Polycyclic aromatic hydrocarbons infrared emission bands

Very small grains and macromolecules are known to be present in the surface layers of some disks as well as in the ISM. They are usually revealed by emission features due to polycyclic aromatic hydrocarbons (PAHs, see also Chapter 6). These particles, with frameworks of six to several thousand carbon atoms, are so small that they can be excited by single ultraviolet photons. Subsequently, they will non-thermally re-radiate the energy in discrete, but broad, bands stretching across the mid-infrared wavelength region. They are found in protoplanetary disks when the ultraviolet radiation field is sufficiently high (Habart et al. 2004), but... 

The polycyclic aromatic hydrocarbons (PAHs) have been inferred to exist in the interstellar dust by the correlation of their general infrared spectral characteristics with observed celestial infrared emission bands [13-15]. 

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. 

AUamandola LJ, Tielens AGGM and Barker JR (1989) Interstellar polycyclic aromatic hydrocarbons the infrared emission bands, the excitation/emission mechanism, and the astrophysical implications. The Astro-physicaljoumal Supplement Series 71 733-775. 

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