4.1. PAH Emission Features
Interstellar dust glows in the infrared, with a significant fraction of the power radiated at 25 µm. In reflection nebulae the surface brightness is often high enough to permit spectroscopy of the emission; the 5-15 µm spectrum of NGC 7023 is shown in Figure 3. The spectrum has 5 very conspicuous emission peaks, at = 12.7, 11.3, 8.6, 7.6, and 6.25 µm; there is an additional emission peak at 3.3 µm (not shown here), as well as weaker peaks at 12.0 and 13.6 µm.
These emission features are in striking agreement with the wavelengths of the major optically-active vibrational modes for polycyclic aromatic hydrocarbon (PAH) molecules: the 5-15 µm features are labelled in Figure 3; the 3.3 µm feature (not shown) is the C-H stretching mode. The vibrational excitation results from single-photon heating (see Section 5). The strength of the observed emission requires that PAH molecules be a major component of interstellar dust. Modeling the observed emission in reflection nebulae indicates that the PAH species containing 103 C atoms contain ~ 40 ppm C/H - approximately 15% of (C/H) = 246 ± 23 ppm (Allende Prieto et al 2002).
Figure 3. PAH emission features in the 5-15 µm emission spectrum of the reflection nebula NGC 7023 (Cesarsky et al. 1996), and four PAH molecules, with examples of mono, duo, trio, and quartet H sites indicated.
4.2. 2175Å Feature: Graphitic C
By far the strongest feature in the extinction curve is the 2175Å "bump" (see Figures 1 and 2). Stecher & Donn (1965) pointed out that the observed feature coincided closely with the position and width of absorption expected from small spheres of graphite. Graphite consists of parallel sheets of graphene - two-dimensional hexagonal carbon lattices; adjacent graphene layers interact only through a weak van der Waals interaction.
The 2175Å feature arises from a * excitation of a orbital in graphene; in small graphite spheres the feature strength corresponds to an oscillator strength per C of 0.16 (Draine 1989), so that the observed 2175Å feature requires C/H 60 ppm 0.25(C/H) to account for the observed strength of the 2175Å feature.
In a large PAH molecule, the interior C atoms have electronic orbitals closely resembling those in graphene, and one therefore expects that large PAH molecules will have a strong absorption feature peaking near 2175Å, with an oscillator strength per C expected to be close to the value for graphite. It is of course interesting to note that the C/H in PAHs required to account for the 3-15 µm IR emission is ~ 2/3 of that required to account for the 2175Å bump. Since it would be entirely natural to have additional PAHs containing > 103 C atoms it is plausible that the 2175Å feature could be entirely due to PAHs, in which case the PAHs contain C/H 60 ppm.
The 2175Å feature is suppressed in graphite grains with a 0.02 µm because the grain becomes opaque throughout the 1800-2600Å range. There can therefore be additional "aromatic" C within a 0.02 µm grains.
4.3. 10 and 18 µm Silicate Features
Interstellar dust has a strong absorption feature at 9.7 µm. While the precise composition and structure of the carrier remains uncertain, there is little doubt that the 9.7 µm feature is produced by the Si-O stretching mode in silicates. In the laboratory, crystalline silicates have multiple narrow features in their 10µm spectra, while amorphous silicate material has a broad profile.
The interstellar 9.7 µm feature is seen in absorption on a number of sightlines. The observed profiles are broad and relatively featureless, indicative of amorphous silicate material. The observed strength of teh absorption feature requires that much, perhaps most, of interstellar Si atoms reside in silicates. Li & Draine (2001a) estimated that at most 5% of interstellar silicate material was crystalline. Conceivably, a mixture of a large number of different crystalline minerals (Bowey & Adamson 2002) could blend together to produce the observed smooth profiles, although it seems unlikely that nature would have produced a blend of crystalline types with no fine structure evident in either absorption or emission, including emission in the far-infrared (Draine 2003a). The 9.7 µm extinction profile does not appear to be "universal" - sightlines through the diffuse ISM show a narrower feature than sightlines through dense clouds (Roche & Aitken 1984; Bowey et al. 1998). Evidently the silicate material is altered in interstellar space.
Identification of the 9.7 µm feature as the Si-O stretching mode is confirmed by the presence of a broad feature centered at ~ 18 µm (McCarthy et al. 1980, Smith et al. 2000) that is interpreted as the O-Si-O bending mode in silicates.
Spectral features of crystalline silicates are seen in some circumstellar disks (Artymowicz 2000, Waelkens et al. 2000) and some comets (Hanner 1999), but even in these objects only a minority of the silicate material is crystalline (e.g., Bouwman et al. 2001).
4.4. 3.4 µm C-H Stretch: Aliphatic Hydrocarbons
Sightlines with sufficient obscuration reveal a broad absorption feature at 3.4 µm that is identified as the C-H stretching mode in aliphatic (i.e., chain-like) hydrocarbons. Unfortunately, it has not proved possible to identify the specific aliphatic hydrocarbon material, and the band strength of this mode varies significantly from one aliphatic material to another. Sandford et al. (1991) suggest that the 3.4 µm feature is due to short saturated aliphatic chains incorporating C/H 11 ppm, while Duley et al. (1998) attribute the feature to hydrogenated amorphous carbon (HAC) material containing C/H 85 ppm.
