11.3 p.m, is identified as a fundamental vibrational mode of this class of molecules, respectively the CH stretch, the C=C stretch, C=C deformation modes of the skeleton, the CH in-plane and finally the CH out-of-plane bending vibrations. 

Ground-based, airborne and space-borne observations have shown that the IR spectra of bright sources with associated dust and gas are dominated by relatively broad emission features at 3.3, 6.2, 7.7, 8.6 and 11.3 pm, which always appear together. Because the carriers of these bands remained unidentified for almost a decade, these bands have become collectively known as the unidentified infrared (UIR) bands. These bands are now unequivocally identified with an aromatic carrier and this name is somewhat of a misnomer. Nevertheless, the abbreviation has stuck and is still widely used. As an example. Figure 7 shows three mid-infrared spectra of carbon-rich outflows from stars in the latest stages of their evolution. It is now known that these emission features are not limited to stellar outflows but are also present in the spectra of disks of newly formed... 

What information can we derive about molecular structure from the vibrational bands of infrared spectra Absorption of radiation in the range of 5000-1250 cm-1 is characteristic of the types of bonds present in the molecule, and corresponds for the most part to stretching vibrations. For example, we know that the C—H bonds of alkanes and alkyl groups have characteristic absorption bands around 2900 cm-1 an unidentified compound that shows absorption in this region will very likely have alkane-type C—H bonds. 

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]... 

The infrared analyses showed absorption bands in the region of 3.4, 6.9, and 7.3 microns caused by carbon-hydrogen bonds bands of 5.8 and 8.3 microns indicative of acetone carbon dioxide bands of 4.3, 13.9, and 15 microns and a doublet at 6.3 microns, caused possibly by organic nitrates. Several minor unidentified bands were also present. The ultraviolet spectra showed no significant absorption bands. 

It has been suggested that free polycyllc aromatic hydrocarbon (PAH) molecules can account for unidentified Interstellar emission features In the infrared (8) and for the diffuse Interstellar absorption bands In the visible (9), Also, the possibility of the Interstellar and circumstellar presence of hollow, cage-llke polycarbon molecules with spheroidal shells of hexagonal, graphlte-like sheets of carbon atoms has... 

---