4.5. Diffuse Interstellar Bands
In addition to narrow absorption features identified as atoms, ions, and small molecules, the observed extinction includes a large number of broader features - known as the "diffuse interstellar bands", or DIBs. The first DIB was recognized over 80 years ago (Heger 1922), and shown to be interstellar 70 years ago (Merrill 1934). A recent survey by Jenniskens & Desert (1994) lists over 154 "certain" DIBs, with another 52 "probable" features. Amazingly, not a single one has yet been positively identified!
High resolution spectroscopy of the 5797Å and 6614Å DIBs reveals fine structure that is consistent with rotational bands in a molecule with tens of atoms (Kerr et al. 1996, 1998), and there is tantalizing evidence that DIBs at 9577Å and 9632Å may be due to C60+ (Foing & Ehrenfreund 1994; Galazutdinov et al 2000, but see also Jenniskens et al. 1997 and Moutou et al. 1999)
It appears likely that some or all of the DIBs are due to absorption in large molecules or ultrasmall grains. As noted above, a large population of PAHs is required to account for the observed IR emission, and it is reasonable to suppose that these PAHs may be responsible for many of the DIBs. What is needed now is laboratory gas-phase absorption spectra for comparison with observed DIBs. Until we have precise wavelengths (and band profiles) from gas-phase measurements, secure identification of DIBs will remain problematic.
4.6. Extended Red Emission
Interstellar dust grains luminesce in the far-red, a phenomenon referred to as the "extended red emission" (ERE). The highest signal-to-noise observations are in reflection nebulae, where a broad featureles emission band is observed to peak at wavelength 6100 p 8200Å, with a FWHM in the range 600-1000Å (Witt & Schild 1985, Witt & Boroson 1990). The ERE has also been seen in H II regions (Darbon et al. 2000), planetary nebulae (Furton & Witt 1990), and the diffuse interstellar medium of our Galaxy (Gordon et al. 1998, Szomoru & Guhathakurta 1998).
The ERE is photoluminescence: absorption of a starlight photon raises the grain to an excited state from which it decays by spontaneous emission of a lower energy photon. The reported detection of ERE from the diffuse interstellar medium (Gordon et al. 1998, Szomoru & Guhathakurta 1998) appears to require that the interstellar dust mixture have an overall photoconversion efficiency of order ~ 10% for photons shortward of ~ 5000 Å. If the overall efficiency is ~ 10%, the ERE carrier itself must contribute a significant fraction of the overall absorption by interstellar dust at 5000 Å.
Candidate ERE carriers which have been proposed include PAHs (d'Hendecourt et al. 1986) and silicon nanoparticles (Ledoux et al. 1998, Witt et al. 1998, Smith & Witt 2002). While some PAHs are known to luminesce, attribution of the ERE to PAHs is difficult because of nondetection of PAH emission from some regions where ERE is seen (Sivan & Perrin 1993, Darbon et al 2000) and nondetection of ERE in some reflection nebulae with PAH emission (Darbon et al 1999). Oxide-coated silicon nanoparticles appear to be ruled out by nondetection of infrared emission at ~ 20 µm (Li & Draine 2002a). The search to identify the ERE carrier continues.
4.7. X-Ray Absorption Edges
Dust grains become nearly transparent at X-ray energies, so that the measured X-ray absorption is sensitive to all of the atoms, not just those in the gas phase. Consider, for example, K shell absorption by an oxygen atom, where photoabsorption excites one of the 1s electrons to a higher (initially vacant) energy level. Transitions to a bound state (2p, 3p, 4p, ...) produce a series of absorption lines, and transitions to unbound "free" (i.e, "photoelectron") states result in a continuum beginning at the "absorption edge". If the atom is in a solid, the absorption spectrum is modified because the available bound states and free electron states are modified by the presence of other nearby atoms. High resolution X-ray absorption spectroscopy of interstellar matter could thereby identify the chemical form in which elements are bound in dust grains (Forrey et al. 1998, and references therein).
Figure 4 shows the structure expected for scattering, absorption, and extinction near the major X-ray absorption edges in a grain model composed of sp2-bonded carbon grains plus amorphous MgFeSiO4 grains (Draine 2003c). Spectroscopy with 1 eV energy resolution near these absorption edges for both interstellar sightlines and laboratory samples can test this model for the composition of interstellar dust. The Chandra X-Ray Observatory has measured the wavelength-dependent extinction near the absorption edges of O (Paerels et al. 2001, Takei et al. 2002) and O, Mg, Si, and Fe (Schulz et al. 2002), but it has not yet proved possible to identify the chemical form in which the solid-phase Mg, Si, Fe, and O reside.
Figure 4. Scattering and absorption cross sections per H nucleon near principal X-ray absorption edges, for an interstellar dust model consisting of graphite and amorphous silicate grains (from Draine 2003c).